Physiology of Bone Formation, Remodeling, and Metabolism 2

Usha Kini and B. N. Nandeesh

Contents 2.1 Introduction 2.1 Introduction ...... 29 Bone is a highly specialized supporting frame- 2.2 Physiology of Bone Formation ...... 30 2.2.1 Bone Formation ...... 30 work of the body, characterized by its rigidity, 2.2.2 Osteoblasts ...... 31 hardness, and power of regeneration and repair. It 2.2.3 Bone Matrix ...... 31 protects the vital organs, provides an environ- 2.2.4 Bone Minerals ...... 32 ment for marrow (both blood forming and fat 2.2.5 Osteocytes ...... 32 2.2.6 Intramembranous (Mesenchymal) storage), acts as a mineral reservoir for calcium Ossification ...... 32 homeostasis and a reservoir of growth factors and 2.2.7 Intracartilaginous (Endochondral) cytokines, and also takes part in acid–base bal- Ossification ...... 33 ance (Taichman 2005 ) . Bone constantly under- 2.2.8 Biological Factors Involved in Normal Bone Formation and Its Regulation ...... 37 goes modeling (reshaping) during life to help it 2.2.9 Bone Modeling ...... 37 adapt to changing biomechanical forces, as well 2.2.10 Determinants of Bone Strength ...... 37 as remodeling to remove old, microdamaged 2.3 Physiology of Bone Metabolism ...... 38 bone and replace it with new, mechanically stron- 2.3.1 Cellular and Intracellular Calcium ger bone to help preserve bone strength. and Phosphorus Metabolism ...... 38 The bones have two components – the cortical 2.3.2 Regulation of Skeletal Metabolism ...... 38 bone which is dense, solid, and surrounds the 2.4 Bone Remodeling ...... 42 marrow space and the trabecular bone which is 2.4.1 Mediators of Remodeling ...... 43 composed of a honeycomb-like network of trabe- 2.4.2 Remodeling Phases ...... 44 2.4.3 Regulatory Factors in Bone Remodeling ..... 46 cular plates and rods interspersed in the bone 2.4.4 Markers of Bone Metabolism ...... 51 marrow compartment. Cortical bone has an outer 2.4.5 Pathophysiology of Bone Remodeling ...... 53 periosteal surface and inner endosteal surface. References ...... 55 The periosteum is a fi brous connective tissue sheath that surrounds the outer cortical surface of bone, except at joints where bone is lined by articular cartilage. It contains blood vessels, nerve fi bers, osteoblasts, and osteoclasts. It protects, nourishes, and aides in bone formation. It U. Kini , M.D., DCP, DNB, FICP () plays an important role in appositional growth B. N. Nandeesh , M.D., DNB and fracture repair. The endosteum is a membra- Department of Pathology , nous structure covering the inner surface of corti- St. John’s Medical College and Hospital , Koramangala , Bangalore , Karnataka 560034 , India cal and cancellous bone and the blood vessel e-mail: [email protected] ; [email protected] canals (Volkmann’s canals) present in bone.

I. Fogelman et al. (eds.), Radionuclide and Hybrid Bone Imaging, 29 DOI 10.1007/978-3-642-02400-9_2, © Springer-Verlag Berlin Heidelberg 2012 30 U. Kini and B.N. Nandeesh

Fig. 2.1 Woven bone/ immature bone (W ) rimmed by osteoblasts (B ) with peripheral lamellar bone (L ) showing a Haversian canal (H )

Further, based on the pattern of collagen form- within the matrix. During life, the bones undergo ing the osteoid, two types of bone are identifi ed: processes of longitudinal and radial growth, mod- woven bone, which is characterized by a haphaz- eling (reshaping), and remodeling (Clarke 2008 ) . ard organization of collagen fi bers (Eriksen et al. Longitudinal growth occurs at the growth plates, 1994 ) , and lamellar bone, which is characterized where cartilage proliferates in the epiphyseal and by a regular parallel alignment of collagen into metaphyseal areas of long bones, before subse- sheets (lamellae) (Fig. 2.1 ). Lamellar bone, as a quently undergoing mineralization to form pri- result of the alternating orientations of collagen mary new bone. fi brils, has a signifi cant mechanical strength similar to plywood. This normal lamellar pattern is absent in woven bone, in which the collagen 2.2.1 Bone Formation fi brils are laid down in a disorganized manner. Hence, the woven bone is weaker than lamellar Ossi fi cation (or osteogenesis) is the process of for- bone. Woven bone is produced when osteoblasts mation of new bone by cells called osteoblasts. produce osteoid rapidly. This occurs initially in These cells and the bone matrix are the two all fetal bones and in fracture healing, but the most crucial elements involved in the formation resulting woven bone is replaced by a process of bone. This process of formation of normal called remodeling by the deposition of more healthy bone is carried out by two important pro- resilient lamellar bone. Virtually all the bone in cesses, namely: the healthy mature adult is lamellar bone. 1. Intramembranous ossi fi cation characterized by laying down of bone into the primitive connective tissue (mesenchyme) resulting in 2.2 Physiology of Bone Formation the formation of bones (skull, clavicle, mandible). It is also seen in the healing process of Bone is composed of support cells, namely, fractures (compound fractures) treated by osteoblasts and osteocytes ; remodeling cells, open reduction and stabilization by metal namely, osteoclasts ; and non-mineral matrix of plate and screws. collagen and noncollagenous proteins called 2. Endochondral ossi fi cation where a cartilage osteoid, with inorganic mineral salts deposited model acts as a precursor (e.g., femur, tibia, 2 Physiology of Bone Formation, Remodeling, and Metabolism 31

humerus, radius). This is the most important Mesenchymal stem cell process occurring during fracture healing when treated by cast immobilization. Hemopoietic If the process of formation of bone tissue stem cell occurs at an extraskeletal location, it is termed as Osteoblast heterotopic ossi ﬁ cation . precursor Three basic steps involved in osteogenesis Osteoclast precursors are: Osteoblasts Osteoclast (a) Synthesis of extracellular organic matrix (osteoid) (b) Matrix mineralization leading to the forma- Bone tion of bone (c) Remodeling of bone by the process of resorption and reformation Fig. 2.2 Diagram to show evolution of osteoblasts and osteoclasts in the formation of bone

2.2.2 Osteoblasts tors, physical activity, and other stimuli act mainly through osteoblasts to bring about their Osteoblasts originate from mesenchymal stem effects on bone (Harada and Rodan 2003 ) . cells (osteoprogenitor cells) (Fig. 2.2 ) of the bone marrow stroma and are responsible for bone 2.2.2.1 Wnt Pathway on matrix synthesis and its subsequent mineraliza- Osteoblastogenesis tion. Commitment of mesenchymal stem cells to Wnts are secreted glycoproteins that regulate a the osteoblast lineage requires the canonical Wnt/ variety of cellular activities, such as cell fate, deter- j3-catenin pathway and associated proteins mination, proliferation, migration, survival, polar- (Logan and Nusse 2004 ) . ity and gene expression (Cadigan and Liu 2006 ; Osteoblasts are mononucleated, and their shape Caetano-Lopes et al. 2007 ) . This pathway is essen- varies from fl at to plump, re fl ecting their level of tial for the differentiation of mature osteoblasts cellular activity, and, in later stages of maturity, and consequently for bone formation. Reduced lines up along bone-forming surfaces (Fig. 2.1 ). Wnt signaling has been associated with osteoporo- Osteoblasts are responsible for regulation of sis (Krishnan et al. 2006 ) . The Wnt system is also osteoclasts and deposition of bone matrix (Mackie important in chondrogenesis and hematopoiesis 2003 ) . As they differentiate, they acquire the and may be stimulatory or inhibitory at different ability to secrete bone matrix. Ultimately, some stages of osteoblast differentiation. osteoblasts become trapped in their own bone matrix, giving rise to osteocytes which, gradually, stop secreting osteoid. Osteocytes are the 2.2.3 Bone Matrix most abundant cells in bone; these cells communicate with each other and with the surrounding The structure of bone is constituted by: medium through extensions of their plasma mem- (a) Inorganic (69 %) component, consisting of brane. Therefore, osteocytes are thought to act as hydroxyapatite (99 %) mechanosensors, instructing osteoclasts where (b) Org anic (22 %), constituted by collagen (90 %) and when to resorb bone and osteoblasts where and noncollagen structural proteins which and when to form it (Boulpaep and Boron 2005 ; include proteoglycans, sialoproteins, gla- Manolagas 2000 ) . The osteoblasts, rich in alka- containing proteins, and 2HS-glycoprotein line phosphatase, an organic phosphate-splitting The functional component of the bone includes enzyme, possess receptors for parathyroid hor- growth factors and cytokines. The hardness and mone and estrogen. Also, hormones, growth fac- rigidity of bone is due to the presence of mineral 32 U. Kini and B.N. Nandeesh salt in the osteoid matrix, which is a crystalline important mediators of calcium regulation, and complex of calcium and phosphate (hydroxyapa- vitamin D defi ciency or hyperparathyroidism will tite). Calci fi ed bone contains about 25 % organic lead to depletion of the bone minerals. matrix, 5 % water, and 70 % inorganic mineral (hydroxyapatite). Collagen 1 constitutes 90–95 % of the organic matrix of bone. Osteoblasts synthe- 2.2.5 Osteocytes size and lay down precursors of collagen 1 (Brodsky and Persikov 2005 ) . They also produce Osteocytes represent terminally differentiated osteocalcin, which is the most abundant noncol- osteoblasts and function within syncytial net- lagenous protein of bone matrix, and the proteo- works to support bone structure and metabolism. glycans of ground substance. The collagen 1 Osteocytes maintain connection with each other formed by osteoblasts is deposited in parallel or and the bone surface via their multiple fi lipodial concentric layers to produce mature (lamellar) cellular processes. Osteocytes are linked meta- bone. When bone is rapidly formed, as in the bolically and electrically through gap junctions fetus or certain pathological conditions (e.g., composed primarily of connexin (Bonewald fracture callus, fi brous dysplasia, hyperparathy- 1999; Plotkin et al. 2002) . The presence of empty roidism), the collagen is not deposited in a paral- lacunae in aging bone suggests that osteocytes lel array but in a basket-like weave resulting in may undergo apoptosis, probably caused by dis- woven, immature, or primitive bone (Fig. 2.1 ). ruption of their intercellular gap junctions or cell- Osteoblasts also synthesize and secrete non- matrix interactions. Osteocyte apoptosis in collagenous protein, such as proteoglycans, gly- response to estrogen de fi ciency or glucocorticoid cosylated proteins, glycosylated proteins with treatment is harmful to bone structure. Estrogen potential cell-attachment activities, and g -carbox- and bisphosphonate therapy and physiologic ylated (gla) proteins. The main glycosylated pro- loading of bone may help prevent osteoblast and tein present in bone is alkaline phosphatase, osteocyte apoptosis (Plotkin et al. 2005 ; Xing which plays an as-yet-undefi ned role in mineral- and Boyce 2005 ) . ization of bone (Whyte 1994 ) .

