Skeletal System Part II

1. Skeletal System Part II Chapter 6

2. Microscopic Structures of Compact Bone • Osteon (Haversian system) – weight bearing pillars • Central canal – lined by endosteum, contains BVs • Perforating (Volkmann’s) canals – connect blood and nerve supply of the peristeum to the central canal and marrow cavity • Osteocytes – spider-shaped mature bone cells • Lacunae – small cavities that contain osteocyte bodies • Canaliculi (‘little canals’) – tubes that contain osteocyte ‘legs’ connecting adjacent lacunae and capillaries • Lamellae (‘little plate’) - layer of bone matrix in which collagen fibers and mineral crystals align to run in a single direction - Concentric, Interstial, Circumferential

3. Microscopic Structure of Compact Bone Fig 6.6

4. Spongy Bone • Trabeculae contain several layers of lamelae and osteocytes but are too small for osteons or vessels • Osteocytes receive nutrients from capillaries in the endosteum surround the trabeculae Fig 6.8

5. Chemical Composition of Bone • 35% organic components - cells, fibers, and organic substances - abundant collagen • 65% inorganic mineral salts (hydroxyapatites) - primarily calcium phosphate crystals - pack tightly providing exceptional hardness - resists compression • Bone is composed of the proper combination of organic and inorganic elements - durable, strong, and resilient without being brittle

6. Bone Development • Ossification (osteogenesis) – bone-tissue formation - embryo  childhood  adolescence full-grown skeleton - slower rate in adult as remodeling • Intramembranous: membrane bones developed from mesechyme (most skull bones and clavicles) - week 8, mesechyme cells cluster in CT to become osteoblasts that secrete organic bone matrix (osteoid) • Endochondral: bones (base of the skull down) that developed from hyaline cartilage - early week 8 bone begins as cartilage by end of week 8 pericondrium becomes vascularized to become bone-forming periosteum

7. Intramembranous Ossification Fig 6.9

9. Endochondral Ossification Fig 6.10

10. 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 - this pushes the epiphysis away from the diaphysis - lengthens entire long bone • Older chondrocytes signal surrounding matrix to calcify, then they die and disintegrate - leaves long trabeculae of calcified cartilage on diaphysis side - trabeculae partially eroded by osteoclasts - osteoblasts then cover trabeculae with bone tissue - trabeculae trimmed away from their tips by osteoclasts

11. Epiphyseal Plate of Growing Long Bone Fig 6.11

12. Endochondral Bone - Postnatal Growth • In chldhood and adolescence bones lengthen entirely by growth of the epiphyseal plates • Growth hormone produced by the pituitary gland stimulates epiphyseal plates • Thyroid hormone ensures the the skeleton retains proper proportions • Sex hormones (estrogen and testosterone) promote bone growth - later induces closure of epiphyseal plates

13. Adolescence to Adulthood • At the end of adolescence, chondroblasts divide less often and epiphyseal plates become thinner - cartilage stops growing and is replaced by bone tissue • Long bones stop lengthening when the diaphysis and epiphysis fuse • Bone is dynamic living tissue - 500 mg of calcium may enter or leave the adult skeleton each day - • Cancellous bone of the skeleton is replaced every 3-4 years • Compact bone is replaced every 10 years

14. Bone Remodeling • Growing bones widen as they lengthen - osteoblasts add bone tissue to the external surface of the diaphysis - osteoclasts – remove bone from the internal surface of the diaphysis • Appositional growth – growth of a bone by addition of bone tissue to its surface • Bone deposit and removal - occurs at periosteal and endosteal surfaces • Bone remodeling - bone deposition accomplished by osteoblasts - bone reabsorption accomplished by osteoclasts

15. Spongy Bone Remodeling Fig 6.12

16. Osteoclast – A Bone-Degrading Cell • A giant cell with many nuclei • Crawls along bone surfaces • Breaks down bone tissue - secretes concentrated HCl - lysosomal enzymes are released Fig 6.13

17. Repair of Bone Fractures • Simple fracture – bone breaks cleanly and does not penetrate the skin • Compound fracture – broken ends of the bone protrude through the skin • Treatment by reduction - realignment of the broken bone ends - closed, bone ends coaxed back into position - open, bone ends are joined surgically with pins or wires

18. Stages of Healing a Fracture Fig 6.14

22. Characterized by low bone mass • Bone reabsorption outpaces bone desposition • Occurs most often in women after menopause

23. Skeleton Throughout Life • Cartilage grows quickly in youth • Skeleton shows fewer chondrocytes in the elderly • Timetable from birth to death - mesoderm gives rise to embryonic mesenchyme - mesenchyme produces membranes and cartilage - membranes and cartilage ossify • Skeleton grows until the age of 18-21 years - in children and adolescents bone formation exceeds rate of bone reabsorption - in old age reabsorption predominates - bone mass declines with age

24. Clincial Terms • Osteomalacia – bones are inadequately mineralized • Rickets – Vit D or calcium phosphate • Paget’s disease • Bone Graft • Bony Spur • Osteosarcoma • Ostealgia • Osteomyelitis • Pathologic Fracture • Traction