Chapter 6 & 7: The Skeletal System

1. Chapter 6 & 7: The Skeletal System

2. Skeletal Cartilages: structures, types & locations • Skeletal cartilage – • Made from cartilage • Consists primarily of water • Allows for resilience • No nerves or blood vessels • Surrounded by a layer of dense irregular connective tissue – perichondrium • Resists outward expansion when compressed • Source of blood vessels – feeds the matrix & chondrocytes • Hyaline cartilage – • Hyaline cartilage is the most abundant skeletal cartilage, and includes the articular (cover bone ends @ movable joints), costal (connects ribs to sternum), respiratory (larynx & reinforce passageways), and nasal (external nose) cartilages. • Provide support & flexibility (due to collagen fibers)

3. Skeletal cartilages cont. • Elastic cartilage – • More flexible than hyaline • Contains more elastic fibers • Located in the external ear & epiglottis • Fibrocartilage – • Located in areas that need to withstand a great deal of pressure & stretch • Chondrocytes & collagen fibers • Knee & intervertebral discs

4. Growth of cartilage • Appositional – • “growth from the outside” • Outward expansion due to production of cartilage matrix on the outside of tissue • Secrete new matrix against the external surface of the existing cartilage • Occurs in the shafts of long bones • Interstitial – • “growth from the inside” • Expansion within the cartilage matrix due to divisions of lacunae-bound chondrocytes & secretions of the matrix • Occurs in the ends of bone

5. Classifications of bones • 206 bones in the body • 2 divisions – • Axial – • Consists of: • The skull, vertebral column, & rib cage • Involved in protection, support, or carrying other body parts • Appendicular – • Consists of: • The bones of the upper & lower limbs & the girdles (shoulder & hip bones) that attach them to the axial skeleton

6. Shape • Long bones – • Longer than they are wide • Have a definite shaft & two ends • Consist of all limb bones except: • Patellas, carpals, & tarsals • Named for their shape not size (fingers are long bones even though they are small) • Short bones – • Somewhat cube-shaped • Include – • the carpals & tarsals • Sesamoid – bones that form with in tendons (patella)

7. Shape cont. • Flat bones – • Thin, flattened, and often curved bones • Include – • Skull bones, sternum, scapulae, and ribs • Irregular bones – • Complicated shapes • Don’t fit into any other class • Include – • Vertebrae • Hip bones & coxae

8. Functions • 5 main functions – • Support – • Support body • Cradle soft organs • Protection – • Protect vital organs • Movement – • Allow movement • Muscles attach to bones acting as levers for movement • Mineral storage – • Store calcium & phosphate • Released into the blood stream as ions for distribution to the body • Blood cell formation – • House hematopoietic tissue

9. Bone structure: gross anatomy • Bone markings – • Projections; • Muscle attach to and pull • Modified for where bones meet (joints) • E.g. heads, trochanters, spines • Depressions and openings; • Allows passages of nerves and blood vessels • E.g. fossae, sinuses, foramina, grooves • Table 6.1 pg. 179

10. Bone Textures • External layer = compact bone • Internal layer = spongy bone • Made of trabeculae

11. Long bones cont. • Diaphysis – • The bone shaft • Contains cavity with yellow marrow • Epiphysis – • Ends of long bones • Typically wider than diaphysis (shaft) • Consist of internal spongy bone & outer layer of compact bone • Ends are covered with hyaline (protects bone ends where they meet at the joint) • Epiphyseal line/plate – • Between epiphysis & diaphysis • Line = remnant of plate (hyaline cartilage disc in young adults that lengthens bone)

12. Long bones cont. • The external surface of bone is covered by the periosteum • Double layered membrane • Covers all bones except joint surfaces • Contains osteoblasts & osteoclasts • Richly supplied w/ blood, nerve fibers, & lymphatic vessels – enter bone shaft via nutrient foramen • Secured to bone shaft by – Sharpey’s fibers – tufts of collagen fibers • Provides insertion points for tendons and ligaments • The internal surface of bone is lined by a connective tissue membrane called the endosteum • Covers trabeculae of spongy bone • Lines canals that run through compact bone • Also contains osteoblasts & osteoclasts

13. Short, flat, & irregular bones • Short, flat, & irregular bones consist of thin plates of periosteum-covering compact bone on the outside, and endosteum-covered spongy bone inside, which houses bone marrow between the trabeculae • No shaft or epiphyses • Flat bones – internal layer of spongy bone = diploë

14. Hematopoietic tissue • Hematopoietictissue = red bone marrow • Located within trabecular cavities of the spongy bone, in diploëof flat bones & epiphysis of long bones • Infants – all areas of spongy bone contain red marrow • Adults – epiphysis of long bones – diaphysis – yellow marrow

15. Gross anat. cont. • 2 types of bone texture – • Compact – • Appears dense, smooth & solid • Contains passageways for blood vessels & nerves • Osteon – • structural unit of bones • tiny weight bearing pillars • arranged like tree rings • Each matrix tube = lamella • Collagen fibers run in same direction – in opposing lamella they run in opposing directions – allow extra strength • Haversian canal – • Center of osteon • Contain blood vessels & nerves • Lacunae – • Contain osteocytes – mature bone cells • Canaliculi– connect lacunae to each other and the central canals • Interstitial lamellae – • Incomplete lamellae between osteons • Circumferential lamellae – • Deep to the periosteum • Superficial to endosteum • Resist twisting of long bones

16. Microscopic anatomy • Compact bone – dense and solid • Structural unit = osteon • Contains lamellae, Haversian canal, & blood vessels and nerves • Volkmann’s canals – • Lie at right angles to the long bone axis • Connect blood & nerve supplies of the periosteum to the central canals & medullary cavity • Osteocytes – • Occupy lacunae & lamella junctions • Connected by canaliculi • Lamellae – • Circumferential – • Beneath periosteum • Interstitial – • Between osteons

