The Skeletal and Muscular Systems

Bone Structure, Composition, and Remodeling

Bone is dynamic living tissue that continuously reorganizes itself. At the microscopic level, compact bone is organized into repeating structural units called osteons, or Haversian systems. Each osteon features circular layers of calcified bone matrix known as concentric lamellae, which encircle a central Haversian canal. Within these dense layers are small pockets called lacunae that house mature bone cells called osteocytes. These cells stay nourished and connected through canaliculi, which are microscopic canals that bridge neighboring lacunae to facilitate nutrient and waste exchange.

On a gross anatomical level, a typical long bone consists of a tubular shaft called the diaphysis and expanded terminal ends known as epiphyses. In growing children, the epiphyseal plate, composed of hyaline cartilage, serves as the site of longitudinal bone growth. While compact bone provides dense strength along outer surfaces, spongy bone (also called cancellous or trabecular bone) forms an internal lattice of thin plates called trabeculae. Bones also house marrow: red bone marrow functions as the site of blood cell production, whereas yellow bone marrow consists mainly of fat-storing adipocytes.

Bone tissue undergoes perpetual maintenance through cellular remodeling. Osteoblasts are responsible for bone formation; they synthesize and secrete the organic components of the bone matrix. Conversely, osteoclasts are large, multinucleated cells derived from monocytes that break down and resorb the matrix. These opposing cells are regulated by hormonal signals to maintain homeostatic blood calcium levels. When circulating calcium drops too low, parathyroid hormone indirectly stimulates osteoclast activity, which increases the release of calcium from the skeletal matrix into the bloodstream. Calcitonin can inhibit osteoclast activity and lower blood calcium levels, although it plays a limited role in calcium regulation in adult humans.