Osteoblast

An osteoblast is a bone-forming cell. Osteoblasts work in groups to form new bone and to regulate levels of calcium and minerals deposited in bones. They make up the largest cellular portion of bones. If osteoblasts fail to perform normally or are out of balance with other cell types, bones do not grow or regenerate properly.

Bone Formation

In most animals, bones are necessary to provide structure, support, mobility, and shielding for more delicate portions of the body (such as the skull's protection of the brain). Bones grow and change along with the body. What begins in infancy as cartilage, a firm but flexible connective tissue, converts to solid, rigid bone through the action of osteoblasts. Osteoblasts create a protein called osteoid, which forms new bone. They also support and foster the bone-building process by adjusting the amounts of minerals and calcium that are deposited. This process, which hardens the skeletal cartilage, is known as ossification.

A system of canaliculi, or small canals, connects groups of osteoblasts. These canals transport nutrient-filled fluids to the osteoblasts and help create a framework for new bone. As new osteoid is laid into this framework, calcium and mineral deposits cause it to ossify.

Once a group of osteoblasts has completed the ossification process, some of the osteoblasts become osteocytes, or mature bone cells. These cells rest in small holes embedded in the bone, known as lacunae, and still receive nourishment through the canaliculi. Their new function is to maintain bone health and structure.

Osteoblasts work in conjunction with other cells called osteoclasts. Like osteoblasts, osteoclasts form in bone marrow, but they work to break down or dissolve bone. These two types of cells together are responsible for the ongoing regeneration of bone and allow the body's skeleton to grow over time. The integrated efforts of osteoblasts and osteoclasts also allow bone to rebuild and repair itself when it breaks. This process continues for as long as the body is alive.

Life Cycle

Osteoblasts form from cells called mesenchymal progenitors, which have the ability to become several types of tissue. The cells from which osteoblasts form can become either bone cells or fat cells. A number of factors influence which form the cells take.

Cells that become osteoblasts are shaped like elongated cubes and sit near the surface of bone. They have special receptors for vitamin D, parathyroid hormone, and estrogen, all of which affect the quality of bone made. Osteoblasts communicate with one another through gap junctions found between individual cells. They also communicate with other parts of the body to regulate levels of calcium and minerals that reach the bone. Additionally, they communicate with osteoclasts to initiate the process of dissolving bone.

Osteoblasts that have completed the ossification process experience one of several fates. Some become osteocytes. Others become part of the bone, but unlike osteocytes, they rest on the bone surface rather than in lacunae within the bone. Now flat and pancake-like, these osteoblasts are known as lining cells, and they regulate the amount of calcium that enters and exits the bone. They also protect the bone from chemicals in the blood that can affect bone structure. In addition, lining cells serve as hormone receptors and contribute to the activation of osteoclasts that begins the remodeling process that constantly breaks down and rebuilds bone. Finally, some osteoblasts undergo apoptosis, or "cell suicide," in which a gene within a cell causes the cell to stop functioning and dissolve. Apoptosis is the ultimate fate of both osteoblasts and osteoclasts when they have completed their part in the bone building and rebuilding process.

Diseases

Bones do more than provide structure, support, and protection for the body. Bone marrow is the source of new blood cells, and the bones play a key role in regulating calcium in the body. Therefore, when the tight regulation and communication between osteoblasts and osteoclasts is out of balance, a number of problems can arise. This balance may be altered by changes in diet or activity, age, hormonal changes such as those brought on by menopause, drugs, and disease. When more bone is being made than dissolved and vice versa, a number of health problems can occur. They include the following:

• Osteoporosis, a condition characterized by weak, porous bones that are prone to breaking. It is caused when osteoclasts dissolve bone faster than osteoblasts can make it. The exact reason this happens is not fully understood, but the frequency with which it occurs in women past the age of menopause, when estrogen levels decrease, points to that hormone as a factor.
• Osteopetrosis, a condition that results when bone is made faster than it is broken down. This condition is subdivided into several categories based on clinical symptoms, but it often impedes the throat and lungs, causing breathing problems and other conditions that can be fatal.
• Osteogenesis imperfecta, a condition that results when the collagen protein produced by bone-making cells is defective. This condition causes weak and poorly formed bones. Patients with osteogenesis imperfecta experience deformities and bones that break easily.
• Osteitis deformans, also known as Paget's disease, a condition characterized by bones that are larger than normal and more prone to breakage. This condition is believed to result from a communication issue between osteoblasts and osteoclasts. While its full nature is not understood, scientists believe that osteoclasts that are larger and more active than normal cause bone to break down faster than normal, which triggers a similar increase in bone formation.

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