2.2.6 Intramembranous 2.2.4 Bone Minerals (Mesenchymal) Ossiﬁ cation

Crystalline hydroxyapatite [Ca 10 (PO4 )6(OH)2 ] is Intramembranous ossi fi cation is one of the two the chief mineral component of bone, constituting essential processes during fetal development of approximately about a quarter of the volume and the mammalian skeletal system resulting in the half of the mass of normal adult bone. These min- formation of bone tissue. Intramembranous eral crystals (according to electron microscopy) ossifi cation mainly occurs during formation of are deposited along, and in close relation to, the the fl at bones of the skull but also the mandible, bone collagen fi brils. The calcium and phosphorus maxilla, and clavicles; it is also an essential pro- (inorganic phosphate) components of these crys- cess during the natural healing of bone fractures tals are derived from the blood plasma and which and the rudimentary formation of bones of the in turn is from nutritional sources. Amorphous cal- head. The bone is formed from connective tissue cium phosphate matures through several interme- such as mesenchyme tissue rather than from car- diate stages to form hydroxyapatite. The end result tilage. The steps in intramembranous ossi fi cation is a highly organized amalgam of protein, primar- (Fig. 2.3 ) are: ily collagen, and mineral, primarily hydroxyapa- 1. Formation of ossi fi cation center tite, that has suffi cient structural integrity to serve 2. Calci fi cation the mechanical functions of the skeleton. Vitamin 3. Formation of trabeculae D metabolites and parathyroid hormone (PTH) are 4. Development of periosteum 2 Physiology of Bone Formation, Remodeling, and Metabolism 33

Fig. 2.3 Intramembranous ossi ﬁ cation showing: (a ) stem cell transform to osteoblasts which synthesize Aggregates of osteoprogenotor cells. (b ) Amorphous osteoid in the center of the aggregate. (d ) A rudimentary ground substance and collagen meshwork formed in the bone tissue formed by the osteoblasts and some of these center and in between the cells. (c ) The mesenchymal get incorporated within the osteoid to become osteocytes

The important cell in the creation of bone tissue by As these changes are proceeding in the ossi fi cation membrane ossi fi cation is a mesenchymal stem center, the surrounding mesenchyme condenses cell. Mesenchymal stem cells (MSCs) within as a fi brovascular periosteum around its edges and human mesenchyme or the medullary cavity of a surfaces. The periosteum is thus formed, and bone bone fracture initiate the process of intramembra- growth continues at the surface of trabeculae. nous ossifi cation. An MSC is an unspecialized cell Much like spicules, the increasing growth of tra- whose morphology undergoes characteristic beculae results in interconnection, and this net- changes as it develops into an osteoblast. The pro- work is called woven bone. Eventually, woven cess of membranous ossifi cation, which is essen- bone is replaced by lamellar bone. Extension of tially the direct mineralization of a highly vascular the ossifi cation process occurs through the agency connective tissue, commences at certain constant of stem cells derived from the deeper layers of the points known as centers of ossi fi cation. At such a periosteum. center, the mesenchymal cells (osteoprogenitor cells) proliferate and condense around a profuse capillary network. Between the cells and around 2.2.7 Intracartilaginous the vessels is amorphous ground substance (Endochondral) Ossifi cation (Fig. 2.3 ) with a fi ne meshwork of collagen fi bers. The osteoprogenitor cells differentiate (spe- Endochondral ossi fi cation (Greek: endon , “within,” cialize) into osteoblasts, which create osteoid in chondros , “cartilage”) occurs in long bones and the center of the aggregate. The osteoblasts pro- most of the rest of the bones in the body; it involves duce bone matrix and get surrounded by collagen an initial hyaline cartilage which continues to fi bers and become osteocytes. At this point, the grow. It is also an essential process during the osteoid becomes mineralized, resulting in a nidus growth of the length of long bones and the natural consisting of mineralized osteoid that contains healing of bone fractures (Brighton and Hunt osteocytes and is lined by active osteoblasts. The 1986 ; Netter 1987 ; Brighton et al. 1973 ) . nidus that began as a diffuse collection of MSCs The steps in endochondral ossifi cation has become rudimentary bone tissue. This process (Figs. 2.4 , 2.5 , and 2.6 ) are: of entrapping of osteoblasts proceeds, the trabec- 1. Development of cartilage model ulae gradually thicken, and the intervening vascu- 2. Growth of cartilage model lar spaces (spongy layer) become progressively 3. Development of the primary ossi fi cation center narrowed. Where the bone persists as cancellous 4. Development of the secondary ossi fi cation bone, however, the process slows down, and the center spaces later become occupied by hemopoietic 5. Formation of articular cartilage and epiphy- tissue (Netter 1987 ; Brighton and Hunt 1991 ) . seal plate 34 U. Kini and B.N. Nandeesh

Secondary ossification center Chondrocyte hypertrophy

Bone collar

ab c d e

Fig. 2.4 Intracartilaginous ossi fi cation showing: (a ) center of ossifi cation ( e) bone with medullary cavity and aggregates of osteoprogenitor cells (b ) model of hyaline epiphyseal ends (f ) highlighting feeding blood vessels cartilage (c ) primary center of ossi fi cation ( d ) secondary

Endochondral ossi fi cation begins with points in foreshadowing that of the early bone (e.g., car- the cartilage called “primary ossifi cation cen- pal bones are preceded by appropriately shaped ters.” They mostly appear during fetal develop- cartilaginous “models”). The cartilaginous ment, though a few short bones begin their model is surrounded by a highly vascular con- primary ossifi cation after birth. They are respon- densed mesenchyme or perichondrium, similar sible for the formation of the diaphyses of long in every way to that which precedes and sur- bones, short bones, and certain parts of irregular rounds intramembranous ossifi cation centers bones. Secondary ossifi cation occurs after birth with its deeper layers containing osteoprogeni- and forms the epiphyses of long bones and the tor cells. extremities of irregular and fl at bones. The diaphysis and both epiphyses of a long bone are 2.2.7.2 Growth of the Cartilage Model separated by a growing zone of cartilage (the The cartilage model will grow in length by epiphyseal plate). When the child reaches skele- continuous cell division of chondrocytes, tal maturity (18–25 years of age), all of the carti- which is accompanied by further secretion of lage is replaced by bone, fusing the diaphysis extracellular matrix. This is called interstitial and both epiphyses together (epiphyseal growth. The process of appositional growth closure). occurs when the cartilage model would also grow in thickness which is due to the addition 2.2.7.1 Development of Cartilage Model of more extracellular matrix on the periphery Each long bone is represented in early fetal life cartilage surface, which is accompanied by by a rod of hyaline cartilage which replaces new chondroblasts that develop from the a rod of condensed mesenchyme, its shape perichondrium. 2 Physiology of Bone Formation, Remodeling, and Metabolism 35

Fig. 2.5 Microphotographs to show primary centers of mesenchyme which carries with it hemopoietic cells and ossi fi cation: (a ) fi rst stage where the chondrocytes at the osteoprogenitor cells (f ) showing trabeculae formation by center of ossifi cation undergo apoptosis/death (b – e ) osteoblasts differentiated from the osteoprogenitor cells showing invasion of this ossi fi cation center by vascular and the hemopoietic cells forming the bone marrow

2.2.7.3 Primary Center of Ossifi cation • Formation of Periosteum . Once vascularized, The fi rst site of ossi fi cation occurs in the primary the perichondrium becomes the periosteum. center of ossi fi cation, which is in the middle of The periosteum contains a layer of undifferen- diaphysis (shaft) and is followed by the following tiated cells (osteoprogenitor cells) which later events: become osteoblasts. 36 U. Kini and B.N. Nandeesh

Fig. 2.6 Endochondral A ossi ﬁ cation as seen at B microscopy: A – showing resting hyaline cartilage, B – chondrocytes undergoing hyperplasia with rapid proliferation laid down as distinctive-looking stacks, C – calci ﬁ cation of the matrix occurs, D – chondrocytes are either dying or dead, leaving cavities that will later become invaded by C bone-forming cells

