1. SKELETAL SYSTEM

2. SKELETAL SYSTEM • Form strong & flexible body framework • Bone • 206 bones in human skeleton • Cartilage • All three types • Ligaments

3. SKELETAL CARTILAGE • Cartilage tissue • High water content • Confers resilience • Avascular, no nerves • Surrounded by dense irregular connective tissue • “Perichondrium” • Vascular, supplies nutrients • Prevents outward expansion of cartilage during compression • All three types of cartilage represented • Hyaline cartilage • Elastic cartilage • Fibrocartilage

4. SKELETAL CARTILAGE Hyaline cartilage • Most abundant skeletal cartilage • Chondrocytes, fine collagen fibers • Subtypes • Articular cartilages • Cover ends of most bones at movable joints • Costal cartilages • Connect ribs to sternum • Respiratory cartilages • Form skeleton of larynx • Reinforce other respiratory organs • Nasal cartilages • Support the external nose

5. SKELETAL CARTILAGE Elastic cartilage • Similar to hyaline cartilage • Contain more stretchy elastic fibers • Withstand repeated bending • Locations • External ear • Epiglottis

6. SKELETAL CARTILAGE Fibrocartilage • Highly compressible • Great tensile strength • Intermediate between hyaline and elastic cartilages • Parallel rows of chondrocytes alternating with thick collagen fibers • Occur in areas subjected to heavy pressure and stretch • e.g., knee, between vertebrae

7. CARTILAGE GROWTH • Appositional growth • Cartilage-forming cells in perichondrium secrete new matrix against external face of existing cartilage tissue • “Growth from the outside” • Interstitial growth • Lacunae-bound chondrocytes divide and secrete new matrix • “Growth from the inside” • Cartilage growth typically ends during adolescence

8. BONES • 206 named bones of human skeleton • Divided into two groups • Axial skeleton • Forms long axis of body • Includes bones of skull, vertebral column, and rib cage • Appendicular skeleton • Consists of bones of limbs and girdles attaching these bones to the axial skeleton • Arms, legs, hip bones, shoulder bones

9. Long bones Short bones Flat bones Irregular bones BONE SHAPES

10. BONE SHAPES Long bones • e.g., femur, humerus, tibia, fibula, radius, ulna • Longer than wide • Rigid levers + muscle  movement Short bones • e.g., carpals, tarsals • Similar in length and width • Glide across each other

11. BONE SHAPES Flat bones • e.g., scapula, ribs, sternum, os coxae, most cranial bones • Enclose and protect soft organs • Provide broad surfaces for muscle attachment Irregular bones • e.g., vertebrae, some skull bones (sphenoid, ethmoid) • Elaborate shapes

12. BONE FUNCTIONS • Support • Protection • e.g., ribs, skull • Movement • Muscles use bones as levers to move • Mineral storage • Most importantly Ca & P • Hematopoiesis • Formation of most of the circulating blood cells

13. BONE STRUCTURE • Bones are organs containing various types of tissue • Osseous tissue dominates bones • Bones also contain • Nervous tissue in nerves • Cartilage tissue in articular cartilages • Fibrous connective tissue lining cavities • Muscle and epithelial tissue in blood vessels

14. BONE STRUCTURE Gross Anatomy: Bone Markings • Bone surfaces display various features • “Bone markings” • Depressions, projections, & openings • Sites of muscle, ligament, and tendon attachment • Joint surfaces • Conduits for muscles or nerves

15. BONE STRUCTURE Gross Anatomy: Bone Textures • Dense outer layer • “Compact bone” • Appears smooth to naked eye • Honeycomb-like inner layer • “Spongy bone” • Small needle-like or flat pieces (“trabeculae”) • Spaces between trabeculae filled with bone marrow • Red or yellow bone marrow

16. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Diaphysis • Shaft forming long axis of bone • Relatively thick collar of compact bone • Central medullary cavity • “Marrow cavity” • Yellow bone marrow cavity • Contains fat in adults

17. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Epiphyses • Bone ends • Generally more expanded than diaphysis • Compact bone forms exterior • Spongy bone forms interior • Joint surface covered with thin layer of articular (hyaline) cartilage • Absorbs stress, cushions during movement

18. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Epiphyseal line • a.k.a., “metaphysis” • Found between diaphysis and each epiphysis of adult long bone • Remnant of epiphyseal plate • Hyaline cartilage disk that grows during childhood to lengthen bone

19. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Membranes • Most of external surface of entire bone is covered by a double-layered membrane • “Periosteum” • Absent on joint surfaces

20. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Membranes: Periosteum • Outer fibrous layer is dense irregular connective tissue • Inner osteogenic layer consists mainly of osteoblasts and osteoclasts • Osteoblasts are bone-forming cells • Osteoclasts are bone-destroying cells

21. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Membranes: Periosteum • Numerous nerve fibers, lymphatic vessels, & blood vessels • Enter diaphysis via a nutrient foramen • Secured to underlying bone by perforating fibers • a.k.a., “Sharpey’s fibers” • Tufts of collagen fibers

22. BONE STRUCTURE Gross Anatomy: Long Bone Structure • Membranes: Endosteum • Internal bone surfaces covered with delicate membrane • “Endosteum” • Contains both osteoblasts and osteoclasts

23. BONE STRUCTURE Gross Anatomy: Structure of Short, Irregular, & Flat Bones • Outside: thin plates of periosteum-covered compact bone • Inside: endosteum-covered spongy bone • Not cylindrical • No diaphysis, epiphyses • Contain marrow between trabeculae • No marrow cavity

24. BONE STRUCTURE Gross Anatomy: Location of Hematopoietic Tissue in Bones • “Red marrow” typically found within trabecular cavities of spongy bone within long bones • Medullary cavity of diaphysis also filled with red marrow in newborn • Medullary cavity contains fat in adult long bones • Fat extends into epiphyses • Only head of femur and humerus possess red marrow • Most blood cell production occurs elsewhere • Flat bones (e.g., sternum) & irregular bones (e.g., hip bone) • Yellow marrow in medullary cavity can revert to red marrow in the severely anemic

25. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Compact bone appears dense and solid • Actually contains numerous passageways • Conduits for nerves, blood & lymphatic vessels

26. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Structural unit of compact bone is the osteon • a.k.a., Haversian system • Elongated cylinder parallel to long axis of bone • Tiny, weight-bearing pillars

27. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Osteon • Group of concentric hollow tubes • “Lamellae” • (Compact bone is sometimes called “lamellar bone)

28. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Not all lamellae are part of an osteon • Interstitial lamellae are incomplete lamellae between osteons • Circumferential lamellae extend around circumference of diaphysis • Just deep to the periosteum and just superficial to the endosteum

29. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Osteon • Collagen fibers • Parallel within each lamella • Perpendicular in adjacent lamella • Arrangement withstands twisting force ( torsion) • Crystals of bone salts align with collagen fibers

30. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Osteon • Central canal runs through osteon core • a.k.a., Haversian canal • Lined with endosteum • Contains nerve fibers and small blood vessels

31. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Osteon • Perforating canals lie at right angles to long axis of bone • a.k.a., Volkman’s canals • Lined with endosteum • Connect to blood and nerve supply of periosteum • (Also connects to that of medullary cavity)

32. BONE STRUCTURE Microscopic Anatomy: Compact Bone • Osteon • Osteocytes occupy lacunae between lamellae • Mature bone cells • Lacunae connected by canaliculi • Hair-like canals • Also connect to central canal

33. BONE STRUCTURE Microscopic Anatomy: Compact Bone Formation • Osteoblasts surround blood vessels • Maintain contact through gap junctions • Osteoblasts secrete bone matrix • Mature cells become trapped as matrix hardens • Canaliculi form around processes • Osteocytes are all connected through these canaliculi • Nutrients shared, wastes jointly removed

34. BONE STRUCTURE Microscopic Anatomy: Spongy Bone • Appears poorly organized • Trabeculae contain irregularly arranged lamellae and osteocytes • No osteons • Trabeculae align along lines of stress • Help bone resist stress • (similar to flying buttresses of a Gothic cathedral) • Nutrients: capillaries of endosteum  canaliculi  osteocytes

35. BONE COMPOSITION Organic Components • Cells • Osteoblasts, -clasts, & -cytes • Osteoid • Organic ~1/3 portion of matrix • Ground substance • Proteoglycans & glycoproteins • Collagen fibers • Contribute to structure, flexibility, and tensile strength

