Osteoblasts, osteocytes, and osteoclasts are all types of cells closely associated with bones. Here’s more about these cells, what functions they perform, and how they differ from each other.
Did You Know?
The prefix ‘osteo’ comes from the Greek word for bone. It is used in many bone-related words apart from the subjects of this write-up; for instance, osteoporosis and osteosarcoma are both conditions related to the bone.
Our bones are usually considered little more than lifeless pillars on which to base our more important systems. However, contrary to popular misconception, bones are as alive as the muscles and organs that cling on to them for support, and are buzzing hives of living cells.
Osteoblasts, osteocytes, and osteoclasts are interrelated types of cells found in the bone. Named after the Greek word for ‘bone’, these cells perform specific functions within the bone. Here’s a summary of the roles the three perform.
Osteoblasts, Osteocytes, and Osteoclasts
Osteocytes are cells that form the bones themselves, osteoblasts are responsible for the formation of new osteocytes, whereas osteoclasts are responsible for the resorption of old bone matter. Thus, between them, the three types of bone cells regulate the formation, sustenance, and decay of bones. It is a constant process and is carried out for an individual’s entire lifetime. A disorder related to either one of the three is disastrous for bone health, since all three, even the osteoclasts, are vital.
These three are part of an osteon, which is a functional unit of compact bone matter. Bones have two types of tissues: the hard, strong exterior and the spongy interior marrow. Osteocytes, osteoblasts, and osteoclasts are found on the outer side of bones.
Osteoblasts are formed from stem cells known as mesenchymal cells. These stem cells can also form cartilage tissues, as well as numerous other types of tissue. Osteoblasts are one of the end products of mesenchymal stem cells.
They release collagen and some bone-forming proteins. This is part of the bone matrix, known as organic matrix. Calcium and phosphorus compounds, such as hydroxyapatite, are then added to this organic matter to form a firm but flexible bone matrix. An osteoblast that becomes covered in its own matrix secretion is called an osteocyte.
Osteocytes are formed from osteoblasts, and become part of the bone (and, as discussed above, ‘become’ osteocytes) when they mature.
They send out long tendrils (as seen in the figure) which connect numerous osteocytes to each other. They produce bone matrix, including collagen and calcium/phosphorus compounds, that eventually covers them. The space occupied by each osteocyte and its matrix is known as a lacuna. Osteocytes maintain bone mass, and are also speculated to act as the command centers of the bones when experiencing stress, using their connection with other osteocytes. The osteocytes direct osteoclasts to the site of the damage, hastening healing.
While osteoblasts and osteocytes have the same source, and are, in fact, different stages of the same cells, osteoclasts are derived from cells in the bone marrow. Osteoclasts perform the job of breaking down the composite material in bones, with the help of an acid and collagenase proteins. The calcium in the bones acted on by osteoclasts is then sent back into the bloodstream.
Osteoclast production is regulated mainly by the thyroid gland. They are produced when more blood calcium is needed, and suppressed when there is no deficiency of calcium in the body. They are also vital in repairing mechanical breaks (fractures) to the bone.
These cells form the cycle of bone remodeling, as illustrated in the diagram below:
The function of all three of these cells is crucial to healthy bones. One may think osteoclasts are actually harmful for the body, but bones aren’t the only sites in the body where calcium is needed, and osteoclasts do the sometimes-vital task of making calcium available. If sufficient calcium intake is maintained and weight-bearing activities are performed regularly, osteoclasts are suppressed and the number of osteoblasts increases, resulting in a greater bone density.