Chapter 6: Bones and Bone Structure

The skeletal system comprises approximately 206 bones divided into the axial skeleton, which includes the skull, thorax, and vertebral column, and the appendicular skeleton, consisting of the limbs and girdles. Bones serve multiple critical functions including structural support, mineral storage, hematopoiesis within the marrow, organ protection, and mechanical leverage for movement. Bones are classified into six structural categories based on morphology, with long bones exemplifying the typical bone design through distinct regions including the epiphysis, diaphysis, metaphysis, and medullary cavity. Bone tissue comprises a composite material of flexible collagen fibers and rigid hydroxyapatite crystals maintained by four specialized cell types: osteogenic cells that serve as progenitors, osteoblasts that synthesize new matrix, osteocytes that maintain existing matrix within lacunae, and osteoclasts that resorb bone through enzymatic degradation. Two primary bone tissue types exist, with compact bone forming the dense outer walls organized into osteons containing concentric lamellae around central vessels, while spongy bone forms an open network of trabeculae that distributes stress multidirectionally. Bone development occurs through two distinct ossification pathways: endochondral ossification replaces a cartilage template to form long bones and permits longitudinal growth via the epiphyseal plate until closure at puberty, while intramembranous ossification generates bone directly from mesenchymal tissue to form dermal bones. Appositional growth increases bone diameter as the periosteum adds surface layers while the endosteum enlarges the medullary cavity. The skeleton functions as the body's primary calcium reservoir, storing ninety-nine percent of total body calcium under hormonal regulation by parathyroid hormone and calcitonin, which maintain blood calcium homeostasis through effects on osteoclast activity, intestinal absorption, and renal excretion. Genetic and metabolic disorders can disrupt skeletal development, producing conditions ranging from growth deficiencies to pathological ossification. Fractures represent breaks in bone continuity categorized as closed or open depending on skin integrity, and healing progresses through sequential stages of hematoma formation, callus development, spongy bone replacement, and long-term remodeling by osteoblasts and osteoclasts.