• Formation of Bone Collar . The osteoblasts Osteoclasts, formed from macrophages, break secrete osteoid against the shaft of the carti- down spongy bone to form the medullary lage model (appositional growth). This serves (bone marrow) cavity. as support for the new bone. • Calci fi cation of Matrix . Chondrocytes in the 2.2.7.4 Secondary Center of Ossifi cation primary center of ossi fi cation begin to grow About the time of birth, a secondary ossi fi cation (hyperplasia) (Fig. 2.6 ). They stop secreting center appears in each end (epiphysis) of long collagen and other proteoglycans and begin bones. Periosteal buds carry mesenchyme and secreting alkaline phosphatase, an enzyme blood vessels in, and the process is similar to that essential for mineral deposition. Then, occurring in a primary ossi fi cation center. The calci fi cation of the matrix occurs, and apopto- cartilage between the primary and secondary sis of the hypertrophic chondrocytes occurs. ossifi cation centers is called the epiphyseal plate, This creates cavities within the bone. and it continues to form new cartilage, which is • Invasion of Periosteal Bud . The hypertrophic replaced by bone, a process that results in an chondrocytes (before apoptosis) secrete vascu- increase in length of the bone. Growth continues lar endothelial cell growth factor that induces until the individual is about 21 years old or until the sprouting of blood vessels from the per- the cartilage in the plate is replaced by bone. The ichondrium. Blood vessels forming the point of union of the primary and secondary periosteal bud invade the cavity left by the ossi fi cation centers is called the epiphyseal line. chondrocytes and branch in opposite directions Appositional Bone Growth . The growth in diam- along the length of the shaft. The blood vessels eter of bones around the diaphysis occurs by depo- carry hemopoietic cells, osteoprogenitor cells, sition of bone beneath the periosteum. Osteoclasts and other cells inside the cavity. The hemopoi- in the interior cavity continue to degrade bone until etic cells will later form the bone marrow. its ultimate thickness is achieved, at which point • Formation of Trabeculae . Osteoblasts, differ- the rate of formation on the outside and degrada- entiated from the osteoprogenitor cells that tion from the inside is constant. The cartilaginous entered the cavity via the periosteal bud, use the extremity (where an epiphysis usually forms) con- calcifi ed matrix as a scaffold and begin to secrete tinues to grow in pace with the rest of the bone by osteoid, which forms the bone trabecula. appositional and interstitial mechanisms. 2 Physiology of Bone Formation, Remodeling, and Metabolism 37

When the whole bone is reaching maturity, gradual adjustment of the skeleton to the forces that epiphyseal and metaphyseal ossi ﬁ cation gradu- it encounters. Bones may widen or change axis by ally encroach upon this growth plate, and ﬁ nal removal or addition of bone to the appropriate sur- bony fusion occurs with cessation of growth. faces by independent action of osteoblasts and osteoclasts in response to biomechanical forces. Bones normally widen with aging in response to 2.2.8 Biological Factors Involved periosteal apposition of new bone and endosteal in Normal Bone Formation resorption of old bone. Wolff’s law describes the and Its Regulation observation that long bones change shape to accom- modate stresses placed on them. Bone modeling is There is a rapid formation of bone mass in the less frequent than remodeling in adults (Kobayashi fetus and infant. This slows somewhat during et al. 2003) . Modeling may be increased in childhood until age 11 in females and a year or so hypoparathyroidism (Ubara et al. 2005 ) , renal later in boys. During the growth spurt that accom- osteodystrophy (Ubara et al. 2003 ) , or treatment panies adolescence, tremendous bone formation with anabolic agents (Lindsay et al. 2006 ) . occurs. The vast majority of adult levels of bone mass are achieved by age 18 or so, with only a small amount added until about 28 years old. 2.2.10 Determinants of Bone Strength

2.2.8.1 Environmental Factors In fl uencing Bone strength depends on bone mass, geometry Normal Bone Formation and composition, material properties, and micro- Physical activity and good nutrition are the most structure. Bone mass accounts for 50–70 % of important of these environmental factors. People bone strength (Pocock et al. 1987 ) . Bone geom- who are affected by any of these factors will likely etry and composition are important, however, have a lower bone mineral density (BMD) than their because larger bones are stronger than smaller healthier peers. Poor activity levels and nutrition bones, even with equivalent bone mineral den- during the years of bone formation may prevent the sity. As bone diameter expands radially, the normal growth of bones, which may cause them to strength of bone increases by the radius of the be less dense. Smoking during these years may also involved bone raised to the fourth power. The decrease the amount of bone formed. A signifi cant amount and proportion of trabecular and cortical illness during the teenage years that causes pro- bone at a given skeletal site affect bone strength longed bed rest and lack of exercise will also pre- independently. Bone material properties are vent the complete acquisition of bone density. important for bone strength. Some patients with osteoporosis have abnormal bone matrix. 2.2.8.2 Hormones Mutations in certain proteins may cause bone There are a number of hormones that are impor- weakness (e.g., collagen defects cause decreased tant to this rapid formation of bone during the bone strength in osteogenesis imperfecta, fi rst two decades of life. These hormones include impaired g -carboxylation of gla proteins). Bone estrogen in females, testosterone in males, growth strength can be affected by osteomalacia, fl uoride hormone, and others. They are discussed in detail therapy, or hypermineralization states. Bone in Sect. 2.3.2 . microstructure affects bone strength also. In some pathological conditions, the thickness and extent of the osteoid layer may be increased (hyperos- 2.2.9 Bone Modeling teoidosis) or decreased. Hyperosteoidosis may be caused by conditions of delayed bone mineraliza- Modeling (reshaping) is the process by which bones tion (as in osteomalacia/rickets, vitamin D change their overall shape in response to physio- de fi ciency) or of increased bone formation (as in logic in fl uences or mechanical forces, leading to fracture callus, Paget’s disease of bone, etc.). 38 U. Kini and B.N. Nandeesh

2.3 Physiology of Bone 2.3.1 Cellular and Intracellular Metabolism Calcium and Phosphorus Metabolism Bone carries out some of the important metabolic functions; the important ones being: Both calcium and phosphorous, as well as mag- (a) Mineral Reservoir. Bones act as homeostatic nesium, are transported to blood from bone, renal, reservoir of minerals important for the body, and gastrointestinal cells, and vice versa. Mineral most important ones being calcium and phos- homeostasis requires the transport of calcium, phorus (Deftos 1998, 2001 ; Deftos and Gagel magnesium, and phosphate across their target 2000 ) . These bone minerals can be mobilized cells in bone, intestine, and kidney. Ninety-nine to maintain systemic mineral homeostasis. percent of body calcium, 85 % of phosphorus, This metabolic function of bone prevails over and 65 % percent of total body magnesium are its structural function in that calcium and contained within the bones. other minerals are removed from and replaced The regulation of bone and bone mineral in bone to serve systemic homeostatic needs metabolism results from the interactions among irrespective of loss of skeletal structural three important hormones – parathyroid hormone integrity. (PTH), calcitonin, and vitamin D – at three target (b) Growth Factor and Cytokine Depository . organs, bone, kidney, and GI tract, to regulate the Mineralized bone matrix is also a depository bone minerals (calcium and phosphorus). for certain cytokines and growth factors that can be released upon its resorption and can exert their effects locally and systemically; 2.3.2 Regulation of Skeletal notable among these are insulin-like growth Metabolism (Fig. 2.7 ) factors (IGF), transforming growth factor-b (TGF- b) and bone morphogenetic proteins Skeletal metabolism is regulated by bone cells (BMP) (see Sect. 2.4.3.1 ). and their progenitors. Among the population of (c) Fat Repository . The yellow bone marrow acts bone cells are osteoblasts, osteocytes, and osteo- as a storage reserve of fatty acids. clasts. Monocytes, macrophages, and mast cells (d) Acid – base Equilibrium. Bone buffers the may also mediate certain aspects of skeletal blood against excessive pH changes by metabolism. absorbing or releasing alkaline salts. Osteoblasts express receptors to many bone- (e) Detoxi ﬁ cation. Bone tissues are capable of active agents such as PTH, parathyroid hor- storing heavy metals and other extraneous mone-related protein (PTHrP), vitamin D elements, thus removing them from the circu- metabolites, gonadal and adrenal steroids, and lation and helping in reducing their effects on certain cytokines and growth factors. The major other tissues. product of osteoblasts is type 1 collagen, which, (f) Endocrine Function . Bone controls metabo- along with other proteins, forms the organic lism of phosphate by releasing ﬁ broblast osteoid matrix that is mineralized to hydroxy- growth factor (FGF-23), which acts on kid- apatite. These osteoblasts become encased in neys to reduce phosphate reabsorption. A hor- bone during its formation and mineralization mone called osteocalcin is also released by and reside in the resulting lacuna as osteocytes . bone, which contributes to the regulation of While their synthetic activity decreases, the cells blood glucose and fat deposition. Osteocalcin develop processes that communicate as canali- enhances both the insulin secretion and sensi- culi with other osteocytes, osteoblasts, and vas- tivity, in addition to boosting up the number culature. These osteocytes, thus, present acres of of insulin-producing b cells and reducing cellular syncytium that permit translocation of stores of fat. bone mineral during times of metabolic activity 2 Physiology of Bone Formation, Remodeling, and Metabolism 39

25-(OH)D3

1,25-(OH)2D3

PTH PTH

Ca2+ Ca2+ Calcium PTH 1,25-(OH)2D3

PO4 Phosphate PO4

Gut PTH

PO4

Fig. 2.7 Figure to show a close interlink between calcium, phosphate, and parathyroid hormone with vitamin D in bone metabolism and can provide minute-to-minute exchanges of into the skeleton through the process of mineral- minerals from bone matrix. ization of the organic matrix of bone, osteoid. Skeletal calcium is controlled through the ECF calcium is also ﬁ ltered by the kidney at a regulatory pathways of the gastrointestinal (GI) rate of about 6 g/day, where up to 98 % of it is tract and the kidney, and this regulation is medi- reabsorbed. ated in bone by osteoblast and osteoclast. The various regulators playing an important Calcium reaches the skeleton by being absorbed role in skeletal metabolism are: from the diet in the GI tract. Unabsorbed cal- Calcitropic hormones cium passes into the feces, which also contains Parathyroid hormone (PTH) the small amount of calcium secreted into the GI Calcitonin (CT) tract. Minor losses occur through perspiration Vitamin D [1,25(OH2 )D] and cell sloughing. In pregnancy, substantial PTHrP losses can occur across the placenta to the devel- Other hormones oping fetus and through breast milk. Absorbed Gonadal and adrenal steroids dietary calcium then enters the extracellular Thyroid hormones ﬂ uid (ECF) space and becomes incorporated Growth factors and cytokines 40 U. Kini and B.N. Nandeesh

