The Skeletal System Consists of Bones, Cartilages, and Joints

PowerPoint® Lecture Slides The Skeletal System consists of bones, prepared by Leslie Hendon cartilages, and joints. University of Alabama, Birmingham Review of Cartilage • Location and basic structure • Found throughout adult body C H A P T E R 6 • Ear and epiglottis Part 1 • Articular cartilages and costal cartilage • Larynx, trachea, and nose • Intervertebral discs, pubic symphysis, and articular discs Bones and • Is surrounded by perichondrium (dense irreg. C.T.)-- Skeletal functions as a girdle (resists outward pressure) and in growth & repair Tissues • Consists primarily of water • Resilient tissue—it springs back to original shape

Types of Cartilage (review) Microscopic appearance of cartilage

Chondrocyte in a lacuna Chondrocyte Matrix in a lacuna • All cartilages share some similarities Elastic fibers Lacuna • Cell type is the chondrocyte Gelatinous ground substance • Chondrocytes are located within lacunae Perichondrium • Matrix contains (a) Hyaline cartilage (180×) (b) Elastic cartilage (470×) • Fibers

• Jellylike ground substance Chondrocyte in a lacuna

Collagen fibers

Types of Cartilage (review) Cartilage: Where to find different types of cartilage

Epiglottis Cartilage in Cartilages in • Hyaline cartilage (glassy) external ear nose Thyroid Larynx cartilage Most abundant cartilage Articular cartilage Cricoid • of a joint cartilage Trachea • Provides support through flexibility & resilience Costal cartilage Lung Cartilage in intervertebral disc • Elastic cartilage—contains many elastic fibers • Able to tolerate repeated bending

Pubic symphysis • Fibrocartilage—resists strong compression and strong Meniscus (padlike cartilage in tension knee joint) Respiratory tube cartilages Articular cartilage in neck and thorax • A intermediate between hyaline and dense regular of a joint Cartilages C.T. Hyaline cartilages Elastic cartilages Fibrocartilages

1 Growth of Cartilage Bones are the organs of the skeletal system • Appositional growth • Chondroblasts in surrounding perichondrium • Bones contain 3 types of tissues produce new cartilage • Dominated by bone CT • Interstitial growth • Contain nervous tissue and blood CT • Chondrocytes within cartilage divide and secrete new matrix • Contain cartilage in articular cartilages • Cartilage stops growing when the skeleton stops growing • Contain epithelial tissue lining blood vessels • Cartilage regenerates and heals poorly

Function of Bones/ Skeletal System Bone Tissue • Bone tissue consists of: • Support—provides hard framework • Organic components—cells, fibers, and ground substance • Movement—skeletal muscles use bones as levers • Inorganic components—mineral salts that invade • Protection of underlying organs bony matrix • Mineral storage—reservoir for important minerals • Unique composition of the extracellular matrix… • Blood-cell formation—bone contains red marrow • Gives bone exceptional properties • Energy metabolism—osteoblasts secrete osteocalcin: • 35%—organic components hormone that stimulates insulin release and release of • Contributes to flexibility and tensile strength fat from fat cells • 65%—inorganic components • Provide exceptional harness, resists compression

Cell types in Bone… A Classification ‘system’ for Bones

• Three types of cells in bone produces or maintain bone • Long bones—longer than wide; a shaft plus ends • Osteogenic cells—stem cells that differentiate into osteoblasts • Osteoblasts—actively produce and secrete bone matrix • Short bones—roughly cube-shaped • Bone matrix is osteoid • Osteocytes—keep bone matrix healthy (maintain the matrix) • Flat bones—thin and flattened, usually curved

• One type of cell breaks down or resorbs bone: • Irregular bones—various shapes, do not fit into other Osteoclasts categories • Responsible for resorption of bone • Are derived from a line of white blood cells • Secrete hydrochloric acid (dissolves mineral component of matrix) and lysosomal enzymes (digests organic component)

2 Classification of Bones Gross Anatomy of Bones:  two different ways of organizing the matrix/ cells and …  different areas in bones where these types are located

