20.2: Bone Structure
Within the skeletal system, the structure of a bone, or osseous tissue, can be exemplified in a long bone, like the femur, where there are two types of osseous tissue: cortical and cancellous.
Covering the cortical, or compact bone, is a membrane called the periosteum, which contains connective tissue, capillaries, and nerves. The outer, solid layer—found along the diaphysis, the shaft—forms a dense protective shell around the medullary canal—the cavity that stores yellow bone marrow, composed primarily of fat cells. This space is also covered in a thin lining—the endosteum in which bone growth, remodeling, and repair occur.
Within the dense layer of cortical bone are osteons—structural units, arranged in concentric rings called lamellae, that contain osteoblasts—cells critical for bone formation and growth. These cells eventually mature into osteocytes in the hollow space, the lacuna. Through the center of each osteon runs the Haversian canal, which contains more blood and lymphatic vessels, as well as nerve fibers.
Towards the rounded ends of the long bone, the epiphyses is the second type of osseous tissue, known as the cancellous, or spongy, bone. This inner layer is composed of a honeycomb-like network of trabeculae—grouped arrangements that form along the lines of stress points to maximize strength with minimal mass. Between each trabecular pore is red bone marrow, which contains hematopoietic stem cells that form into red and white blood cells and platelets that ultimately enter the circulatory and lymphatic systems.