Major Skeletal Functions: Body support, mineral and lipid storage, blood cell production, soft tissue protection, and mechanical leverage for movement.
Classification of Bones by Shape:
Sutural (Wormian) bones: Small, flat, oddly shaped bones found between flat bones of the skull.
Irregular bones: Complex shapes with short, flat, notched, or ridged surfaces (e.g., vertebrae, pelvic bones, several skull bones).
Short bones: Boxlike in appearance (e.g., carpal bones, tarsal bones).
Flat bones: Thin, parallel surfaces providing protection and broad muscle attachment areas (e.g., roof of skull, sternum, ribs, scapulae).
Long bones: Long and slender, located in limbs, hands, feet, fingers, and toes (e.g., femur, the largest and heaviest bone in the body).
Sesamoid bones: Small, round, and flat, found near joints of knees, hands, and feet (e.g., patellae).
Gross Anatomy of Bones
Long Bone Anatomy:
Diaphysis: Tubular shaft with a wall of compact bone enclosing a hollow medullary cavity.
Epiphysis: Expanded articular ends consisting mostly of spongy (trabecular) bone.
Metaphysis: Narrow region connecting the diaphysis and epiphysis.
Flat Bone Anatomy:
Core of spongy bone sandwiched between two layers of compact bone (cortex).
Diploë: Internal layer of spongy bone within cranial flat bones.
Bone Surface Coverings:
Periosteum: Outer membrane covering bone surfaces (except at joints), containing an outer fibrous layer and inner cellular layer. Secured to bone by perforating (Sharpey's) fibers.
Endosteum: Incomplete cellular lining of the medullary cavity, trabeculae, and central canals; contains osteogenic cells, osteoblasts, and osteoclasts.
Bone Tissue Composition & Cells
Extracellular Matrix Composition:
Specialized bone cells constitute only 2% of total bone mass.
Inorganic Components (67% / two-thirds): Hydroxyapatite crystals formed from calcium phosphate, calcium carbonate, sodium, magnesium, and fluoride; provides compression resistance and hardness.
Osteogenic (osteoprogenitor) cells: Stem cells located in periosteum and endosteum that divide to produce osteoblasts; active in fracture repair.
Osteoblasts: Immature cells that secrete osteoid (organic matrix) and promote calcification during osteogenesis; differentiate into osteocytes when enclosed by matrix.
Osteocytes: Non-dividing mature cells in lacunae connected by canaliculi; maintain protein and mineral content and assist in bone repair.
Osteoclasts: Multinucleate cells that secrete acids and enzymes to dissolve matrix and release minerals during osteolysis.
Histology of Compact and Spongy Bone
Cortical (Compact) Bone:
Osteon (Haversian System): Basic functional unit composed of concentric lamellae around a central canal containing blood vessels parallel to the surface.
Perforating (Volkmann's) Canals: Vascular passages running perpendicular to the bone surface.
Lamellae Types: Concentric (encircle central canal), Interstitial (fill spaces between osteons), and Circumferential (located at outer and inner bone surfaces).
Trabecular (Spongy) Bone:
Lacks osteons; organized as an interconnected network of trabeculae nourished via diffusion through canaliculi.
Contains red bone marrow (site of blood cell production) or yellow bone marrow (adipose storage).
Resists multi-directional stress and transfers weight along the femur shaft (medial side under compression, lateral side under tension).
Bone Formation, Growth, and Remodeling
Ossification Processes:
Endochondral Ossification: Replaces a hyaline cartilage model. Primary ossification center develops in diaphysis; secondary centers form in epiphyses.
Intramembranous Ossification: Bone forms directly from mesenchyme or fibrous connective tissue, creating dermal bones (e.g., skull roof, mandible, clavicle).
Bone Growth Types:
Interstitial Growth: Increases bone length at the epiphyseal cartilage (plate) within the metaphysis; stops at epiphyseal closure, leaving an epiphyseal line.
Appositional Growth: Increases bone diameter; periosteal osteoblasts add circumferential lamellae while endosteal osteoclasts enlarge the medullary cavity.
Bone Remodeling & Adaptations:
Continuous matrix recycling balanced between osteoblasts and osteoclasts.
Weight-bearing physical activity and mechanical stress stimulate osteoblast activity, increasing bone density and thickness.
Calcium Homeostasis & Hormonal Control
Mineral Storage: Skeleton stores 99% of total body calcium.
Parathyroid Hormone (PTH):
Released by parathyroid glands when blood calcium drops below 8.5mg/dL.
Increases blood Ca2+ by stimulating osteoclast activity, enhancing calcitriol synthesis for intestinal calcium absorption, and reducing renal calcium excretion.
Calcitonin:
Secreted by thyroid C cells when blood calcium rises above 11mg/dL.
Decreases blood Ca2+ by inhibiting osteoclasts, reducing intestinal absorption, and increasing renal calcium excretion.
Fractures, Clinical Conditions, and Aging
Fracture Types: Open (compound, breaks through skin) vs. Closed (simple, completely internal).
Fracture Repair Sequence:
Fracture hematoma formation: Extensive bleeding creates a blood clot; dead bone extends along shaft due to disrupted circulation.
Callus formation: Endosteum and periosteum cells divide to form internal callus (spongy bone) and external callus (cartilage and bone).
Spongy bone formation: Osteoblasts replace central cartilage of external callus with spongy bone.
Compact bone formation: Osteoblasts and osteoclasts remodel the site into compact bone.
Pathological & Growth Conditions:
Pituitary growth failure: Inadequate growth hormone leading to short stature.
Gigantism / Acromegaly: Excess growth hormone before puberty (gigantism) or after epiphyseal closure (acromegaly).
