Bone Diseases

BONE IS THE main weight—bearing tissue of the body and functions to withstand mechanical forces several times the weight of the body. Despite the numerous shapes and sizes of the bones in the body, bone tissue is structurally and microscopically similar throughout. Bone tissue exists in two general forms: Cortical bone composes the shell of many of the long bones and has low porosity and high density and strength. Cancellous (or tra—becular) bone is found at the ends of long bones or in low—weight bearing areas (such as the lower jaw) and is composed of microscopic interconnecting bony trabecula, giving macroscopically high porosity and low density.

Although bone tissue is generally thought of as a rigid structure, it contains both elastic and stiff components. The bone extracellular matrix is approximately 65 percent inorganic, mainly calcium and phosphate in the form of hydroxyapatite, and 35 percent organic matrix (osteoid), mainly collagen, which is a ropelike fiber. This general composition gives bone marked rigidity while retaining some elasticity, with the collagen fibers of the organic matrix providing high tensile strength to resist pulling forces and the inorganic mineral providing high compressive strength to resist crushing forces.

Contrary to the popular belief that bones are inert structures, bone tissue is remarkably active. Bone is constantly being remodeled to maintain optimal structure, which includes two parallel processes. Bone résorption, or breaking down of bone tissue, is carried out by the osteoclasts. Bone formation is carried out by the osteoblasts, which lay down new bone to replace the old. A small number of osteoblasts become entrenched in their own calcified matrix and become stationary osteocytes, which have recently been identified as playing a role in initiating and coordinating the remodeling process. Approximately 99 percent of calcium is also stored in the bones; this calcium is constantly being removed for use throughout the body, to be replaced later.

Within the hollow interior of the long bones and among the trabecula of cancellous bone lies the bone marrow. Bone marrow is separated into red and yellow marrow. Red marrow is the myeloid, or blood, portion of the marrow and is where most new red blood cells are produced. Yellow marrow is made mostly of fat cells but also contains multiple other types of stromal cells, including fibro—blasts, osteoblasts, and blood vessel cells. Some of these cells play important roles in maintaining specific microenvironments within the marrow, such as the hematopoietic stem cell compartment, which provides the multipotent cells that form all types of blood cells. Bone marrow stromal cells, or mesen—chymal stem cells, have also been isolated that have the ability to differentiate into bone, fat, and cartilage lineages. Cells with a similar differentiation ability, called pericytes, are localized in the bone marrow immediately adjacent to blood vessels.