36. BONE COMPOSITION Inorganic Components • ~65% of bone mass • Hydroxyapatites • a.k.a., Mineral salts • Mainly calcium phosphate • Form tiny, tightly-packed crystals surrounding collagen fibers • Impart hardness • Ability to resist compression

37. BONE DEVELOPMENT Ossification / osteogenesis • Process of bone formation • Formation of the bony skeleton in the embryo • Bone growth until early adulthood • Bone remodeling throughout life

38. SKELETON FORMATION • Prior to week 8 • Embryonic skeleton is comprised of hyaline cartilage and fibrous membranes • Week 8 and beyond • Bone tissue begins to develop • Majority of fibrous or cartilaginous structures eventually replaced with bone • Fibrous membrane  (membrane) bone • Intramembranous ossification • Hyaline cartilage  (cartilage) bone • Endochondral ossification

39. OSSIFICATION Intramembranous Ossification • Formation of clavicles and cranial bones

40. OSSIFICATION Endochondral Ossification • Formation of almost all bones

41. POSTNATAL GROWTH • Long bones lengthen by interstitial growth of epiphyseal plates • “Growth from the inside” • All bones grow in thickness by appositional growth • “Growth from the outside” • Most bones stop growing during adolescence • Some bones continue very slow growth • e.g. some bones of nose and lower jaw

42. LONG BONE GROWTH • Cartilage of epiphyseal plate • Inactive on side facing epiphysis • Active on side facing diaphysis • Rapid mitosis forms tall columns of chondrocytes • Cells at “top” push epiphysis away from diaphysis • Long bone lengthens • Then… • “Bottom” chondrocytes hypertrophy • Lacunae erode & enlarge • Surrounding cartilage matrix calcifies • Chondrocytes die and deteriorate

43. LONG BONE GROWTH • Long spicules of calcified cartilage • (This is different than bone) • Invaded by marrow elements from medullary cavity • Spicules partially eroded by osteoclasts • Spicules covered with bone matrix • Spongy bone formed • Spicule tips ultimately digested by osteoclasts • Medullary cavity grows longer

44. LONG BONE GROWTH • Epiphyseal plate maintains a constant thickness • Rate of cartilage growth = rate of replacement • Longitudinal growth is accompanied by remodeling of epiphyseal ends • Involves new bone formation • Involves bone reabsorption • (More on this later) • End of adolescence • Epiphyseal plate chondrocytes divide less often • Plates become thinner • Entire replacement  epiphyseal line • “Epiphyseal plate closure” at ~18 (♀) – 21 (♂)

45. LONG BONE GROWTH Growth in Width • Growing bones widen as they lengthen • Appositional growth • “Growth from the outside” • Two processes • Osteoblasts beneath periosteum secrete bone matrix onto external bone surface • Osteoclasts on endosteal surface remove bone

46. BONE GROWTH Hormonal Regulation • Growth of epiphyseal plate stimulated by growth hormone • Released by pituitary • Activity modulated by thyroid hormone • Regulated by testosterone and estrogens • Growth spurt in adolescence • Masculinization / feminization of skeleton • Later induce epiphyseal plate closure

47. BONE REMODELING • Bone tissue is active and dynamic • ~5-7% of bone mass recycled weekly • Spongy bone replacement every ~3-4 years • Compact bone replacement every ~10 years • Bone deposition and absorption • Occur at surfaces • Periosteum and endosteum • Coordinated by packets of cells • Osteoblasts and osteoclasts

48. BONE DEPOSITION • Occurs when bone is injured • Occurs when added bone strength is required

49. BONE RESORPTION • Accomplished by osteoclasts • Giant multinucleate cells • Arise from hematopoietic stem cells • Same cells give rise to macrophages • Dig grooves into bone surface • “Resorption bays” • Release of HCl and lysosomal enzymes • Solubilizes calcium salts • Phagocytosis of demineralized matrix

50. REMODELING CONTROL Hormonal negative feedback mechanism • Maintains blood Ca2+ homeostasis • Calcium is important in many processes • Nerve impulses • Muscle contraction • Blood coagulation • Secretion • Cell division • Etc.