2.3.2.1 Parathyroid Hormone primary and secondary hyperparathyroidism, Parathyroid hormone (PTH) is an 84-amino acid increase osteoclastic bone resorption. Low levels, peptide secreted by two pairs of parathyroid especially if delivered episodically, seem to glands located posterior to thyroid gland. The increase osteoblastic bone formation. The skele- mature PTH is packaged into dense secretory tal effects of PTH are mediated through the osteo- granules for regulated secretion. The major regu- blast, since they are the major expressor of the latory signal for PTH secretion is serum calcium. PTH receptor. However, osteoblasts communi- Serum calcium inversely affects PTH secretion, cate with osteoclasts to mediate PTH effects. with the steep portion of the sigmoidal response This communication seems mediated through the curve corresponding to the normal range of both. RANK-OPG pathway (see Sect. 2.4.1.3 ). The parathyroid gland senses the concentra- PTH increases the reabsorption of calcium in tion of extracellular ionized calcium through a the kidney, predominantly in the distal convo- cell-surface calcium-sensing receptor (CSR) for luted tubule, and inhibits the reabsorption of which calcium is an agonist. The same sensor phosphate in the renal proximal tubule, causing also regulates the responses to calcium of thyroid hypercalcemia and hypophosphatemia. C cells, which secrete calcitonin in direct rela- PTH mediates these effects through the PTH tionship to extracellular calcium; the distal receptor which is an 80,000-MW membrane gly- nephron of the kidney, where calcium excretion coprotein of the G protein receptor superfamily, is regulated; the placenta, where fetal-maternal while the parathyroid hormone-related protein calcium ﬂ uxes occur; and the brain and GI tract, (PTHrP) is the major humoral mediator of the where its function is unknown; and the bone cells. hypercalcemia of malignancy and is secreted by Serum phosphate has an inverse effect on calcium many types of malignant tumors, notably by concentration, and ambient phosphate directly breast and lung cancer. Both PTH and PTHrP increases 1,25-D production. Thus, serum phos- generate cyclic adenosine monophosphate phate may directly and indirectly regulate PTH (cAMP) as a cellular second messenger by acti- expression. vating protein kinase A (PKA), and the phospho- lipase C effector system increasing cellular IP3 Metabolism and Clearance and calcium and activating protein kinase C of Parathyroid Hormone (PKC). Parathyroid hormone, with a circulating half-life of less than 5 min, is metabolized in the liver, kid- 2.3.2.2 Calcitonin ney, and blood. The carboxyl-terminal fragments Calcitonin (CT) is a 32-amino acid peptide whose are cleared by glomerular ﬁ ltration, so they accu- main effect is to inhibit osteoclast-mediated bone mulate in renal failure. As a result of the biosyn- resorption. CT is secreted by parafollicular C thesis, secretion, and metabolism of PTH, the cells of the thyroid and other neuroendocrine circulation contains several forms of the mole- cells. In contrast to PTH, hypercalcemia increases cule. Overall, 10–20 % of circulating PTH immu- secretion of hypocalcemia-inducing CT, while noreactivity comprises the intact hormone, with hypocalcemia inhibits secretion. It inhibits the remainder being a heterogeneous collection resorption of bone, increases calcium and phos- of peptide fragments corresponding to the middle phorus excretion, and decreases the blood levels and carboxy regions of the molecule. of calcium and phosphorus (Deftos 1998 ) .

Biologic Effects of Parathyroid 2.3.2.3 Vitamin D Hormone (Fig. 2.7) Vitamin D is a secosterol hormone that is pres-

Parathyroid hormone regulates serum calcium ent in humans in an endogenous ( vitamin D3 ) and phosphorus concentrations through its recep- and exogenous (vitamin D2 ) form. The endoge- tor-mediated, combined actions on bone, intes- nous form of vitamin D, cholecalciferol (vita- tine, and kidney. High levels of PTH, as seen in min D3 ), is synthesized in the skin from the 2 Physiology of Bone Formation, Remodeling, and Metabolism 41

UV rays

7,dehydroxy cholesterol Provitamin D3 Skin

Vitamin D3

25 hydroxycholecalciferol 7 dehydroxy (25 OH D ) cholesterol 3 Liver

Cholesterol

Plasma 25 hydroxycholecalciferol Blood Plasma cholesterol vessel

25 OH D3 Kidney

Bone 1,25 dihydroxycholecalciferol Gut

Fig. 2.8 Synthesis and sources of vitamin D 3 and its hydroxylation to form the hormone 1,25-dihydroxycholecalciferol with its main sites of action being highlighted cholesterol metabolite 7-dehydrocholesterol Effects of 1,25-Dihydroxyvitamin D under the inﬂ uence of ultraviolet radiation on Mineral Metabolism (Deftos 1998) (Fig. 2.8). The exogenous form of (a) Intestinal Calcium Absorption . Vitamin D vitamin D 2 (ergocalciferol) is produced by ultra- increases intestinal calcium absorption, pri- violet irradiation of the plant sterol ergosterol marily in the jejunum and ileum, by increas- and is available through the diet. Both forms of ing calcium uptake through the brush border vitamin D require further metabolism to be membrane of the enterocyte. In a vitamin activated. d-de ﬁ cient state, only 10–15 % of dietary 42 U. Kini and B.N. Nandeesh

calcium is absorbed by the gastrointestinal added (a sub-process called ossi fi cation or bone tract, but with adequate vitamin D, adults formation). Remodeling involves continuous absorb approximately 30 % of dietary cal- removal of discrete packets of old bone, replace- cium (Deftos 1998 ) . During pregnancy, lacta- ment of these packets with newly synthesized tion, and growth, increased circulating proteinaceous matrix, and subsequent mineral- concentrations of 1,25-D promote the ization of the matrix to form new bone (Fernández- effi ciency of intestinal calcium absorption by Tresguerres-Hernández-Gil et al. 2006 ; Fraher as much as 50–80 %. Vitamin D also regu- 1993) . These processes also control the reshaping lates skeletal metabolism through the RANK or replacement of bone during growth and fol- pathway. 1,25-D also increases the effi ciency lowing injuries like fractures but also microdam- of dietary phosphorus absorption by about age (prevents accumulation of bone microdamage 15–20 %. through replacement of old bone with the new (b) Bone. Vitamin D promotes the mineralization one) (Turner 1998 ) which occurs during normal of osteoid and causes bone resorption by activity. Remodeling responds also to functional mature osteoclasts, but this effect is indirect, demands of the mechanical loading. As a result, requiring cell recruitment and interaction with bone is added where needed and removed where osteoblasts and the fusion of monocytic pre- it is not required. This process is essential in the cursors to osteoclasts. Vitamin D also regu- maintenance of bone strength and mineral homeo- lates the expression of several bone proteins, stasis. The skeleton is a metabolically active notably osteocalcin. It promotes the transcrip- organ that undergoes continuous remodeling tion of osteocalcin and has bidirectional throughout life. This remodeling is necessary effects on type I collagen and alkaline phos- both to maintain the structural integrity of the phatase gene transcription. skeleton and to subserve its metabolic functions (c) Kidney . The kidney decreases calcium and as a storehouse of calcium and phosphorus. phosphorus excretion. Normal bone remodeling cycle requires that (d) Blood. Blood increases calcium and phospho- the process of bone resorption and bone forma- rus levels. tion take place in a coordinated fashion, which in turn depends on the orderly development and 2.3.2.4 Other Hormones activation of osteoclasts and osteoblasts, respec- In addition to the primary calcemic hormones, tively. This property of bone, which constantly other hormones play an important role in calcium resorbs the old bone and forms new bone, makes and skeletal metabolism. Gonadal steroids main- the bone a very dynamic tissue that permits the tain skeletal mass. Glucocorticoids are deleteri- maintenance of bone tissue, the repair of dam- ous to all skeletal functions. Insulin, growth aged tissue, and the homeostasis of the phospho- hormone, androgens, and thyroid hormones pro- calcic metabolism. The bone remodeling cycle mote skeletal growth and maturation. Excess pro- involves a series of highly regulated steps that duction of the latter can cause hypercalcemia depend on the interactions of two cell lineages, (Deftos 1998 ; Kawaguchi et al. 1994 ) . While in the mesenchymal osteoblastic lineage and the adults bone metabolism is basically limited to hematopoietic osteoclastic lineage (Fraher 1993 ) . bone maintenance, the most obvious feature of The balance between bone resorption and bone bone metabolism in children and adolescents is deposition is determined by the activities of these increase in bone size in all three dimensions. two principle cell types, namely, osteoclasts and osteoblasts. Osteoblasts and osteoclasts, coupled together via paracrine cell signaling, are referred 2.4 Bone Remodeling to as bone remodeling units. In the young skeleton, the amount of resorbed Bone remodeling is a lifelong process wherein bone is proportional to the newly formed. For this old bone is removed from the skeleton (a sub- reason, it is referred to as a balanced process, process called bone resorption), and new bone is linked in both space and time under normal 2 Physiology of Bone Formation, Remodeling, and Metabolism 43

