Skeletal System

Since most bone begins as cartilage, it must be converted to bone through a process called ossification. The key players in bone development are cartilage cells (chondrocytes), bone precursor cells (osteoprogenitor cells), bone deposition cells (osteoblasts), bone resorption cells (osteoclasts) and mature bone cells (osteocytes).

During ossification, blood vessels invade the cartilage and transport osteoprogenitor cells to a region called the center of ossification. At this site, the cartilage cells die, leaving behind small cavities. Osteoblast cells form from the progenitor cells and begin depositing bone tissue, spreading out from the center. Through this process, both the spongy textured cancellous bone and the smooth outer compact bone forms. Two types of bone marrow, red and yellow, occupy the spaces in cancellous bone. Red marrow produces red blood cells while yellow marrow stores fat in addition to producing blood cells. Eventually, in compact bone, osteoblast cells become trapped in their bony cavities, called lacunae, and become osteocytes. Neighboring osteocytes form connections with each other and thus are able to transfer materials between cells. The osteocytes are part of a larger system called the Haversian system. These systems are like long tubes, squeezed tightly together in compact bone. Blood vessel, lymph vessels and nerves run through the center of the tube, called the Haversian canal, and are surrounded by layers of bone, called lamellae, which house the osteocytes. Blood vessels are connected to each other by lateral canals called Volkmann's canals. Blood vessels are also found in spongy bone, without the Haversian system. A protective membrane called the periosteum surrounds all bones.

Bone development is a complex process, but it is only half the story. Bones must grow, and they do so via a process called remodeling. Remodeling involves resorption of existing bone inside the bone (enlarging the marrow cavities) and deposition of new bone on the exterior. The resorptive cells are the osteoclasts and osteoblast cells lay down the new bone material. As remodeling progresses in long bones, a new center of ossification develops, this one at the swollen ends of the bone, called the epiphysis. A thin layer of cartilage called the epiphyseal plate separates the epiphysis from the shaft and is the site of bone deposition. When growth is complete, this cartilage plate disappears, so that the only cartilage remaining is that which lines the joints, called hyaline cartilage. Remodeling does not end when growth ends. Osteocytes, responding to the body's need for calcium, resorb bone in adults to maintain a calcium balance. This process can sometimes have detrimental affects on the skeleton, especially in pregnant women and women who bear many children.