• Compact bone— • dense outer layer of bone • Spongy (cancellous) bone— • internal network of bone • Contains trabeculae: little “beams” of bone • Open spaces between trabeculae are filled with marrow

Structure of a Typical Long Bone Structure of a Long Bone Articular cartilage

Compact bone Proximal “ ” epiphysis • Diaphysis— shaft of a bone Spongy bone Endosteum Epiphyseal • Epiphysis—ends of a bone line Endosteum Periosteum • Blood vessels—well vascularized Compact bone Medullary cavity (lined • Medullary cavity—hollow cavity filled with by endosteum) (b)

yellow marrow Diaphysis Yellow bone marrow • Membranes Compact bone Periosteum • Periosteum, perforating fibers (Sharpey’s Perforating (Sharpey’s) fibers fibers), and endosteum Nutrient arteries

Distal epiphysis (a) (c)

Structure of Short, Irregular, and Flat Bones Gross Anatomy of Bones

Load here (body weight) • Flat bones, short • Bone design and bones, and stress

irregular bones • Anatomy of a bone Head of • Contain bone femur Spongy bone reflects applied marrow but no (diploë) stresses marrow cavity Compact bone • Compression and • Diploë tension greatest at • Internal spongy Tension Compression here here Trabeculae external surfaces bone of flat Point bones of no stress

(a)

3 Figure 6.6b Bone anatomy and bending stress. Load here Bone Markings

Compression lines Tension lines • Superficial surfaces of bones reflect stresses on them • There are three broad categories of bone markings: • Projections for muscle attachment • Surfaces that form joints Trabeculae of spongy • Depressions and openings bone

Bone Markings

Microscopic Structure of Compact Bone • Compact Bone • Contains passage ways for blood vessels, lymph vessels, and nerves • Matrix is organized in Osteons (Haversian canals) —long cylindrical structures • Function in support • Structurally—resembles rings of a tree in cross-section • Osteons contain: • Lamellae • Central canal • Perforating canals • Canaliculi

4 Figure 6.7 Microscopic structure of compact bone. Compact bone Spongy bone Microscopic Structure of Compact Bone

Compact bone Spongy bone

Perforating Central (Volkmann’s) canal Perforating (Haversian) canal Central (Volkmann’s) (Haversian) canal Endosteum lining bony canals canal Osteon Endosteum lining and covering trabeculae Osteon bony canals and (Haversian system) (Haversian system) covering trabeculae Circumferential Circumferential lamellae lamellae

(a) Perforating (Sharpey’s) fibers Lamellae Periosteal blood vessel Perforating collagen fiber bundles Periosteum Lamellae Periosteal blood vessel Periosteum

Nerve Osteocyte Nerve within lacuna Vein Vein Lamellae Artery Lamellae Lamellae Central Artery Central canal Central canal Central Canaliculi canal Canaliculi Lacunae canal Interstitial Osteocyte Lacunae Osteocyte in a lacuna Lacunae lamella Interstitial Lacuna (with in a lacuna (b) (c) lamellae osteocyte)

Microscopic structure of a single osteon Microscopic Structure of Spongy Bone Artery with capillaries Structures in the Vein central Nerve fiber • Spongy Bone canal • Is less complex than compact bone Lamellae • Trabeculae contain layers of lamellae and Collagen osteocytes fibers run in different • Are too small to contain osteons directions

Microscopic Structure of Compact Bones Bone Development

Marrow • Ossification (osteogenesis)—bone-tissue space formation Trabecula

Osteocytes • Membrane bones—formed directly from mesenchyme, via… • Intramembranous ossification • Other bones—develop initially from hyaline

Endosteum cartilage, via… (a) Osteoblasts • Endochondral ossification

(b)