conditions. The average lifespan of each remod- 2.4.1 Mediators of Remodeling eled unit in humans is 2–8 months, the greater part of this time being taken up by bone formation. 2.4.1.1 Osteoclasts Bone remodeling occurs throughout life, but Osteoclasts are the only cells that are known to be only up to the third decade is the balance posi- capable of resorbing bone. They are typically tive. It is precisely in the third decade when the multinucleated. Osteoclasts are derived from bone mass is at its maximum, and this is main- mononuclear precursor cells of the monocyte- tained with small variations until the age of 50. macrophage lineage (hematopoietic stem cells From then on, resorption predominates and the that give rise to monocytes and macrophages) bone mass begins to decrease. Bone remodeling (Boyle et al. 2003 ) . Mononuclear monocyte- increases in perimenopausal and early postmeno- macrophage precursor cells have been identiﬁ ed pausal women and then slows with further aging in various tissues, but bone marrow monocyte- but continues at a faster rate than in premeno- macrophage precursor cells are thought to give pausal women. rise to most osteoclasts. Although cortical bone makes up 75 % of the total volume, the metabolic rate is ten times 2.4.1.2 Osteoblasts higher in trabecular bone, since the surface area- Osteoblasts can be stimulated to increase bone to-volume ratio is much greater (trabecular bone mass through increased secretion of osteoid and surface representing 60 % of the total). Therefore, by inhibiting the ability of osteoclasts to break approximately 5–10 % of total bone is renewed down osseous tissue. Bone building through per year. increased formation of osteoid is stimulated by Osteoclasts are endowed with highly active ion the secretion of growth hormone by the pituitary, channels in the cell membrane that pump protons the thyroid hormone and the sex hormones (estro- into the extracellular space, thus lowering the pH gens and androgens). The functional aspects of in their own microenvironment. This drop in pH osteoblasts have been discussed in detail in the dissolves the bone mineral (Blair et al. 1989 ) . earlier section (Sect. 2.2.1 ) on bone formation. The bone remodeling cycle involves a complex series of sequential steps (coupling of bone 2.4.1.3 RANK formation and bone resorption). Bone balance is The cell surface receptor called RANK (for recep- the difference between the old bone resorbed and tor activator of NFkB) prods osteoclast precursor new bone formed. Periosteal bone balance is cells to develop into fully differentiated osteoclasts mildly positive, whereas endosteal and trabecular when RANK is activated by its cognate partner bone balances are mildly negative, leading to cor- RANK ligand (RANKL). RANKL belongs to the tical and trabecular thinning with aging. These TNF superfamily and is critical for osteoclast for- relative changes occur with endosteal resorption mation. It is one of the key signaling molecules outstripping periosteal formation. that facilitate cross talk between the osteoblasts The main recognized functions of bone remod- and osteoclasts and help coordinate bone remodel- eling include preservation of bone mechanical ing. RANKL and macrophage CSF (M-CSF) are strength by replacing older, microdamaged bone two cytokines that are critical for osteoclast for- with newer, healthier bone and calcium and phos- mation. Both RANKL and M-CSF are produced phate homeostasis. The relatively low adult corti- mainly by marrow stromal cells and osteoblasts in cal bone turnover rate of 2–3 %/year is adequate membrane-bound and soluble forms, and osteo- to maintain biomechanical strength of bone. The clastogenesis requires the presence of stromal rate of trabecular bone turnover is higher, more cells and osteoblasts in bone marrow (Teitelbaum than required for maintenance of mechanical and Ross 2003 ; Cohen 2006 ) . Osteoprotegerin is strength, indicating that trabecular bone turnover another protein released by osteoblasts that acts as is more important for mineral metabolism. a decoy to prevent RANK and RANKL from com- Increased demand for calcium or phosphorus ing in contact (Asagiri and Takayanagi 2007 ; may require increased bone remodeling units. Boyle et al. 2003; Gori et al. 2000; Lacey et al. 44 U. Kini and B.N. Nandeesh

1998; Suda et al. 1999; Theill et al. 2002 ; Yasuda osteoblasts communicate with each other through et al. 1998 ) (see Sect. 2.4.1.4 ). the release of various “signaling” molecules. Osteoblast precursors express a molecule Osteoclasts are apparently activated by “sig- called TRANCE, or osteoclast differentiation nals” from osteoblasts. For example, osteoblasts factor, which can activate cells of the osteoclast have receptors for PTH, whereas osteoclasts do lineage by interacting with a receptor called not, and PTH-induced osteoclastic bone resorp- RANK (Horwood et al. 1998 ; Yasuda et al. tion is said not to occur in the absence of osteo- 1998 ) . blasts. The action of osteoblasts and osteoclasts is controlled by a number of chemical factors 2.4.1.4 Osteoprotegerin which either promote or inhibit the activity of the Osteoprotegerin (OPG), also known as osteoclast bone remodeling cells, controlling the rate at inhibiting factor (OCIF) or osteoclast binding fac- which bone is made, destroyed, or changed in tor (OBF), is a key factor inhibiting the differen- shape. The cells also use paracrine signaling to tiation and activation of osteoclasts, and is, control the activity of each other, described in therefore, essential for bone resorption. Sect. 2.4.3 . Osteoprotegerin is a dimeric glycoprotein belong- ing to the TNF receptor family. Osteoprotegerin inhibits the binding of RANK to RANKL and thus 2.4.2 Remodeling Phases inhibits the recruitment, proliferation, and activation of osteoclasts. Abnormalties in the balance of Bone remodeling can be divided into the follow- OPGL/RANK/OPG system lead to the increased ing six phases (Fig. 2.9 ), namely, quiescent, acti- bone resorption that underlies the bone damage of vation, resorption, reversal, formation, and postmenopausal osteoporosis, Paget’s disease, mineralization. Activation precedes resorption bone loss in metastatic cancers, and rheumatoid which precedes reversal, with mineralization as arthritis. Bone resorption depends on osteoclast the last step. These occur at remodeling sites secretion of hydrogen ions and cathepsin K which are distributed randomly but also are tar- enzyme. H+ ions acidify the resorption compart- geted to areas that require repair (Burr 2002 ; ment beneath osteoclasts to dissolve the mineral Fernández-Tresguerres-Hernández-Gil et al. component of bone matrix, whereas cathepsin K 2006 ; Par ﬁ tt 2002 ) . digests the proteinaceous matrix, which is mostly 1 . Quiescent Phase . It is the state/phase of the composed of type I collagen (Boyle et al. 2003 ) . bone when at rest. The factors that initiate the Osteoclasts bind to bone matrix via integrin recep- remodeling process remain unknown. tors in the osteoclast membrane linking to bone 2 . Activation Phase. The ﬁ rst phenomenon that matrix peptides. They digest the organic matrix, occurs is the activation of the bone surface resulting in formation of saucer-shaped Howship’s prior to resorption, through the retraction of lacunae on the surface of trabecular bone and the bone lining cells (elongated mature osteo- Haversian canals in cortical bone. The resorption blasts existing on the endosteal surface) and is completed by mononuclear cells after the multi- the digestion of the endosteal membrane by nucleated osteoclasts undergo apoptosis (Eriksen collagenase action. The initial “activation” 1986 ; Reddy 2004; Teitelbaum et al. 1995 ; stage involves recruitment and activation of Vaananen et al. 2000 ) . mononuclear monocyte-macrophage osteo- The boundary between the old and new bone clast precursors from the circulation is distinguished in an hematoxylin and eosin sec- (Bruzzaniti and Baron 2007; Roodman et al. tion by a blue (basophilic) line called a cement 1992) , resulting in interaction of osteoclast line or reversal line. and osteoblast precursor cells. This leads to the differentiation, migration, and fusion of 2.4.1.5 Paracrine Cell Signaling the large multinucleated osteoclasts. These At various stages throughout this process of cells attach to the mineralized bone surface remodeling, the precursors, osteoclasts, and and initiate resorption by the secretion of 2 Physiology of Bone Formation, Remodeling, and Metabolism 45

f b

e c

Fig. 2.9 Phases of bone remodeling: (a ) quiescent phase (d ) shows formation phase where the osteoclasts are where ﬂ at bone lining cells are seen lining the endosteal replaced by osteoblasts with underlying new osteoid membrane (b ) showing activation phase characterized by matrix (e ) shows mineralization of osteoid matrix (f ) cell retraction with resultant membrane resorption (c ) shows formation of bone structure unit with progression shows activated osteoclasts resorbing the underlying bone to quiescent phase

hydrogen ions and lysosomal enzymes, par- 4 . Reversal Phase . During the reversal phase, ticularly cathepsin K, which can degrade all bone resorption transitions to bone formation. the components of bone matrix, including At the completion of bone resorption, resorp- collagen, at low pH. tion cavities contain a variety of mononuclear 3 . Resorption Phase (Fig. 2.10). The osteoclasts cells, including monocytes, osteocytes then begin to dissolve the mineral matrix and released from bone matrix, and preosteo- decompose the osteoid matrix. This process is blasts, recruited to begin new bone formation. completed by the macrophages and permits The coupling signals linking the end of bone the release of the growth factors contained resorption to the beginning of bone formation within the matrix, fundamentally transforming are as yet unknown, but proposed coupling growth factor-b (TGF-b ), platelet-derived signal candidates include bone matrix-derived growth factor (PDGF), and insulin-like growth factors such as TGF-/3, IGF-1, IGF-2, bone factor I and II ( IGF- I and II). Osteoclastic morphogenetic proteins, PDGF, or ﬁ broblast resorption produces irregular scalloped cavi- growth factor (Bonewald and Mundy 1990 ; ties on the trabecular bone surface, called Hock et al. 2004 ; Locklin et al. 1999 ) . Howship’s lacunae, or cylindrical Haversian 5 . Formation Phase. Once osteoclasts have canals in cortical bone. Osteoclast-mediated resorbed a cavity of bone, they detach from bone resorption takes only approximately the bone surface and are replaced by cells of 2–4 weeks during each remodeling cycle. the osteoblast lineage which in turn initiate 46 U. Kini and B.N. Nandeesh

Fig. 2.10 Figure to show Bone a Osteoclasts normal bone remodeling ( ) lining cells vs. defective resorption (b ) Osteoblasts seen in disease states