17. Compact Bone

18. Gross anat. cont. • Spongy – • Internal to compact bone • Honeycomb, needle-like, flat pieces = trabeculae • Align along the lines of stress • Help the bone to resist stress • Contain irregularly arranged lamellae & osteocytes connected by canaliculi • No osteons present • Nutrients – diffused from canaliculi from capillaries in the endosteum

19. Chemical Composition of Bone • Organic components • Cells (osteogenic cells, osteoblasts, osteocytes, and osteoclasts) • Osteoid – ground substance and collagen fibers • Contribute to bone’s structure and flexibility • Inorganic components • 65% mineral salts (calcium phosphates) • Tightly packed crystals around collagen fibers • Contribute to bone’s hardness – resists compression

20. Formation of the Bony Skeleton • Ossification or osteogenesis = process of bone formation • Before week 8, skeleton made up of fibrous membranes and hyaline cartilage • Flexible and resilient, can accommodate mitosis • Intramembranous ossification – • Formation of cranial bones of the skull and clavicles • Endochondral ossification – • All bones below base of the skull (except clavicles) • Hyaline cartilage broken down as ossification proceeds

21. Intramembranous Ossification

22. Endochondral Ossification

23. Postnatal bone growth • During youth bones lengthen entirely by interstitial growth from the epiphyseal plates • Growth in length – • The cartilage cells at the top of the epiphyseal plate (closest to the epiphysis) push the epiphysis away from the diaphysis causing the bone to grow • Old chondrocytes (closer to the diaphysis) calcify & replace the cartilage with bone tissue • Growth in width – • Occurs through appositional growth • Bone growth due to deposition of bone matrix by osteoblasts beneath the periosteum

24. Growth in Length of Long Bones

25. Postnatal Bone Growth • Hormonal regulation • Infancy and childhood – growth hormone stimulates epiphyseal plate activity • Released by anterior pituitary gland • Thyroid hormones regulate the activity of growth hormone ensuring proper bone proportions • Testosterone and estrogens are released in increasing amounts at puberty • Initially – growth spurt, later induce epiphyseal plate closure

26. Bone homeostasis • Bone remodeling – • Weekly recycle 5-7% of bone mass • Spongy bone replaced every 3-4 yrs • Compact bone replaced every 10 yrs • Adults – • Balanced due to deposit & removal • Bone deposit occurs at a greater rate when bone is injured • Bone resorption allows minerals to be absorbed into the blood • Vit C (collagen synthesis), Vit D (absorption of dietary calcium), Vit A (needed for balance between deposit & removal of bone) • Bone Modeling & Remodeling • Control of bone remodeling – • Hormones – maintain blood calcium homeostasis • Mechanical stress & gravity – affect bone growth & allow bone to withstand stresses

27. Bone Remodeling • Response to mechanical stress • Wolff’s Law: a bone grows or remodels in response to the demands placed on it • Long bones are thickest midway along the diaphysis (where bending stresses are greatest) • Curved bones are thickest where they are most likely to buckle • Trabeculae of spongy bone form trusses or struts along lines of compression • Large, bony projections occur where heavy, active muscles attach

29. Bone repair • Classification of fractures – • Position of bone ends after fracture • Nondisplaced fractures = bone ends retain normal position • Displaced fracture = bone ends out of normal alignment • Completeness of break • Complete fracture = bone broken through • Incomplete = not broken all the way through • Orientation of break relative to the long axis of bone • Linear = parallels the long axis • Transverse = perpendicular to the bones long axis • Whether the bone ends penetrate the skin • Open/compound = bone ends penetrate the skin • Closed/simple = bone ends don’t penetrate the skin

30. Bone repair • Fractures are treated by reduction – • Closed (external) reduction • Bone ends coaxed into position by physician’s hands • Open (internal) reduction • Bone ends secured together surgically with pins or wires • Check out Fig 6.2 on pg. 192 for other types • Comminuted – common in elderly (brittle bones) • Compression – common in porous bones • Spiral – common sports fracture • Epiphyseal • Depressed – typical skull fracture • Greenstick – common in children (more organic matter)

31. Spiral fractures

32. Comminuted

33. Compression

34. Dislocations & open fractures

35. Bone repair cont. • 4 stages of fracture repair – • 1. Hematoma formation – • Blood vessels are torn during the break, blood clot forms • Nearby bone cells deprived of nutrition and die • 2. Fibroncartilaginous callus formation – • Vessels begin to form • Phagocytic cells clean up debris • Fibroblasts (produce collagen fibers that reconnect the bone) & osteoblasts (begin forming spongy bone) begin to reform the bone • 3. Bony callus formation – • Bone trabeculae convert callus into bone • Begins 3-4 weeks after injury • Continues for about 2-3 months • 4. Remodeling of bony callus – • Excess material removed • Compact bone is laid down

36. Steps of Bone Repair

37. Homeostatic Imbalances • Osteomalacia & Rickets (in children) • Bones inadequately mineralized • Caused by insufficient calcium or vitamin D deficiency • Osteoporosis • Bone resorption outpaces bone deposit, bone mass reduced • Spongy bone of spine most vulnerable and neck of femur (“broken hip”) • Caucasian women most susceptible group • Estrogen helps restrain osteoclast activity • GET ENOUGH CALCIUM WHILE BONES STILL INCREASING IN DENSITY! (Also, drink fluoridated water) • Paget’s Disease • Excessive and haphazard bone deposit and resorption • High ratio of spongy to compact -> spotty weakening