Functions of Bones

• Bones perform seven critical functions: support, movement, protection, mineral storage, blood cell formation, energy storage, and endocrine function.

Functions of Bones

• Support is provided by bones, which form a rigid framework that supports the body and cradles soft organs.
• Bones act as levers for skeletal muscles to pull on, enabling movement.
• Tendons connect skeletal muscles to bones.
• Joints dictate the range and type of motion possible.
• Flat bones such as the cranium protect the brain.
• The vertebrae protect the spinal cord.
• The rib cage protects the heart and lungs.
• Bones store calcium and phosphate.
• Hydroxyapatite helps maintain electrolyte balance in the blood.
• Red bone marrow, mainly in spongy bone of flat and irregular bones, is where blood cell formation occurs.
• Hematopoiesis is the process of blood cell formation.
• Yellow bone marrow in the medullary cavity of long bones stores fat for energy.
• Bones produce osteocalcin, a hormone that regulates blood glucose and fat deposition.

Classification by Shape

• Long bones are longer than they are wide, for example, the femur and humerus.
• Short bones are cube-shaped, exemplified by carpals and tarsals.
• Flat bones are thin and often curved, such as the sternum and skull bones.
• Irregular bones have complex shapes, like vertebrae and pelvic bones.
• Sesamoid bones develop within tendons, such as the patella.

Gross Anatomy of a Long Bone

• The diaphysis is the shaft of a long bone, composed mostly of compact bone.
• Epiphyses are the expanded ends of long bones, that consist mostly of spongy bone covered by articular cartilage.
• The metaphysis is the region between the diaphysis and epiphysis, including the epiphyseal plate in growing bones.
• The medullary cavity is the central cavity of the diaphysis that contains yellow marrow in adults.
• The periosteum is a connective tissue membrane covering the outer surface of bones (except joint surfaces).
• The periosteum consists of an outer fibrous layer and an inner osteogenic layer containing osteoblasts and osteoclasts.
• The endosteum is a thin membrane lining internal bone surfaces, such as the medullary cavity and trabeculae, and contains osteogenic cells.

Microscopic Structure

• Compact bone is made of repeating units called osteons, each containing lamellae, a central canal, lacunae, and canaliculi.
• Lamellae are concentric layers of bone matrix.
• The central canal contains blood vessels and nerves.
• Lacunae are small cavities housing osteocytes.
• Canaliculi are tiny canals connecting osteocytes for nutrient diffusion.
• Perforating canals connect central canals across osteons.
• Spongy bone lacks osteons and is composed of trabeculae.
• Trabeculae are thin plates of bone aligned along stress lines.
• Spaces in spongy bone are filled with red marrow in some bones.
• Osteocytes within trabeculae receive nutrients via canaliculi.

Bone Cells

• Osteoprogenitor cells are stem cells in periosteum and endosteum that differentiate into osteoblasts.
• Osteoblasts are bone-forming cells that secrete bone matrix (osteoid).
• Osteocytes are mature bone cells located in lacunae that maintain bone tissue.
• Osteoclasts are bone-resorbing cells that dissolve bone matrix by secreting hydrochloric acid and lysosomal enzymes.

Matrix Composition

• The organic component of bone matrix (35%) includes collagen fibers for tensile strength and flexibility.
• The inorganic component of bone matrix (65%) is mainly hydroxyapatite crystals (calcium phosphate) that provide hardness and resistance to compression.