Gross Anatomy of Bone

A long bone consists of three primary regions: the diaphysis, a long central shaft of compact bone; the metaphysis, a transitional zone at each end; and the epiphyses, rounded ends filled with spongy bone. The diaphysis contains a hollow medullary cavity lined by the endosteum, while the epiphyses are covered with articular cartilage that reduces friction during joint movement.

Compact bone forms the thick outer walls of the diaphysis and consists of densely packed osteons, the functional units of compact bone. Spongy bone, found in the epiphyses and metaphysis, is porous with spaces filled with red marrow. This structural difference allows spongy bone to be lighter while maintaining strength through its trabecular arrangement.

The periosteum is a double-layered membrane covering most of the long bone except the epiphyseal ends. Its outer layer, composed of dense irregular connective tissue, provides attachment sites for ligaments and tendons. The inner membrane houses osteogenic stem cells involved in bone growth and repair, and contains blood vessels and nerves that nourish the compact bone.

In adults, the medullary cavity of the diaphysis contains lipid-rich yellow marrow, while the spaces within spongy bone in the epiphyses are filled with red marrow. In infants and children, red marrow fills the medullary cavity, but this transitions to yellow marrow as the individual matures, reflecting changes in bone metabolism and hematopoietic activity.

The epiphyseal plate, located at the metaphysis, is a layer of hyaline cartilage that enables bone lengthening during growth. When bone growth ceases in early adulthood, this cartilage is replaced by osseous tissue, becoming the epiphyseal line. Understanding this transition is key to bone formation by endochondral ossification, which drives skeletal development.

Articular cartilage is a thin layer of hyaline cartilage covering the epiphyseal ends where bones meet to form joints. It reduces friction between articulating surfaces and acts as a shock absorber during movement. Unlike the periosteum, articular cartilage is avascular and cannot heal as quickly as other bone tissues.

Flat bones, such as those of the cranium, consist of spongy bone sandwiched between two layers of compact bone, rather than having a distinct diaphysis and epiphyses. This arrangement allows flat bones to protect internal organs while remaining relatively lightweight, demonstrating how bone structure varies across the skeletal system.