5 Intramembranous Ossification Intramembranous Ossification

Mesenchyme Fibrous condensing periosteum to form the Osteoblast Mesenchymal Osteoblast periosteum cell Plate of Collagen Osteoid Trabeculae of compact bone fiber woven bone Ossification Osteocyte Diploë (spongy center Blood vessel bone) cavities Newly calcified contain red Osteoid bone matrix marrow 4 Lamellar bone replaces woven bone, just Osteoblast 3 Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic deep to the periosteum. Red marrow appears. 1 Ossification centers appear in the fibrous 2 Bone matrix (osteoid) is secreted within the blood vessels in a random manner. The result is a • Trabeculae just deep to the periosteum thicken and connective tissue membrane. fibrous membrane and calcifies. network (instead of lamellae) of trabeculae called are later replaced with mature lamellar bone, forming • Selected centrally located mesenchymal cells cluster • Osteoblasts begin to secrete osteoid, which is woven bone. compact bone plates. and differentiate into osteoblasts, forming an calcified within a few days. • Vascularized mesenchyme condenses on the external • Spongy bone (diploë), consisting of distinct ossification center. • Trapped osteoblasts become osteocytes. face of the woven bone and becomes the periosteum. trabeculae, persists internally, and its vascular tissue becomes red marrow.

Endochondral Ossification Stages in Endochondral Ossification

Week 9 Month 3 Birth Childhood to adolescence

Articular Forms all bones except some bones of the cartilage • Secondary ossification Spongy bone skull and clavicles center

Epiphyseal Area of blood vessel • Bones are modeled in hyaline cartilage deteriorating Epiphyseal cartilage matrix plate cartilage Hyaline • Begins forming late in the second month of cartilage Spongy Medullary bone cavity embryonic development formation

Bone Blood collar • Continues forming until early adulthood vessel of Primary periosteal ossification bud center

1 Bone collar forms 2 Cartilage in the 3 The periosteal 4 The diaphysis 5 The epiphyses around hyaline center of the bud invades the elongates and a ossify. When completed, cartilage model. diaphysis calcifies internal cavities, medullary cavity forms hyaline cartilage and then develops and spongy bone as ossification continues. remains only in the cavities. begins to form. Secondary ossification epiphyseal plates and centers appear in the articular cartilages. epiphyses in preparation for stage 5.

Anatomy of Epiphyseal Growth Areas

• In epiphyseal plates of growing bones: • Cartilage is organized for quick, efficient growth • Cartilage cells form tall stacks • Chondroblasts at the top of stacks divide quickly • Pushes the epiphysis away from the diaphysis • Lengthens entire long bone

6 Anatomy of Epiphyseal Growth Areas Organization of Cartilage within Epiphyseal Plate of Growing Long Bone

• Older chondrocytes signal surrounding matrix to calcify Resting zone • Older chondrocytes then die and disintegrate

1 Proliferation zone • Leaves long trabeculae (spicules) of calcified Cartilage cells undergo mitosis. cartilage on diaphysis side

• Trabeculae are partly eroded by osteoclasts 2 Hypertrophic zone Older cartilage cells enlarge. • Osteoblasts then cover trabeculae with bone tissue • Trabeculae finally eaten away from their tips by 3 Calcification zone Matrix becomes calcified; osteoclasts cartilage cells die; matrix begins deteriorating. Calcified cartilage spicule

Osseous tissue 4 Ossification zone New bone formation is occurring.

Figure 6.12 Organization of the cartilage within the epiphyseal plate of a growing long bone. Postnatal Growth of Endochondral Bones Resting zone

1 Proliferation zone • During childhood and adolescence: Cartilage cells undergo mitosis. • Bones lengthen entirely by growth of the

2 Hypertrophic epiphyseal plates (endochondral [‘in the zone Older cartilage cells enlarge. cartilage’] growth)

X-ray image of right knee, 3 Calcification • Cartilage is replaced with bone CT as quickly anterior view. Proximal zone epiphyseal plate of the Matrix becomes tibia enlarged in calcified; cartilage as it grows part (b). Calcified cells die; matrix cartilage begins deteriorating. spicule • Epiphyseal plate maintains constant thickness Osseous 4 Ossification tissue zone • Whole bone lengthens New bone is forming.

