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Home , Endosteum, Lacuna (histology)

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Clinical Practice Keywords Skeletal system/ physiology/Musculoskeletal health Systems of life This article has been Skeletal system double-blind peer reviewed In this article... ● The key functions and structure of bone ● Bone formation and growth, and the process of remodelling ● Diet and lifestyle factors that can affect bone structure

Skeletal system 1: the anatomy and physiology of

Key points Author Jennie Walker is principal lecturer, Nottingham Trent University. Bones are key to providing the body Abstract The skeletal system is formed of bones and , which are connected with structural by ligaments to form a framework for the remainder of the body tissues. This article, support and the first in a two-part series on the structure and function of the skeletal system, enabling movement reviews the anatomy and physiology of bone. Understanding the structure and purpose of the bone allows nurses to understand common pathophysiology and Most of the body’s consider the most-appropriate steps to improve musculoskeletal health. minerals are stored in the bones Citation Walker J (2020) Skeletal system 1: the anatomy and physiology of bones. Nursing Times [online]; 116: 2, 38-42. Diet and lifestyle can affect the quality of bone formation he skeletal system is composed of Protection bones and cartilage connected by Bones provide protective boundaries for After bones have ligaments to form a framework for soft organs: the cranium around the brain, formed they Tthe rest of the body tissues. There the vertebral column surrounding the undergo constant are two parts to the : spinal cord, the ribcage containing the remodelling l Axial skeleton – bones along the axis of heart and lungs, and the pelvis protecting the body, including the skull, vertebral the urogenital organs. Changes in the column and ribcage; remodelling process l Appendicular skeleton – appendages, Mineral homoeostasis can result in such as the upper and lower limbs, As the main reservoirs for minerals in the pathology such as pelvic girdle and shoulder girdle. body, bones contain approximately 99% of Paget’s disease of the body’s calcium, 85% of its phosphate bone or osteoporosis Function and 50% of its magnesium (Bartl and Bartl, As well as contributing to the body’s 2017). They are essential in maintaining overall shape, the skeletal system has sev- homoeostasis of minerals in the blood with eral key functions, including: minerals stored in the bone are released in l Support and movement; response to the body’s demands, with l Protection; levels maintained and regulated by hor- l Mineral homeostasis; mones, such as parathyroid hormone. l Blood-cell formation; l Triglyceride storage. Blood-cell formation (haemopoiesis) Blood cells are formed from haemopoietic Support and movement stem cells present in red . Bones are a site of attachment for ligaments Babies are born with only red bone and tendons, providing a skeletal frame- marrow; over time this is replaced by work that can produce movement through yellow marrow due to a decrease in eryth- the coordinated use of levers, muscles, ten- ropoietin, the hormone responsible for dons and ligaments. The bones act as stimulating the production of erythro- levers, while the muscles generate the cytes (red blood cells) in the bone marrow.

FRANCESCA CORRA FRANCESCA forces responsible for moving the bones. By adulthood, the amount of red marrow

