Bones Cell and Bone structure

Osteoblasts

WHAT: Bone-forming cells. FUNCTION: Build new bone by producing osteoid, the organic bone matrix. EXAMPLES: Bone growth; fracture repair.

Osteoclasts

WHAT: Cells that break down bone. FUNCTION: Resorb bone during remodeling and help release stored calcium into the blood. EXAMPLES: Replacing old bone; helping regulate blood calcium.

Osteocytes

WHAT: Mature bone cells. FUNCTION: Maintain bone tissue and sense mechanical stress. EXAMPLES: Found in compact bone; found in spongy bone.

Osteogenic cells

WHAT: Stem cells that can develop into osteoblasts. FUNCTION: Help produce new bone-forming cells. EXAMPLES: Found in the periosteum; found in the endosteum.

Osteoid

WHAT: The soft, unmineralized organic matrix made by osteoblasts. FUNCTION: Provides the framework that later hardens with minerals. EXAMPLES: Contains collagen; becomes mineralized during bone formation.

Periosteum

WHAT: Tough connective tissue membrane covering the outer surface of most bones. FUNCTION: Nourishes and protects bone and helps with growth and repair. EXAMPLES: Covers the femur; covers the humerus.

Endosteum

WHAT: Thin membrane lining internal bone surfaces. FUNCTION: Contains cells involved in bone growth, repair, and remodeling. EXAMPLES: Lines the medullary cavity; covers surfaces of trabeculae.

Compact bone

WHAT: Dense, hard bone tissue. FUNCTION: Provides strength and resists stress. EXAMPLES: Outer layer of most bones; shaft of the femur.

Spongy bone

WHAT: Porous bone tissue made of a network of trabeculae. FUNCTION: Reduces bone weight and provides space for marrow. EXAMPLES: Ends of the femur; inside the sternum.

Medullary cavity

WHAT: Hollow space inside the shaft of a long bone. FUNCTION: Houses bone marrow and reduces bone weight. EXAMPLES: Femur; humerus.

Red bone marrow

WHAT: Blood-cell-producing tissue. FUNCTION: Makes red blood cells, white blood cells, and platelets. EXAMPLES: Pelvis; sternum.

Yellow bone marrow

WHAT: Marrow containing mostly fat cells. FUNCTION: Stores energy as fat. EXAMPLES: Medullary cavity of an adult femur; medullary cavity of an adult humerus.

Epiphyseal plate

WHAT: A plate of hyaline cartilage in a growing bone. FUNCTION: Allows a long bone to grow in length. EXAMPLES: Growing femur; growing tibia.

Epiphyseal line

WHAT: The bony remnant of the growth plate after lengthwise growth stops. FUNCTION: Marks the former location of the growth plate. EXAMPLES: Adult femur; adult humerus.