Photomicrograph Diagram of the zones of cartilage in the within the epiphyseal epiphyseal plate (125×). plate.

Postnatal Growth of Endochondral Bones Postnatal Growth of Endochondral Bones

• As adolescence draws to an end: • Growing bones also widen as they lengthen • Chondroblasts divide less often • Osteoblasts—add bone tissue to the external • Epiphyseal plates become thinner surface of the diaphysis • Cartilage stops growing • Osteoclasts—remove bone from the internal • Replaced by bone tissue surface of the diaphysis • Long bones stop lengthening when diaphysis • This widening is called appositional growth and epiphysis fuse —growth of a bone by addition of bone tissue to its surface

7 Bone Growth influenced by hormones Bone Remodeling (also nutrition)

• Growth hormone—produced by the pituitary gland • Bone is dynamic living tissue • Stimulates epiphyseal plates • 500 mg of calcium may enter or leave the adult skeleton each day • Thyroid hormone—ensures that the skeleton retains proper proportions • Bone matrix and osteocytes are continually removed and replaced • Sex hormones (estrogen and testosterone) • Cancellous bone of the skeleton is replaced Promote bone growth • every 3–4 years • Later induces closure of epiphyseal plates • Compact bone is replaced every 10 years

Bone Remodeling Remodeling, Spongy Bone

• Bone deposit and removal

Compact Spongy bone • Occurs at periosteal and endosteal surfaces bone Osteoclast Osteoblast Osteoblast • Bone remodeling New • Bone deposition—accomplished by bone osteoblasts • Bone reabsorption—accomplished by Trabeculae of Resorption of Deposition of osteoclasts spongy bone bone matrix new bone by by osteoclasts osteoblasts

Osteoclast—A Bone-Degrading Cell Repair of Bone Fractures

• A giant cell with many nuclei • Simple and compound fractures • Crawls along bone surfaces • Treatment by reduction-- realignment of the • Breaks down bone tissue broken ends of the bone • Secretes • Closed reduction-- manually concentrated HCl Bone matrix • Open reduction-- surgically with pins or wires • Lysosomal enzymes are Osteocyte within a lacuna released Ruffled border of osteoclast • Derived from Nuclei hematopoietic stem cells

8 Stages of Healing a Fracture Common Types of Fractures

Hematoma External callus Bony callus of spongy bone Internal New blood Healed callus fracture (fibrous vessels tissue and Spongy cartilage) bone trabecula

1 A hematoma forms. 2 Fibrocartilaginous 3 Bony callus forms. 4 Bone remodeling callus forms. occurs.

Common Types of Fractures Common Types of Fractures

Disorders of Bones Osteoporosis

• Osteoporosis • Characterized by low bone mass • Bone reabsorption outpaces bone deposition • Occurs most often in women after menopause

9 Figure 6.17 Rickets. Disorders of Bones

• Osteomalacia • Occurs in adults—bones are inadequately mineralized (not enough vitamin D or calcium phosphate in diet) • Rickets • Occurs in children—analogous to osteomalacia & same cause

Disorders of Bones The Skeleton Throughout Life

• Paget’s disease • Cartilage grows quickly in youth • Characterized by excessive rate of bone • Skeleton shows fewer chondrocytes in the deposition & resorption--- results in immature elderly matrix • Bones are a timetable • Osteosarcoma • Mesoderm • A form of bone cancer • Gives rise to embryonic mesenchyme cells • Mesenchyme • Produces membranes and cartilage • Membranes and cartilage ossify

Figure 6.18 Primary ossification centers in the skeleton of a 12-week-old fetus. Parietal The Skeleton Throughout Life bone Frontal bone of skull Occipital • Skeleton grows until the age of 18–21 years bone Mandible In children and adolescents, bone formation Clavicle • Scapula exceeds rate of bone reabsorption • In young adults, bone formation and bone Radius Ulna reabsorption are in balance Humerus • In old age, reabsorption predominates Femur

• Bone mass declines with age Tibia Ribs

Vertebra Ilium