Osseous Tissue and Bone Structure

1. Chapter 6 Osseous Tissue and Bone Structure

2. Bones and Cartilages of the Human Body Hyaline Most abundant skeletal cartilage: articular, costal, respiratory, nasal Elastic External ear, epiglottis Fibrocartilage IVD, knee menisci

3. Functions of the skeletal system 1. Support - framework for the body 2. Protection - skull, vertebrae, ribcage 3. Leverage - bones are levers, joints are fulcrums 4. Mineral storage � (calcium) 5. Lipid Storage � (yellow marrow) 6. Blood cell formation - hematopoiesis

4. Classification of Bones Axial skeleton � bones of the skull, vertebral column, and rib cage Appendicular skeleton � bones of the upper and lower limbs, shoulder, and hip

5. Classification of Bones: By Shape Long bones longer than they are wide (e.g., humerus)

6. Classification of Bones: By Shape Short bones Cube-shaped bones of the wrist and ankle

7. Classification of Bones: By Shape Flat bones thin, flattened, and a bit curved (e.g., sternum, and most skull bones)

8. Classification of Bones: By Shape Irregular bones bones with complicated shapes (e.g., vertebrae and hip bones) Sesamoid- form w/in a tendon Wormian- form in sutures

9. Classification of Bones: By Shape

10. Structure of a typical long bone 1. Diaphysis- shaft of the bone Dia=through; physis=growth Contains medullary cavity (yellow in adults) 2. Epiphysis- epi=above; physis=growth Epiphyseal line- where diaphysis joins epiphysis Located at the metaphysis Articular cartilage- thin layer of hyaline cart to reduce friction & cushion joint

11. Bone Membranes Periosteum � double-layered membrane Outer fibrous layer is dense regular connective tissue Inner osteogenic layer is composed of osteoblasts and osteoclasts Richly supplied with nerve fibers, blood, and lymphatic vessels, which enter the bone via nutrient foramina Secured to underlying bone by Sharpey�s fibers Endosteum � delicate membrane covering internal surfaces of bone Active in bone repair

12. Structure of other bones No shaft or epiphysis Contain bone marrow but no cavity Thin plates of periosteum covered compact bone b/t endosteum covered spongy bone w/in Internal layer of spongy bone= diploe

13. Types of bone 1. Compact- has osteon 2. Spongy- no osteons Lattice of plates called trabeculae (contain lacuna) w/in the trabeculae are marrow B.V.�s from periosteum penetrate into spongy bone & osteocytes are nourished directly from blood in marrow cavity

14. Bone cell types (~ 2% of bone mass) 1. osteoprogenitor cells (osteogenic) Derived from mesenchyme & have ability to differentiate into osteoblasts; assist in fracture repair Found in the osteogenic layer of periosteum/endosteum, & through Volkmann�s canals 2. osteoblasts No mitotic potential; found in osteogenic layer of peri/endosteum Secrete the organic components & some of the mineral salts involved in bone formation (called osteoid); mature into osteocytes 3. osteocytes-cannot divide but maintain cellular activity Principle cells of bone tissue; maintain bone matrix & repair damaged bone 4. osteoclasts-found on inner peri/endosteum Derived from circulating monocytes and function to resorb bone

15. Bone (Osseous) Tissue Figure 6�3 Types of Bone Cells.

16. Intercellular substance 1. Matrix Proteins- organic matrix (primarily collagen, some glycoproteins) Somewhat flexible Approx. 35% of content Osteoblasts, osteocytes, osteoclasts 2. Mineral salts (hydroxyapatites)- primarily calcium phosphate, Ca3(PO4)2 Approx 65% of content Extremely strong, responsible for bone hardness and its resistance to compression

17. Microscopic Structure of Compact Bone Osteon, or Haversian system� the structural unit of compact bone Lamella � weight-bearing, column-like matrix tubes composed mainly of collagen Haversian, or central canal � central channel containing blood vessels and nerves Volkmann�s (perforating) canals � channels lying at right angles to the central canal, connecting blood and nerve supply of the periosteum to that of the Haversian canal Lacunae � small cavities in bone that contain osteocytes Canaliculi � hairlike canals that connect lacunae to each other and the central canal

18. Microscopic Structure of Compact Bone

19. Compact and Spongy Bone Figure 6�4a The Histology of Compact Bone.

20. Spongy Bone Does not have osteons The matrix forms an open network of trabeculae Trabeculae have no blood vessels The space between trabeculae is filled with red bone marrow: which has blood vessels forms red blood cells and supplies nutrients to osteocytes In some bones, spongy bone holds yellow bone marrow which stores fat