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Fig 1. Bone structure flexibility to withstand the daily forces exerted on them. This flexibility and ten- sile strength of bone is derived from the collagen fibres. Over-mineralisation of the fibres or impaired collagen production can yaline artilage iysis increase the brittleness of bones – as with the genetic disorder osteogenesis imper- fecta – and increase bone fragility (Ralston iyseal line and McInnes, 2014). e bone marrow Structure Bone architecture is made up of two types arrow aity of bone tissue: l Cortical bone; l Cancellous bone. ellow bone marrow Cortical bone Also known as compact bone, this dense outer layer provides support and protec- tion for the inner cancellous structure. eriosteum iaysis Cortical bone comprises three elements: l (Fig 1); l Intracortical area; l (Bartl and Bartl, 2017). The periosteum is a tough, fibrous utrient outer membrane. It is highly vascular and oramen omat almost completely covers the bone, except bone for the surfaces that form joints; these are ite o covered by . Tendons and enosteum ligaments attach to the outer layer of the periosteum, whereas the inner layer con- tains (bone-forming cells) and (bone-resorbing cells) respon- sible for bone remodelling. ongy The function of the periosteum is to: bone iysis l Protect the bone; l Help with fracture repair; l Nourish bone tissue (Robson and Syndercombe Court, 2018). It also contains Volkmann’s canals, small channels running perpendicular to the of the bone (Fig 1); these has halved, and this reduces further to and 10% other proteins, such as glycopro- convey blood vessels, lymph vessels and around 30% in older age (Robson and Syn- tein, osteocalcin, and proteoglycans (Bartl nerves from the periosteal surface through dercombe Court, 2018). and Bartl, 2017). It forms the framework for to the intracortical layer. The periosteum bones, which are hardened through the has numerous sensory fibres, so bone inju- Triglyceride storage deposit of the calcium and other minerals ries (such as fractures or tumours) can be Yellow bone marrow (Fig 1) acts as a poten- around the fibres (Robson and Synder- extremely painful (Drake et al, 2019). tial energy reserve for the body; it consists combe Court, 2018). The intracortical bone is organised into largely of adipose cells, which store triglyc- Mineral salts are first deposited between structural units, referred to as or erides (a type of lipid that occurs naturally in the gaps in the collagen layers with once Haversian systems (Fig 2). These are cylin- the blood) (Tortora and Derrickson, 2009). these spaces are filled, minerals accumulate drical structures, composed of concentric around the collagen fibres, crystallising and layers of bone called lamellae, whose struc- Bone composition causing the tissue to harden; this process is ture contributes to the strength of the cor- Bone matrix has three main components: called (Tortora and Derrickson, tical bone. (mature bone cells) l 25% organic matrix (); 2009). The hardness of the bone depends on sit in the small spaces between the concen- l 50% inorganic mineral content the type and quantity of the minerals avail- tric layers of lamellae, which are known as (mineral salts); able for the body to use; hydroxyapatite is lacunae. Canaliculi are microscopic canals l 25% water (Robson and Syndercombe one of the main minerals present in bones. between the lacunae, in which the osteo- Court, 2018). While bones need sufficient minerals to cytes are networked to each other by fila- Organic matrix (osteoid) is made up of strengthen them, they also need to prevent mentous extensions. In the centre of each

FRANCESCA CORRA FRANCESCA approximately 90% type-I collagen fibres being broken by maintaining sufficient is a central (Haversian) canal