Osteocytes Calcified bone matrix

New bone

bone formation. The preosteoblast grouping When the cycle is completed, the amount of phenomenon is produced and attracted by the bone formed should equal the amount of bone growth factors liberated from the matrix resorbed. which act as chemotactics and in addition stimulate their proliferation (Lind et al. 1995) . The preosteoblasts synthesize a 2.4.3 Regulatory Factors in Bone cementing substance upon which the new tis- Remodeling sue is attached and express bone morphogenic proteins (BMP) responsible for differentia- The balance between bone resorption and formation (refer Sect 2.4.3.2 ). A few days later, the tion is inﬂ uenced by such interrelated factors as already differentiated osteoblasts synthesize genetic, mechanical, vascular, nutritional, hor- the osteoid matrix which ﬁ lls the (resorption monal, and local. cavity) perforated areas (Lind et al. 1995 ) . The remaining osteoblasts continue to syn- 2.4.3.1 Systemic Regulation of Bone thesize bone until they eventually stop and Remodeling transform to quiescent lining cells that com- 1 . Genetic Factors pletely cover the newly formed bone surface These are important in determining the maxi- and connect with the osteocytes in the bone mum bone mass, since between 60 and 80 % of matrix through a network of canaliculi. this bone mass is genetically determined. Thus, 6 . Mineralization Phase. The process begins Negroes have a greater bone mass than Whites, 30 days after deposition of the osteoid, ending who in turn have a higher mass than Asians. at 90 days in the trabecular and at 130 days in Bone mass is a characteristic transmitted from the cortical bone. The quiescent or “at rest” parents to children, which is why daughters of phase then begins again. mothers with osteoporosis are more predisposed 2 Physiology of Bone Formation, Remodeling, and Metabolism 47

to having this condition themselves(Grant and in the decreased bone density in de-nerved Ralston 1997 ; Pocock et al. 1987 ) . mandibles. 2 . Mechanical Factors 4 . Nutritional Factors Remodeling is regulated by mechanical load- A minimum amount of calcium is needed for ing, allowing bone to adapt its structure in mineralization, which the majority of authors response to the mechanical demands. Physical put at 1,200 mg/day to the age of 25, not less activity is essential for the correct develop- than 1 g/day from 25 to 45, and following ment of bone. It is believed that muscular menopause should be at least 1,500 mg/day. action transmits tension to the bone, which is Likewise, it is known that toxic habits such as detected by the osteocyte network within the smoking, caffeine, alcohol, and excess salt osseous fl uid. On the other hand, the absence constitute risk factors for osteopenia. of muscular activity, rest, or weightlessness 5 . Hormonal Factors has an adverse effect on bone, accelerating Normal development of the skeleton is condi- resorption. It is well-known that trabeculae tioned by the correct functioning of the endo- tend to align with maximum stresses in many crine system. The most important hormones bones. Mechanical stress improves bone in bone remodeling are: strength by infl uencing collagen alignment as (a) Thyroid Hormones . Thyroid hormones can new bone is being formed. Cortical bone tis- also stimulate bone resorption and formation sue located in regions subject to predomi- (possess two opposing actions on bone) and nantly tensile stresses has a higher percentage are critical for maintenance of normal bone of collagen fi bers aligned along the bone long remodeling (Kawaguchi et al. 1994 ) . In the axis. In regions of predominant compressive fi rst place, they stimulate the synthesis of the stresses, fi bers are more likely to be aligned osteoid matrix by the osteoblasts and its min- transverse to the long axis. eralization, favoring the synthesis of IGF-I. 3 . Vascular / Nerve Factors For this reason, in congenital hypothyroidism Vascularization is fundamental for normal (cretinism), short stature is produced by the bone development, supplying blood cells, alteration in bone formation. In the second oxygen, minerals, ions, glucose, hormones, place, a contrary effect is produced, stimulat- and growth factors. Vascularization consti- ing resorption with the increase in number tutes the fi rst phase in ossi fi cation: the blood and function of the osteoclasts. The clinical vessels invade the cartilage and later produce manifestation of this effect is the appearance resorption via the osteoclasts originating from of bone loss in hyperthyroidism. the nearby vessels. In the same way, vascular (b) Parathyroid Hormone ( PTH ). It controls the neoformation is the fi rst event in the repair of homeostasis of calcium by direct action on fractures or bone regeneration (Trueta 1963 ) . the bone and the kidneys and indirectly on the Innervation is necessary for normal bone intestine. It is produced by the parathyroid physiology. The bone is innervated by the glands in response to hypocalcemia. Continual autonomous nervous system and by sensorial supply of PTH would stimulate bone resorp- nerve fi bers. Autonomous fi bers have been tion through the synthesis of a factor favoring found in periosteum, endosteum, and cortical osteoclastogenesis (RANKL) on the part of bone and associated with the blood vessels of the osteoblastic cells, while at intermittent the Volkmann conduit, and likewise neuro- doses it would stimulate the formation of peptides and their receptors in bone (Wheeless bone, associated with an increase of the 2011 ) . Examples of the importance of inner- above-mentioned growth factors and with a vation in bone physiology are found in decrease in the apoptosis of the osteoblasts. osteopenia and the bone fragility present in PTH regulates serum calcium concentration. patients with neurological disorders, and also It is a potent stimulator of bone resorption and 48 U. Kini and B.N. Nandeesh

has biphasic effects on bone formation. There muscle mass at older age. Androgens protect is an acute inhibition of collagen synthesis men against osteoporosis via maintenance of with high concentrations of PTH, but pro- cancellous bone mass and expansion of corti- longed intermittent administration of this hor- cal bone. mone produces increased bone formation, a (f) Estrogens. Estrogens are essential for the clo- property for which it is being explored clini- sure of the growth plates and have an impor- cally as an anabolic agent (Dempster et al. tant role in the development of the skeleton. 1993 ) . Plasma PTH tends to increase with Estrogens have a dual effect on bone metabo- age, and this may produce an increase in bone lism: on the one hand, they favor bone forma- turnover and a loss of bone mass, particularly tion, increasing the number and function of of cortical bone (see Sect. 2.3.2.1 ). the osteoblasts, and on the other, they reduce (c) Calcitonin . Produced by the parafollicular C resorption. Estrogen receptors have been cells of the thyroid, it is an inhibitor of bone described in human osteoblasts, osteocytes, resorption, reducing the number and activity and osteoclasts. Recent investigations have of the osteoclasts (see Sect. 2.3.2.1 ). However, found that estrogens can increase the levels of this is a transitory action, since the osteoclasts osteoprotegerin (OPG), a protein produced by seem to become “impermeable” to calcitonin osteoblasts that inhibits resorption, so they within a few days. may play an important role in the regulation

(d) 1 , 25 ( OH ) 2 Vitamin D 3 or Calcitriol. A steroid of osteoclastogenesis (Hofbauer et al. 1999 ) . hormone, by favoring the intestinal absorp- Alternatively, estrogen may inhibit local fac- tion of calcium and phosphate, favors bone tors that impair bone formation or enhance mineralization. It is necessary for normal local factors that stimulate bone formation. growth of the skeleton. Some authors believe For this reason, estrogen de ﬁ ciency during it may be produced by lymphocytic or mono- menopause constitutes the most important cytic bone cells, playing an important role as pathogenic factor in bone loss associated with a local regulator of osteoclast differentiation osteoporosis. Loss of estrogens or androgens (Raisz 1993 ) . increases the rate of bone remodeling by (e) Androgens . Androgens have an anabolic removing restraining effects on osteoblasto- effect on bone through the stimulation of the genesis and osteoclastogenesis and also osteoblast receptors. Likewise, they act as causes a focal imbalance between resorption mediators of the growth hormone in puberty. and formation by prolonging the lifespan of While androgen deﬁ ciency is associated with osteoclasts and shortening the lifespan of lower bone density, the administration of tes- osteoblasts. tosterone in young people before the closure (g) Progesterone. Progesterone also has an ana- of the epiphyses increases bone mass. In the bolic effect on bone, either directly, through same way, women with an excess of andro- the osteoblasts which possess hormone recep- gens present higher bone densities. Androgens tors, or indirectly, through competition for the increase cortical bone size via stimulation of osteoblastic receptors of the glucocorticoids. both longitudinal and radial growth. First, (h) Insulin. Insulin stimulates matrix synthesis androgens, like estrogens, have a biphasic effect both directly and indirectly, increasing the on endochondral bone formation: at the start of hepatic synthesis of IGF-I (insulin-like growth puberty, sex steroids stimulate endochondral factor) (see Sect. 2.4.3.2 ). bone formation, whereas they induce epiphy- (i) Glucocorticoids. Glucocorticoids are neces- seal closure at the end of puberty. This effect of sary for bone cell differentiation during devel- androgens may be important because bone opment, but their greatest postnatal effect is to strength in males seems to be determined by inhibit bone formation (at high doses, they relatively higher periosteal bone formation and, have a catabolic effect on bone), since they therefore, greater bone dimensions, relative to inhibit the synthesis of IGF-I by the osteoblasts 2 Physiology of Bone Formation, Remodeling, and Metabolism 49

