Bone Structure and Composition - Bone is composed of a flexible matrix and bound minerals. - The matrix is mostly elastic collagen fibers. - Inorganic mineral salt, calcium phosphate, gives bones rigidity. - Bones are constructed and remodeled by osteoblasts and osteoclasts. - Cortical bone is denser and forms the hard exterior of bones.

Cortical Bone - Cortical bone is the hard outer layer of bones. - It accounts for 80% of the total bone mass. - It supports the body, protects organs, and provides levers for movement. - Cortical bone consists of microscopic columns called osteons. - It is covered by a periosteum on the outer surface and an endosteum on the inner surface.

Trabecular Bone - Trabecular bone is the internal tissue of skeletal bones. - It is less dense and more flexible than cortical bone. - Trabecular bone has a higher surface-area-to-volume ratio. - It is found at the ends of long bones, near joints, and in the interior of vertebrae. - Trabecular bone contains bone marrow and hematopoietic stem cells.

Bone Marrow - Bone marrow is found in bones that hold cancellous tissue. - In newborns, all such bones are filled with red marrow. - As a child ages, the hematopoietic fraction decreases and marrow adipose tissue increases. - In adults, red marrow is mostly found in the femur, ribs, vertebrae, and pelvic bones. - Marrow contains hematopoietic stem cells and gives rise to blood cells.

Vascular Supply and Bone Cells - Bone receives about 10% of cardiac output. - Blood enters the endosteum and flows through the marrow. - Blood oxygen tension in bone marrow is lower compared to arterial blood. - Vascular supply in bone is important for nutrient and oxygen delivery. - Small vessels in the cortex allow blood to exit the bone. - Osteoblasts are mononucleate bone-forming cells. - Osteoblasts synthesize osteoid, a protein mixture that mineralizes to become bone. - Osteocytes are cells of mesenchymal origin that are trapped and surrounded by a bone matrix. - Osteocytes communicate with other bone cells through gap junctions. - Osteoclasts are large multinucleate cells responsible for bone resorption.