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Fig 2. Anatomy of cortical bone Box 1. Types of bones l Long bones – typically longer than Canaliculi they are wide (such as humerus, radius, tibia, ), they comprise a diaphysis (shaft) and epiphyses at Inner circumferential lamella the distal and proximal ends, joining Lymphatic vessel Osteon at the . In growing bone, Concentric lamellae Outer this is the site where growth occurs circumferential and is known as the epiphyseal lamella growth plate. Most long bones are erioseum Medullary located in the appendicular skeleton Inner osteogenic cavity and function as levers to produce layer movement Outer fibrous l Short bones – small and roughly layer Trabeculae cube-shaped, these contain mainly Central canal cancellous bone, with a thin outer Perforating canal layer of cortical bone (such as the bones in the hands and tarsal bones Periosteal vein in the feet) Spongy bone Periosteal artery l Flat bones – thin and usually slightly Compact bone curved, typically containing a thin layer of cancellous bone surrounded by cortical bone (examples include the skull, ribs and scapula). Most are through which the blood vessels, lymph Blood vessels in bone are necessary for located in the axial skeleton and offer vessels and nerves pass. These central canals nearly all skeletal functions, including the protection to underlying structures tend to run parallel to the axis of the bone; delivery of oxygen and nutrients, homoeo- l Irregular bones – bones that do not Volkmann’s canals connect adjacent stasis and repair (Tomlinson and Silva, fit in other categories because they osteons and the blood vessels of the central 2013). The blood supply in long bones is have a range of different canals with the periosteum. derived from the and the characteristics. They are formed of The endosteum consists of a thin layer of periosteal, epiphyseal and metaphyseal cancellous bone, with an outer layer connective tissue that lines the inside of the arteries (Iyer, 2019). of cortical bone (for example, the cortical surface (Bartl and Bartl, 2017) (Fig 1). Each artery is also accompanied by nerve vertebrae and the pelvis) fibres, which branch into the marrow cavi- l Sesamoid bones – round or oval Cancellous bone ties. Arteries are the main source of blood bones (such as the patella), which Also known as spongy bone, cancellous and nutrients for long bones, entering develop in tendons bone is found in the outer cortical layer. It through the nutrient foramen, then is formed of lamellae arranged in an irreg- dividing into ascending and descending ular lattice structure of trabeculae, which branches. The ends of long bones are sup- ossification or endochondral ossification gives a honeycomb appearance. The large plied by the metaphyseal and epiphyseal (replacing cartilage with bone). gaps between the trabeculae help make the arteries, which arise from the arteries from Bones are classified according to their bones lighter, and so easier to mobilise. the associated joint (Bartl and Bartl, 2017). shape (Box 1). Flat bones develop from Trabeculae are characteristically ori- If the blood supply to bone is disrupted, membrane (membrane models) and sesa- ented along the lines of stress to help resist it can result in the death of bone tissue moid bones from tendon (tendon models) forces and reduce the risk of fracture (Tor- (osteonecrosis). A common example is fol- (Waugh and Grant, 2018). The term intra- tora and Derrickson, 2009). The closer the lowing a fracture to the femoral neck, membranous ossification describes the trabecular structures are spaced, the greater which disrupts the blood supply to the direct conversion of mesenchyme struc- the stability and structure of the bone (Bartl femoral head and causes the bone tissue to tures to bone, in which the fibrous tissues and Bartl, 2017). Red or yellow bone marrow become necrotic. The femoral head struc- become ossified as the mesenchymal stem exists in these spaces (Robson and Synder- ture then collapses, causing pain and dys- cells differentiate into osteoblasts. The combe Court, 2018). Red bone marrow in function. osteoblasts then start to lay down bone adults is found in the ribs, sternum, verte- matrix, which becomes ossified to form brae and ends of long bones (Tortora and Growth new bone. Derrickson, 2009); it is haemopoietic tissue, Bones begin to form in utero in the first Long, short and irregular bones develop which produces erythrocytes, leucocytes eight weeks following fertilisation (Moini, from an initial model of hyaline cartilage (white blood cells) and platelets. 2019). The embryonic skeleton is first (cartilage models). Once the cartilage formed of mesenchyme (connective tissue) model has been formed, the osteoblasts Blood supply structures; this primitive skeleton is gradually replace the cartilage with bone Bone and marrow are highly vascularised referred to as the skeletal template. These matrix through endochondral ossification and account for approximately 10-20% of structures are then developed into bone, (Robson and Syndercombe Court, 2018).

FRANCESCA CORRA FRANCESCA cardiac output (Bartl and Bartl, 2017). either through intramembranous Mineralisation starts at the centre of the

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Fig 3. Growth plate zones growth stops (Ralston and McInnes, 2014). Males are on average taller than females because male puberty tends to occur later, nges in ro le ones so male bones have more time to grow onroes (Waugh and Grant, 2018). Over-secretion of human growth hormone during child- Resting or hood can produce gigantism, whereby the quiescent zone Matrix production person is taller and heavier than usually expected, while over-secretion in adults results in a condition called acromegaly. Growth or If there is a fracture in the epiphyseal proliferation zone Mitosis growth plate while bones are still growing, this can subsequently inhibit bone growth, resulting in reduced bone formation and the bone being shorter. It may also cause misalignment of the joint surfaces and cause a predisposition to developing sec- ondary arthritis later in life. A discrepancy Hypertrophic zone Matrix calcification in leg length can lead to pelvic obliquity, with subsequent scoliosis caused by trying to compensate for the difference.