and directly suppress BMP-2, critical factors an important role in the production of prostaglan- in osteoblastogenesis. This is the major patho- dins and nitric oxide, as well as cytokines and genetic mechanism in glucocorticoid-induced growth factors. osteoporosis. Indirect effects of glucocorti- The important local factors acting on the skel- coids on calcium absorption and sex hormone eton are as follows and are tabulated in Table 2.1 : production may, however, increase bone resorption (Lukert and Kream 1996 ; Growth Factors Manolagas 2000 ) . Bone contains a large number of growth factors. (j) Growth Hormone . Growth hormone acts both These are polypeptides produced by the bone directly and indirectly on bone. Growth hor- cells themselves or in extra-osseous tissue and mone acts directly on the osteoblasts with act as modulators of the cellular functions, fun- hormone receptors, stimulating their activity, damentally growth, differentiation, and prolif- thus increasing the synthesis of collagen, eration. Among the most abundant are the IGFs, osteocalcin, and alkaline phosphate. The indi- which, with their associated binding proteins, rect action is produced through an increase in may be important modulators of local bone synthesis of IGF-I and II by the osteoblasts. remodeling (Fraher 1993 ; Hakeda et al. 1996 ; These factors stimulate the proliferation and Rosen and Donahue 1998 ) . Transforming differentiation of the osteoblasts, increasing growth factor-b and the related family of bone their number and function (Harvey and Hull morphogenetic proteins are present in the skel- 1998 ; Rosen and Donahue 1998 ) . eton and have important functions not only in Thus, the hormones that regulate bone metab- remodeling but also in skeletal development. olism are as follows: Other growth factors, such as platelet-derived • Decrease bone resorption growth factor, PTH-related protein, and – Calcitonin ﬁ broblast growth factor, may play an important – Estrogens role in physiologic remodeling and an even • Increase bone resorption more important role in the remodeling associ- – PTH/PTHrP ated with skeletal repair. – Glucocorticoids (a) IGF - I and II ( Insulin - Like Growth Factor I – Thyroid hormones and II). These are polypeptides similar to – High-dose vitamin D insulin; they are synthesized by the liver and • Increase bone formation osteoblasts and found in high concentrations – Growth hormone in the osteoid matrix (Cohick and Clemmons – Vitamin D metabolites 1993 ) . They increase the number and function – Androgens of the osteoblasts, stimulating collagen syn- – Insulin thesis. They circulate linked to IGF-binding – Low-dose PTH/PTHrP proteins ( IGFBP), which in turn can exercise – Progestogens stimulatory or inhibitory effects on bone • Decrease bone formation (Conover 2008) . IGF synthesis is regulated – Glucocorticoids by local growth factors and hormones; thus, growth hormone, estrogens, and progesterone 2.4.3.2 Local Regulators of Bone increase their production, while the glucocor- Remodeling ticoids inhibit it. They also mediate in the Bone remodeling is also regulated by local fac- osteoblast-osteoclast interaction and actively tors, among which principally growth factors and participate in bone remodeling (Hill et al. cytokines, and recently, the bone matrix proteins 1995 ) . IGF-II is the most abundant factor in have been implicated as modulators of other local the bone matrix and is important during factors (Raisz 1999 ; Fernández-Tresguerres- embryogenesis (Canalis et al. 1989 ; Mohan Hernández-Gil et al. 2006 ) . Bone cells also play and Baylink 1991 ) . 50 U. Kini and B.N. Nandeesh

Table 2.1 Local factors in bone remodeling Stimulate bone formation Stimulate bone resorption Inhibit bone resorption Growth factors BMP-2, BMP-4, BMP-6, BMP-7, TNF, EGF, PDGF, FGF, IGF-I, IGF-II TGF-b, FGF, and M-CSF, and GM-CSF PDGF Cytokines IL-4,IL-13, IFN, and OPG IL-1, IL-6, IL-8, IL-11, IFN-y IL-4

PGE 2, PGE1, PGG2, PGI2, and PGH2

(b) Transforming growth factor -b (TGF -b ) is a muscle cells, neovascularization, and colla- superfamily of proteins highly abundant in gen synthesis, therefore favoring scarring bone tissue (second after IGF). They are (Nash et al. 1994 ) . latently present in the matrix and activate dur- (e) Fibroblastic growth factor (FGF ) has an ana- ing osteoclastic resorption. TGF-b is a potent bolic effect on bone, as it is a mitogen of stimulator of bone formation, promoting osteoblasts, vascular endothelial cells, and osteoblastic differentiation and the synthesis fi broblasts. As a practical example of the of the osteoid matrix and inhibiting the syn- effect of FGF, it is known that mutations in its thesis of the proteases, especially the matrix receptors produce alterations in the craniofa- metalloproteinase (MMP ), an enzyme which cial skeleton, such as achondroplasia, Apert’s degrades it (Bonewald and Dallas 1994 ) . syndrome, and Crouzon’s syndrome, among TGF- b inhibits resorption on reducing the for- others (Marie 2003 ) . mation and differentiation of the osteoclasts, (f) Epidermal growth factor ( EGF ) is a powerful as well as mature osteoclast activity, and stim- mitogen of cells of mesodermic or ectodermic ulating their apoptosis (Baylink et al. 1993 ) . origin. It has dual formative and destructive (c) Bone morphogenetic proteins (BMP ) are action, although the latter is the most well included in the TGF-b family. They form a known. group of 15 proteins able to achieve the trans- (g) Vascular endothelial growth factor ( VEGF ) formation of connective tissue into bone tis- induces angiogenesis and vascular endothe- sue, for which they are considered lial proliferation. It produces vasodilation and osteoinductive (Sakou 1998 ) . They stimulate an increase in vascular permeability. It is pro- the differentiation of the stem cells toward duced in hypoxia and is currently considered different cell lines (adipose tissue, cartilage, one of the key factors in the fi rst phases of and bone). They are highly abundant in bone fracture repair and bone regeneration, as well tissue and during embryogenesis participate as in tumor growth. in the formation of bone and cartilage (h) Granulocyte / macrophage - colony stimulating (Yamaguchi et al. 2000 ) . They strongly pro- factor ( GM - CSF) is important in osteoclasto- mote osteoblastic differentiation and are genesis and may play a role in the pathogen- believed to inhibit osteoclastogenesis in addi- esis of osteopetrosis. tion to stimulating osteogenesis (Canalis et al. (i) Macrophage- colony stimulating factor 2003 ) . Another cell surface receptor called ( M- CSF) is produced by osteoblasts and med- the low-density lipoprotein (LDL)-related ullar stromal cells. It is an essential factor in the protein 5 receptor (LRP5) is also important fi rst phases of osteoclastogenesis and is required for bone formation. for the formation of giant multinucleate cells (d) Platelet - derived growth factor (PDGF ), on but have no effect on osteoclastic activity. the one hand, stimulates protein synthesis (j) Tumor Necrosis Factor ( TNF ). Tumor necro- brought about the osteoblasts and, on the sis factor in vitro stimulates resorption and other, favors bone resorption. Other effects has been related with bone loss in arthritis and are the proliferation of fi broblasts and smooth periodontal disease. 2 Physiology of Bone Formation, Remodeling, and Metabolism 51

Matrix Proteins bone resorption (Kawaguchi et al. 1995 ) . The matrix proteins have recently been discov- This could also be important in in fl ammatory ered to act as growth factor modulators (Young bone loss. The fi rst step on PGE2 synthesis 2003 ) . Matrix proteins are found in concentra- is carried out by an enzyme called cyclooxy- tions a 1,000 times higher than growth factors and genase 2 (COX2), and inhibitors of this could therefore play a more important role in the enzyme can prevent bone formation in regulation of the different cell functions (Horowitz response to mechanical stress in animals. 2003 ) . Matrix proteins also participate in regula- PGE2 may be required for exercise-induced tion of the differentiation of the cells contained bone formation. within the matrix. For example, collagen I is one (e) Leukotrienes. Leukotrienes are another set of of the earliest markers which regulates the osteo- lipid molecules that appear to regulate bone progenitor cells, and alkaline phosphatase is a sur- remodeling. face protein that could participate in the regulation Thus, the growth factors that regulate bone of the proliferation, migration, and differentiation remodeling are: of the osteoblastic cells. Osteonectin, fi bronectin, Insulin-like growth factors (IGF) I and II and osteocalcin promote cell attachment, facili- Transforming growth factor-b (TGF- b ) super- tate cell migration, and activate cells. family, including the bone morphogenetic proteins (BMPs) Cytokines Fibroblast growth factors (FGFs) These are polypeptides synthesized in the lym- Platelet-derived growth factors (PDGFs) phocytic and monocytic cells and play an impor- Cytokines tant role in multiple cellular functions, such as the immunological response, infl ammation, and hematopoiesis, having both an autocrine and para- 2.4.4 Markers of Bone Metabolism crine effect. The following are important in bone: (a) Interleukin 1 (IL - 1) directly stimulates osteo- Biochemical markers of bone metabolism pro- clastic resorption, increasing the proliferation vide dynamic information about the turnover of and differentiation of the preosteoblasts, as osseous tissue (Schneider et al. 1998 ) . They can well as the osteoclastic activity, and inhibiting be broadly classifi ed as refl ecting either bone for- the apoptosis of the osteoclasts (Compston mation or bone resorption. 2001) . In reality, they are three different related molecules: IL-1 a , IL-1 b , and the 2.4.4.1 Markers of Bone Formation IL-1 receptor antagonist, the last being the Alkaline Phosphatase inhibitor of the fi rst two. They act both directly Alkaline phosphatase in serum has been used and indirectly on resorption through the syn- for more than 50 years to monitor bone metabo- thesis of prostaglandins. lism and is still the most frequently used marker. (b) Interleukin 6 (IL - 6) stimulates bone resorp- Alkaline phosphatase is an ectoenzyme anchored tion and appears to be implicated in the patho- to the cell surfaces of osteoblasts and other cells genesis of Paget’s disease (Roodman 1999 ) . It (Delmas 1995 ) . However, alkaline phosphatase is believed to play an important role in the ini- is not speci fi c to bone, and ideally selective tial stages of osteoclastogenesis and is pro- measurement of the bone isoenzyme should be duced in response to PTH, IL-1, and used as a marker of bone formation. The clear-

1,25(OH)2 D3 . ance from the circulation is relatively slow, with (c) Interleukin 11 (IL - 11 ). Recently discovered, a half-life in the order of 1–3 days for the bone it is produced in bone marrow and induces isoenzyme. Its values in plasma and serum are osteoclastogenesis. raised in conditions such as Paget’s disease, (d) Prostaglandins (PG ). Prostaglandins, par- osteomalacia, and after fractures or ectopic ticularly prostaglandin E2 (PGE2), stimulate bone formation. 52 U. Kini and B.N. Nandeesh

