Bone Remodeling and Osteoporosis

Osteoporosis is a disease of the skeletal system that causes the bones to become weak and susceptible to fracture. The most common sites for these types of fractures are in the hip, spine and wrist.

There has been no single cause identified in osteoporosis however it is characterized by a loss of bone density. In order to understand how osteoporosis affects the body it is important to understand how the bones function.

While bones may appear to be solid and unchanging they are actually made up of living tissue that is constantly changing, or being remodeled. Bone remodeling occurs as a result of a need for calcium by the extracellular fluid or as a result of mechanical stress on the bone tissue.

Bone Cell Types

There are three types of cells found in bone tissue. Osteoblasts are bone-forming cells. They are connective tissue cells found at the surface of bone. Osteoblasts can be stimulated to proliferate and differentiate as Osteocytes.

Osteocytes are bone cells. They fabricate type I collagen and other substances that make up the extracellular matrix of the bone. Osteocytes will be found enclosed in bone. Osteoclasts are bone-resorbing cells, meaning they break down the bone.

Osteoclasts are derived from stem cells in the bone marrow. Parathyroid hormones, or PTH stimulates the generation of new osteoclasts, a process known as osteoclastogenesis. PTH can trigger the resorption of bones. PTH indirectly stimulates bone resorption by osteoclasts. PTH receptors are located on osteoblasts that signal bone marrow-derived osteoclast precursors to stimulate their fusion, differentiation and activation.

Osteoclast precursors express a cell-surface receptor known as RANK. In doing so, osteoblasts express a secreted factor called osteoprotegerin. Osteoprotegerin protects bone by preventing bone resorption. The ratio of RANK to Osteoprotegerin determines the amount of bone that is resorbed.

Hormones and Bone Cells

All of the factors that stimulate osteoblasts to form bone are as of yet undetermined. It is known however, that the parathyroid hormone stimulation of the osteoblasts ultimately leads to bone deposition. When this pattern is disrupted, or the ratio is not equal, more bone is resorbed then deposited. In this case more bone is lost then gained which may lead to osteoporosis.

Over time if more bone is resorbed then deposited, the bones lose density and become thinner and weaker. This is what causes fractures to occur from minimal trauma such as occurs with osteoporosis. During youth and adolescence bones are growing and forming meaning that bone mass is being added, or deposited. With remodeling, in healthy bones a small amount is lost but is then usually replaced.

Peak bone mass is reached somewhere in the mid to late 20s. After peak bone mass is reached, everyone starts to lose bone mass, yet everyone does not get osteoporosis. Someone with a low peak bone mass is more likely to develop osteoporosis later in life. Hormones play a large role in this process and are important to overall bone health since they suppress the production of signals that promote osteoclastogenesis.

This is why osteoporosis is found most often in the elderly, and particularly of post-menopausal women, because of the loss of sex hormones estrogen and testosterone.