Remodelling Once bone has formed and matured, it Calcification zone Cell death undergoes constant remodelling by osteo- clasts and osteoblasts, whereby old bone tissue is replaced by new bone tissue (Fig 4). remodelling has several functions, spongiosa including mobilisation of calcium and other Zone of ossification minerals from the skeletal tissue to main- tain serum homoeostasis, replacing old Secondary spongiosa tissue and repairing damaged bone, as well as helping the body adapt to different forces, loads and stress applied to the skeleton. Calcium plays a significant role in the body and is required for muscle contrac- cartilage structure, which is known as the this zone become ossified and form tion, nerve conduction, cell division and primary ossification centre. Secondary part of the ‘new diaphysis’ (Tortora and blood coagulation. As only 1% of the body’s ossification centres also form at the epi- Derrickson, 2009). calcium is in the blood, the skeleton acts as physes (epiphyseal growth plates) (Dan- Bones are not fully developed at birth, storage facility, releasing calcium in ning, 2019). The epiphyseal growth plate is and continue to form until skeletal maturity response to the body’s demands. Serum composed of hyaline cartilage and has four is reached. By the end of adolescence around calcium levels are tightly regulated by two regions (Fig 3): 90% of adult bone is formed and skeletal hormones, which work antagonistically to l Resting or quiescent zone – situated maturity occurs at around 20-25 years, maintain homoeostasis. Calcitonin facili- closest to the , this is although this can vary depending on geo- tates the deposition of calcium to bone, composed of small scattered graphical location and socio-economic con- lowering the serum levels, whereas the with a low proliferation ditions; for example, malnutrition may parathyroid hormone stimulates the rate and anchors the growth plate to the delay bone maturity (Drake et al, 2019; Bartl release of calcium from bone, raising the epiphysis; and Bartl, 2017). In rare cases, a genetic muta- serum calcium levels. l Growth or proliferation zone – this tion can disrupt cartilage development, and Osteoclasts are large multinucleated area has larger chondrocytes, arranged therefore the development of bone. This can cells typically found at sites where there is like stacks of coins, which divide and result in reduced growth and short stature active bone growth, repair or remodelling, are responsible for the longitudinal and is known as achondroplasia. such as around the periosteum, within the growth of the bone; The human growth hormone (somato- endosteum and in the removal of calluses l Hypertrophic zone – this consists of tropin) is the main stimulus for growth at formed during fracture healing (Waugh large maturing chondrocytes, which the epiphyseal growth plates. During and Grant, 2018). The cell mem- migrate towards the metaphysis. There puberty, levels of sex hormones (oestrogen brane has numerous folds that face the is no new growth at this layer; and testosterone) increase, which stops surface of the bone and osteoclasts break l Calcification zone – this final zone of cell division within the growth plate. As down bone tissue by secreting lysosomal the growth plate is only a few cells the chondrocytes in the proliferation zone enzymes and acids into the space between thick. Through the process of stop dividing, the growth plate thins and the ruffled membrane (Robson and Syn-

FRANCESCA CORRA FRANCESCA endochondral ossification, the cells in eventually calcifies, and longitudinal bone dercombe Court, 2018). These enzymes

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Fig 4. Bone remodelling process Physical exercise, in particular weight- bearing exercise, is important in main- taining or increasing bone mineral density ining bone ells and the overall quality and strength of the bone. This is because osteoblasts are stim- ulated by load-bearing exercise and so bones subjected to mechanical stresses esting stage one ormation undergo a higher rate of bone remodel- one ling. Reduced skeletal loading is associ- ated with an increased risk of developing steolasts Osteoclast osteoporosis (Robson and Syndercombe recruitment Matrix and activation synthesis Court, 2018).