21. Bone formation (osteogenesis) Osteogenesis occurs throughout life but in different ways 1. embryo responsible for laying down of bony skeleton (ossification well started by 8th week) 2. bone growth continues until early adulthood 3. remodeling & repair continues for life Ossification - The process of replacing other tissues with bone (endochondral and intramembranous) Calcification - The process of depositing calcium salts Occurs during bone ossification and in other tissues

22. 2 types of ossification 1. Intramembranous (dermal ossification) Formation of most of the flat bones of the skull and the clavicles from a fibrous membrane Fibrous connective tissue membranes are formed by mesenchymal cells 2. Endochondral Formation of bone in hyaline cartilage Both lead to the same type of bone Both begin with migration of mesenchymal cells from c.t. to areas of bone formation No blood supply? chondroblasts Blood supply?osteoblasts

23. Intramembranous Ossification: Step 1 Mesenchymal cells aggregate: differentiate into osteoblasts begin ossification at the ossification center develop projections called spicules

24. Intramembranous Ossification: Step 2 Blood vessels grow into the area: to supply the osteoblasts Spicules connect: trapping blood vessels inside bone

25. Intramembranous Ossification: Step 3 Spongy bone develops and is remodeled into: osteons of compact bone periosteum or marrow cavities

26. Endochondral Ossification Begins in the second month of development Uses hyaline cartilage �bones� as models for bone construction Requires breakdown of hyaline cartilage prior to ossification

27. Endochondral Ossification: Step 1 Chondrocytes in the center of hyaline cartilage: enlarge form struts and calcify die, leaving cavities in cartilage

28. Endochondral Ossification: Step 2 Blood vessels grow around the edges of the cartilage Cells in the perichondrium change to osteoblasts: producing a layer of superficial bone around the shaft which will continue to grow and become compact bone (appositional growth)

29. Endochondral Ossification: Step 3 Blood vessels enter the cartilage: bringing fibroblasts that become osteoblasts spongy bone develops at the primary ossification center

30. Endochondral Ossification: Step 4 Remodeling creates a marrow cavity: bone replaces cartilage at the metaphyses

31. Endochondral Ossification: Step 5 Capillaries and osteoblasts enter the epiphyses: creating secondary ossification centers

32. Endochondral Ossification: Step 6 Epiphyses fill with spongy bone: cartilage within the joint cavity is articulation cartilage cartilage at the metaphysis is epiphyseal cartilage

33. Epiphyseal Lines When long bone stops growing, after puberty: Epiphyseal cartilage disappears At young adulthood cartilage division slows and the plate becomes smaller until it is reduced to a fine line �epiphyseal line Females? age 18 Males?age 21

34. Blood Supply of Mature Bones 3 sets of blood vessels develop Nutrient artery and vein: a single pair of large blood vessels enters the diaphysis through the nutrient foramen Metaphyseal vessels: supply the epiphyseal cartilage where bone growth occurs Periosteal vessels provide: blood to superficial osteons secondary ossification centers

35. Mature Bones As long bone matures: osteoclasts enlarge marrow cavity osteons form around blood vessels in compact bone Effects of Exercise on Bone Mineral recycling allows bones to adapt to stress Heavily stressed bones become thicker and stronger Bone Degeneration Bone degenerates quickly Up to 1/3 of bone mass can be lost in a few weeks of inactivity

36. Bone Formation and Growth Figure 6�9 Heterotopic Bone Formation.

37. Effects of Hormones and Nutrition on Bone 1. Growth hormone Single most important stimulus to the epiphyseal plate (dwarfism/gigantism) 2. Thyroid hormone Moderates growth hormone to insure proper proportions of growth 3. Sex hormones (estrogens & androgens) A great �rush� at puberty = growth spurt Lead to a breakdown of cartilage that leads to a closure of plates �steroids! 4. Calcitriol Made in kidneys; synthesis requires cholecalciferol Helps absorb calcium & phosphorus from GI tract

38. Additional dietary regulators Need adequate calcium, phophorus, magnesium, flouride, iron, & manganese Calcium is necessary for: Transmission of nerve impulses Muscle contraction Blood coagulation Secretion by glands and nerve cells Cell division Vitamin D � absorption of calcium from GI Vitamin C � formation of collagen Vitamin A � stimulates osteoblast activity Vitamins K and B12 - help synthesize bone proteins