In clinical practice, the major problem for diag- or production of alkaline phosphatase to an nostic purposes is to distinguish between the equivalent degree. Serum osteocalcin values isoenzymes derived from liver and bone, can re fl ect the age-related increase in bone although the intestinal enzyme may be raised turnover, and the values rise after the menopause after meals and the placental isoenzyme during and fall after treatment with estrogens. Serum pregnancy. The bone isoenzyme can be distin- osteocalcin correlates with skeletal growth at guished based on sialic acid residues. In normal the time of puberty and is increased in a variety individuals, about half of the total alkaline phos- of conditions characterized by increased bone phatase is derived from bone and the rest from turnover, such as primary and secondary hyper- liver. In conditions such as Paget’s disease, the parathyroidism, hyperthyroidism, Paget’s dis- changes in alkaline phosphatase are often very ease, and acromegaly (Fraher 1993 ) . substantial. Procollagen Peptides Osteocalcin Collagen is the major structural protein of bone Osteocalcin, also known as bone gla protein and comprises about 90 % of the organic mate- (BGP), is a bone specifi c protein, which has rial. Collagen clearly contributes to the integrity proven to be a sensitive and speci fi c marker of and strength of bone matrix, and defects in its osteoblast activity in a variety of metabolic production, for example, in osteogenesis imper- bone diseases. Its synthesis is dependent upon the fecta, leads to bone of poor quality, susceptible to presence of active metabolites of vitamin D, espe- fracture. Attempts to measure collagen synthesis cially 1,25-dihydroxyvitamin D and requires vita- is, therefore, a more logical approach than mea- min K for the conversion by carboxylation of suring other less abundant matrix constituents in three glutamate residues to gamma-carboxygluta- the assessment of bone formation. During colla- mate (gla) (Delmas 1995 ) . The post-translational gen synthesis, pro-peptides are released both modi fi cations confer calcium-binding properties from the amino-terminal (“N-terminal”) and car- on osteocalcin. This can be used to differentiate boxyterminal (“C-terminal”) ends of the procol- fully carboxylated from partially carboxylated lagen molecule, after the three individual alpha osteocalcin in the circulation, and it has been chains have formed the triple helix, which will shown that a signi fi cant proportion of osteocalcin become part of the collagen fi bril. Assays for in osteoporotic elderly patients is incompletely both the N- and C-terminal pro-peptides exist. carboxylated. The values are increased during growth and in Measurements of serum osteocalcin by situations of increased bone formation, such as immunoassays show increased values in condi- occur in Paget’s disease, and in response to tions associated with increased bone formation, growth hormone. for example, hyperparathyroidism, hyperthyroidism, and bone metastases. In Paget’s dis- 2.4.4.2 Markers of Bone Resorption ease, however, the rises are less than expected, Biochemical markers used to monitor bone perhaps refl ecting differential incorporation resorption include urinary measurements of: into bone matrix or altered synthesis by osteo- (a) Hydroxyproline- Containing Peptides. Studies blasts (Gallagher 1997 ) . Reduced levels of in relation to osteoporosis show that, when osteocalcin may re fl ect lower rates of bone for- measurements are made carefully, urinary mation, as seen, for example, in myeloma. hydroxyproline values rise after the meno- Osteocalcin values may be substantially reduced pause and fall again when antiresorptive drugs during treatment with glucocorticosteroids, such as estrogens, calcitonins, and bisphos- although in this case, it should be remembered phonates are given. Pyridinoline crosslinks – that glucocorticoids specifi cally suppress osteo- pyridinoline (Pyr) and deoxypyridinoline calcin synthesis by osteoblasts, while not nec- (DPyr) – also called hydroxylysyl pyridinoline essarily similarly depressing collagen synthesis (HL) and lysyl pyridinoline (LP), respectively, 2 Physiology of Bone Formation, Remodeling, and Metabolism 53

are currently receiving considerable attention 2.4.5 Pathophysiology of Bone as the most promising markers of bone resorp- Remodeling (Fig. 2.11 ) tion. Both are non-reducible crosslinks which stabilize the collagen chains within the extra- Abnormalities of bone remodeling can produce a cellular matrix and are formed by the conden- variety of skeletal disorders (Raisz 1999 ) . sation of three lysine and/or hydroxylysine In fl ammatory bone loss in periodontal disease and residues in adjacent alpha chains. arthritis is probably the combined result of stimu- (b) Hydroxylysine glycosides and pyridinoline lation of resorption and inhibition of formation by crosslinks (Delmas 1995 ) are derived from cytokines and prostaglandins. Interleukin 1 (IL- type I collagen. Hydroxylysine, like hydroxy- 1), IL-6, and tumor necrosis factor, as well as proline derived from proline, is produced by a growth factors, have been implicated in patho- post-translational hydroxylation. The subse- logic responses in the skeleton, particularly in quent glycosylation of hydroxylysine differs osteoporosis associated with estrogen defi ciency, in collagens in different tissues. The mono- hyperparathyroidism, and Paget’s disease glycosylated galactosyl hydroxylysine is (Lorenzo 1992 ; Mills and Frausto 1997; Raisz enriched in bone compared with the diglyco- 1999 ; Papanicolaou et al. 1998 ) . A few of the sylated form, glucosyl galactosyl hydroxy- pathological entities wherein there is deranged lysine, which is the major form in skin. This skeletal metabolism/remodeling are addressed may be a useful marker of bone resorption, below, and their abnormalities are shown in for example, in osteoporosis. Table 2.2 . (c) Acid Phosphatase . Acid phosphatase is a lysosomal enzyme which exists in several forms 2.4.5.1 Osteoporosis in different tissues. The type 5 isoenzyme is Osteoporosis is a very common metabolic disor- the one found in osteoclasts, which appear to der of the skeleton, where in the bone mineral be released during bone resorption. Assays of density (BMD) is reduced, the bone microarchi- total tartrate-resistant acid phosphatase tecture is disrupted (perforation of trabecular (TRAP) in the circulation are moderately plates), and the amount and variety of noncollag- raised in disorders associated with increased enous proteins in bone is altered, leading to bone resorption, but the assays are dif fi cult to increased risk of fracture. Osteoporosis may be: perform because of the instability of the • Primary (postmenopausal/senile) enzyme and the relatively small changes • Secondary cause (nutrition, endocrine, drug, observed in pathological states. malignancy, chronic disease, idiopathic) (d) Other Assays. In some circumstances, fasting (Raisz 1997 ) urine calcium can give an indirect measure of The loss of bone mass and strength can be bone resorption rates and may be useful in contributed by: Paget’s disease and in patients with metastatic (a) Failure to reach an optimal peak bone mass as bone disease for following responses to treat- a young adult ment. Other measurements include assays of (b) Excessive resorption of bone after peak mass tartrate-resistant acid phosphatase and free has been achieved gamma-carboxyglutamic acid. Many cytok- (c) An impaired bone formation response during ines and growth factors in fl uence bone metab- remodeling olism (e.g., interleukins 1 & 6, tumor necrosis There is a defect in osteoblast function or factors [TNFs], insulin-like growth factors I because of loss of template from excessive resorp- & II and their binding proteins). These assays tion with perforation of trabecular plates and have been of help with the diagnosis and man- removal of endosteal cortical bone. The defect in agement of patients with fl orid disorders of osteoblast function could be the consequence of bone metabolism, such as Paget’s disease or cellular senescence but also may be the result of a vitamin d -de fi cient osteomalacia. decrease in the synthesis or activity of systemic 54 U. Kini and B.N. Nandeesh

Fig. 2.11 Microphotograph showing osteoclasts (star ) causing endosteal surface resorption of bone

Table 2.2 Abnormalities of remodeling in disease 2.4.5.2 Hyperparathyroidism conditions and Hyperthyroidism Bone In these disorders, bone turnover may be mark- Bone resorption formation edly increased with or without decreased bone Osteoporosis ↑↑ ± ↑ mass. Both parathyroid hormones and thyroid Glucocorticoid ↑ ↓↓ hormones can stimulate bone formation as well osteoporosis as resorption, and if the cells of the osteoblastic ↑↑ ↑↑ Hyperparathyroidism lineage are suf fi ciently responsive, then bone loss ↑↑ ↑↑ Hyperthyroidism will not occur. Increased IL-6 has been reported Paget disease ↑↑ ↑↑ in hyperparathyroidism. In fl ammation ↑↑ ± ↓ Immobilization ↓ ↓↓ 2.4.5.3 Paget’s Disease Paget’s disease is the best example of a disease in and local growth factors. A combination of bio- which biochemical markers of bone metabolism chemical assays, including total alkaline phos- have been extensively used in clinical practice. A phatase and osteocalcin in plasma and remarkable disorder of bone remodeling is Paget hydroxyproline and calcium in fasting urine, have disease (Siris 1998 ) . In this disorder, the osteo- strong predictive power in relation to rates of bone clasts become abnormally activated, possibly by loss subsequently measured by bone densitometry viral infection, and produce a bizarre and irregu- techniques. lar pattern of resorption, to which there is usually The two major needs for the use of markers in an intense osteoblastic response with irregular osteoporosis are: new bone formation often in the form of woven 1. To identify the patients at greatest risk, for bone. Thus, in Paget’s disease there may be example, those with rapid rates of bone loss increased bone density, but because of the irregu- compared with bone formation lar architecture, bone strength is decreased and 2. To monitor the effects of speci fi c treatment pathologic fractures may occur. (e.g., with estrogens, calcitonins and bisphos- Bone markers in Paget’s disease are used to: phonates, or with bone-forming agents) in 1. Assess and to monitor disease activity in indi- individual patients vidual patients 2 Physiology of Bone Formation, Remodeling, and Metabolism 55

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