Osteoblast steoblasts Conclusion recruitment and activation Bones are an important part of the muscu- loskeletal system and serve many core functions, as well as supporting the body’s one resortion Osteoclast removal ransition structure and facilitating movement. Bone is a dynamic structure, which is continu- ally remodelled in response to stresses placed on the body. Changes to this remod- dissolve the minerals and some of the bone disorganised bone remodelling affecting elling process, or inadequate intake of matrix. The minerals are released from the one or more bones. Typical features on X-ray nutrients, can result in changes to bone bone matrix into the extracellular space include focal patches of lysis or sclerosis, structure that may predispose the body to and the rest of the matrix is phagocytosed cortical thickening, disorganised trabeculae increased risk of fracture. Part 2 of this and metabolised in the cytoplasm of the and trabecular thickening. series will review the structure and func- osteoclasts (Bartl and Bartl, 2017). Once the As the body ages, bone may lose some tion of the skeletal system. NT area of bone has been resorbed, the osteo- of its strength and elasticity, making it clasts move on, while the osteoblasts move more susceptible to fracture. This is due to References Bartl R, Bartl C (2017) Structure and architecture in to rebuild the bone matrix. the loss of mineral in the matrix and a of bone. In: Bone Disorder: Biology, Diagnosis, Osteoblasts synthesise collagen fibres reduction in the flexibility of the collagen. Prevention, Therapy. Bit.ly/SpringerBoneDisorder and other organic components that make Danning CL (2019) Structure and function of the up the bone matrix. They also secrete alka- Diet and lifestyle factors musculoskeletal system. In: Banasik JL, Copstead L-EC (eds) Pathophysiology. St Louis, MO: Elsevier. line phosphatase, which initiates calcifica- Adequate intake of vitamins and minerals Drake RL et al (eds) (2019) Gray’s Anatomy for tion through the deposit of calcium and is essential for optimum bone formation Students. London: Elsevier. other minerals around the matrix (Robson and ongoing bone health. Two of the most Iyer KM (2019) Anatomy of bone, fracture, and and Syndercombe Court, 2018). As the important are calcium and vitamin D, but fracture healing. In: Iyer KM, Khan WS (eds) General Principles of Orthopedics and Trauma. osteoblasts deposit new bone tissue many others are needed to keep bones London: Springer. around themselves, they become trapped strong and healthy (Box 2). Moini J (2019) Bone tissues and the skeletal in pockets of bone called lacunae. Once system. In: Anatomy and Physiology for Health Professionals. Burlington, MA: Jones and Bartlett. this happens, the cells differentiate into Box 2. Vitamins and minerals Ralston SH, McInnes IB (2014) Rheumatology and osteocytes, which are mature bone cells needed for bone health bone disease. In: Walker BR et al (eds) Davidson’s that no longer secrete bone matrix. Principles and Practice of Medicine. Edinburgh: The remodelling process is achieved Key nutritional requirements for bone Churchill Livingstone. Robson L, Syndercombe Court D (2018) Bone, through the balanced activity of osteoclasts health include minerals such as calcium muscle, skin and connective tissue. In: Naish J, and osteoblasts. If bone is built without the and phosphorus, as well as smaller Syndercombe Court D (eds) Medical Sciences. appropriate balance of osteocytes, it results qualities of fluoride, manganese, and London: Elsevier in abnormally thick bone or bony spurs. iron (Robson and Syndercombe Court, Tomlinson RE, Silva MJ (2013) Skeletal blood flow in bone repair and maintenance. Bone Research; Conversely, too much tissue loss or calcium 2018). Calcium, phosphorus and vitamin 1: 4, 311-322. depletion can lead to fragile bone that is D are essential for effective bone Tortora GJ, Derrickson B (2009) The skeletal more susceptible to fracture. The larger sur- mineralisation. Vitamin D promotes system: bone tissue. In: Principles of Anatomy and Physiology. Chichester: John Wiley & Sons. calcium absorption in the intestines, face area of cancellous bones is associated Waugh A, Grant A (2018) The musculoskeletal with a higher remodelling rate than cortical and deficiency in calcium or vitamin D system. In: Ross & Wilson Anatomy and Physiology bone (Bartl and Bartl, 2017), which means can predispose an individual to in Health and Illness. London: Elsevier. osteoporosis is more evident in bones with a ineffective mineralisation and increased

high proportion of cancellous bone, such as risk of developing conditions such as CLINICAL the head/neck of femur or vertebral bones osteoporosis and osteomalacia. SERIES Skeletal system (Robson and Syndercombe Court, 2018). Other key vitamins for healthy bones Part 1: Anatomy and physiology include vitamin A for Changes in the remodelling balance may of bones Feb also occur due to pathological conditions, function and vitamin C for collagen Part 2: Structure and function of such as Paget’s disease of bone, a condition synthesis (Waugh and Grant, 2018). the skeletal system Mar

FRANCESCA CORRA FRANCESCA characterised by focal areas of increased and

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