Functions of Bone

Functions of Bone Support of soft tissues Protect internal organs Assist in movement Mineral homeostasis (Ca, phosphorus) Hematopoiesis (blood cell formation1 in red bone marrow)   Triglyceride storage (yellow bone marrow) Bone Classification Individual bones: organs in the skeletal system Shape categories: Long, short, flat, irregular Sesamoid, sutural Structure of Bone Parts of a long bone: Epiphysis, metaphysis, diaphysis Periosteum, endosteum, medullary cavity Articular cartilage Other shapes: spongy bone core surrounded by compact bone Blood and Nerve Supply Bone is a living tissue Foramen: entry points for vessels and nerves Arteries and veins: epiphyseal, metaphyseal, periosteal, nutrient Nerves: innervate bone tissues, send pain messages Bone Histology Osteogenic cells: undifferentiated, develop into osteoblasts Osteoblasts: produce the matrix, develop into osteocytes Osteocytes: mature bone cells Osteoclasts: secrete enzymes that break down bone tissue, develop from monocytes Matrix Mineral salts (Ca, P): strength Water (25%): flexibility Collagen fibers (35%): flexibility Compact Bone Resists physical stress from body weight and movement Ossified tissue arranged into osteons Concentric lamellae Central (Haversian) canals Lacunae, osteocytes, canaliculi Spongy Bone Ossified tissue arranged into trabeculae Lightweight, found in short, flat, and irregular bones Houses red bone marrow Bone Formation (2 types)   Intramembranous Ossification: Mesenchymal cells differentiate into osteoblasts Bone matrix secreted and calcifies Osteoblasts become osteocytes Trabeculae form in matrix Periosteum and compact bone form Used to form most flat bones Endochondral Ossification: Hyaline cartilage model forms Chondrocytes enlarge, matrix calcifies, and cells die Blood vessels invade, bringing osteoblasts Bony collar forms, ossification center forms in diaphysis Ossification continues, epiphyses form Medullary cavity forms Near complete ossification, articular cartilage and epiphyseal plate form Bone Growth: Length (Epiphyseal Plates) 2 main events: Growth of cartilage in epiphyseal plates Zones: resting, proliferating, hypertrophic, calcified cartilage Full replacement of cartilage by bone Growth in length stops when epiphyseal line forms Bone Growth: Width (Thickening) Diameter appositional growth Osteogenic cells in periosteum become osteoblasts New matrix formed Osteoclasts in endosteum break down ossified tissue Diameter of medullary cavity increases   Bone Growth and Remodeling Remodeling: bone is constantly resorbed (osteoclasts) and deposited (osteoblasts) Bone growth requires: Resources: minerals (Ca, P), vitamins (D, K, B12) Hormones (GH, TH, estrogen, androgens, PTH) Calcium Homeostasis Ca2+ stored in bone tissue: normal = 8-10 mg/dL Ca2+ needed for muscle contraction, nerve impulse conduction, and protein function ↓ Ca2+ levels → PTH released Osteoclasts stimulated ↑ Ca2+ levels → Calcitonin released Osteoblasts stimulated Aging and Bone 2 effects of aging, both ↑ risk of fracture: Demineralization: may result in osteoporosis Decreased protein (collagen) synthesis Nutritional supplements, exercise, hormones Bone Fracture (any break in a bone) Simple (closed): broken ends not involved, skin intact Compound (open): broken ends protrude through skin Stress Fracture: microscopic fissures in bone Comminuted: bone is splintered or crushed Greenstick: one side bends, other side is broken Impacted: broken end driven into the other Others: compression, spiral, etc. Fracture Repair: 4 step process restores structure & function Fracture hematoma forms: swollen blood clot outside vessel Cartilaginous callus forms: phagocytic cells clean debris Bony callus forms: strength restored but not all functions Remodeling: excess bone removed, new osteons formed