39. Chemical Composition of Bone

40. Control of Remodeling Two control loops regulate bone remodeling Hormonal mechanism maintains calcium homeostasis in the blood Mechanical and gravitational forces acting on the skeleton

41. Hormonal (-) feedback mechanism 1. PTH (parathyroid hormone) Released in response to dropping blood levels of calcium Stimulates osteoclasts�raise blood Ca levels Increases absorption from digestive tract Causes reabsorption from kidneys 2. Calcitonin (secreted by thyroid) Causes Ca salts to be deposited in bone�inhibits osteoclast activity & increases calcium excretion at kidneys Mechanism is designed to maintain blood Ca at 9-11 mg/100ml Ca is vital for nerve conduction & muscle contraction BIG problems w/ breakdown in homeostasis

42. Calcium Homeostasis Figure 6�16a Factors That Alter the Concentration of Calcium Ions in Body Fluids.

43. Hormones for Bone Growth and Maintenance

44. Response to Mechanical Stress Wolff�s Law = bone remodels in response to stress placed upon it A deforming bone produces a minute electrical current. (-) on side of compression & (+) on side of tension�(-) seems to stimulate osteoblasts & the calcification of bone

45. Bone Fractures (Breaks) Bone fractures are classified by: The position of the bone ends after fracture The completeness of the break The orientation of the bone to the long axis Whether or not the bones ends penetrate the skin

46. Types of Bone Fractures Nondisplaced � bone ends retain their normal position Displaced � bone ends are out of normal alignment Complete � bone is broken all the way through Incomplete � bone is not broken all the way through Linear � the fracture is parallel to the long axis of the bone Transverse � the fracture is perpendicular to the long axis of the bone Compound (open) � bone ends penetrate the skin Simple (closed) � bone ends do not penetrate the skin Comminuted � bone fragments into three or more pieces; common in the elderly Spiral � ragged break when bone is excessively twisted; common sports injury Depressed � broken bone portion pressed inward; typical skull fracture Compression � bone is crushed; common in porous bones Epiphyseal � epiphysis separates from diaphysis along epiphyseal line; occurs where cartilage cells are dying Greenstick � incomplete fracture where one side of the bone breaks and the other side bends; common in children

47. The Major Types of Fractures

48. The Major Types of Fractures

49. Fracture Repair: Step 1 Bleeding: produces a clot (fracture hematoma) establishes a fibrous network Bone cells in the area die

50. Fracture Repair: Step 2 Cells of the endosteum and periosteum: Divide and migrate into fracture zone Calluses stabilize the break: external callus of cartilage and bone surrounds break internal callus develops in marrow cavity

51. Fracture Repair: Step 3 Osteoblasts: replace central cartilage of external callus with spongy bone

52. Fracture Repair: Step 4 Osteoblasts and osteocytes remodel the fracture for up to a year: reducing bone calluses

53. Osteoporosis Bone reabsorption>bone production Osteopenia begins between ages 30 and 40 Women lose 8% of bone mass per decade, men 3% Decrease in bone mass?increase fracture risk Decreased levels of estrogen primarily Most important cause of fracture in women>50 35% of bone mass may be gone by age 70 Vertebrae & femur neck are most affected Risk Factors Body build � short women have less bone mass Weight � thinner at greater risk Smoking � decreases estrogen levels Lack of dietary calcium Exercise � decrease rate of absorption Drugs � alcohol, cortisone, tetracycline Premature menopause

54. Osteopenia Figure 6�19 The Effects of Osteoporosis on Spongy Bone.

55. Homeostatic Imbalances Osteomalacia Bones are inadequately mineralized causing softened, weakened bones Main symptom is pain when weight is put on the affected bone Caused by insufficient calcium in the diet, or by vitamin D deficiency Rickets Bones of children are inadequately mineralized causing softened, weakened bones Bowed legs and deformities of the pelvis, skull, and rib cage are common Caused by insufficient calcium in the diet, or by vitamin D deficiency

56. Developmental Aspects of Bones The embryonic skeleton ossifies in a predictable timetable that allows fetal age to be easily determined from sonograms At birth, most long bones are well ossified (except for their epiphyses) By age 25, nearly all bones are completely ossified In old age, bone resorption predominates A single gene that codes for vitamin D docking determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life

57. Fetal Primary Ossification Centers at 12 weeks of age