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.