All bones comprise an outer layer of compact bone, and an interior made up of spongy bone tissue, also called cancellous or trabecular bone. In long bones, spongy bone tissue is mainly found in the interior of the epiphyses (broad ends of the bone).
Spongy bone is more porous, and less dense compared to compact bone. It is composed of concentric lamellae that are arranged irregularly to form the trabecular network. In some bones, the spaces between trabeculae contain red marrow, where hematopoiesis occurs; in others, they are filled with yellow marrow. Spongy bone is highly vascular, providing nutrients to the osteocytes in the trabeculae via channels called canaliculi. The osteocytes are mature bone cells that regulate bone formation and growth. They are found between the lamellae, in spaces called lacunae.
The trabeculae may appear to be a random network, but each trabecula forms along the lines of stress. The trabecular network develops as the individual learns to walk. It may also dynamically alter when stress lines change, such as when the walking style changes after a leg injury or fracture. The mesh-like network can thus resist compression forces from different directions and provide strength to the bone. This helps transmit forces across joints without damage to the bone. The porous tissue also makes the bones lighter, so muscles can move them more easily.
Part of this text is adapted from Openstax, Anatomy and Physiology 2e, Section 6.3: Bone structure
