What are Osteoclasts?

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Having a broken bone seems to be the outcome of a major accident and most often immediate treatment is recommended. But how many of us know that bones undergo a continual process of breaking and redevelopment as long as we live? Osteoclasts are bone cells that help in breaking down bone tissue and the organic part of the bone matrix in what is called as bone resorption.

What is an Osteoclast?

Osteoclasts are the specialized cells assisting in the function of bone deformation. These are multi-nucleated giant cells that differentiate from cells of haemopoietic origin and are significant for processes like tooth eruption, regulating the amount of bone tissue and dead bone removal. Osteoclasts are primarily formed from a group of the circulating mononuclear cells that come from the blood to the bone surface  and undergo differentiation followed by fusion to form multinucleated You do not have access to view this node.

What are the theories on the origin of Osteoclasts?

Osteoclasts were discovered by Rudolph Albert Von Kolliker in the year 1873. Kolliker believed that osteoclasts are formed by the fusion of precursor cells which were osteoblasts . The term osteoclast, comes from 'osteon' meaning You do not have access to view this node. There are other theories such as the connective tissue theory, biphyletic theory etc.

Connective tissue theory: It was the most popular out of all theories that emerged in the span 1949-1970. Connective tissue theory stated that osteoclasts and osteoblasts belong to the same lineage and osteoblasts fuse forming osteoclasts. Also at times osteoclasts fuse to form osteoblasts and then You do not have access to view this node.

Biphyletic theory: This theory was proposed during the 1970's. It stated that osteoclasts and osteoblasts are of different lineage.

Monocyte phagocytic theory: Monocyte phagocytic theory, proposed in early 1980 stated that the precursor of osteoclasts is the monocyte phagocytic system.

Where are Osteoclasts found in the body?

Osteoblasts are formed by the process of conjoining of numerous cells which are created by the bone marrow. The osteoclasts travel in the circulatory system. Hence the cells contain multiple nuclei. On an average an osteoclast can have 5-20 nuclei but even 200 nuclei are possible. Osteoclasts are found in notches/ indentations of bone surfaces called Howship's lacunae. The Howship's Lacuane are formed by the digestion of the underlying bone when osteoclastic enzymes such as enzyme acid phosphatase act on them. Such cavities or trenches are found virtually on every bone of the skeletal system. When there is an excess activity of osteoclasts, it leads to osteoporosis, a disease in which bone develops pores as too much of the calcium, which is  the primary mineral in the You do not have access to view this node, is removed.

Why are Osteoclasts significant for the body?

An Osteoclast is one of the two primary cell types (the other being Osteoblasts) which are used for the formation and maintenance of You do not have access to view this node. It aids the body in various ways for eg.

• Bone removal by osteoclasts is a prerequisite to repair the microscopic damage and any alterations in the bone shape during growth and tooth formation. Deficit of osteoblasts creates a condition called osteopetrosis where long bones are unable to be remodelled.
• Osteoclast mediated bone resorption helps maintain Calcium levels in the blood by releasing calcium stored in the You do not have access to view this node.

How are Osteoclasts and Osteoblasts interrelated?

While osteoblasts are bone creating You do not have access to view this node, osteoclasts are the cells responsible for breaking down and absorption of bone tissue back into the body. In order to maintain proper functioning of the skeletal system, osteoclasts should work in a synchronized manner with the osteoblasts.  A close contact between osteoclast precursors and osteoblastic cells is required in order that differentiation to fully functional osteoclasts occurs. A molecule called ODF (osteoclast differentiation factor) is located on the surface of osteoblasts. It binds to receptors on the surface of osteoclast precursor cells and induces their progression to osteoclasts.

What is the process of formation of osteoclasts in the body ?

Osteoclasts are formed by the fusion of blood monocytes.These tend to express high levels of tartrate resistant acid phosphatase(TRAP). Osteoclasts have the required intracellular machinery for bone resorption. This process takes place when the cells form a tight sealing zone by attachment of the cell membrane against the bone matrix, creating a bone-resorbing compartment. The steps involved in the formation are:

• formation of pre-osteoclast in presence of transcription factor PU.1
• converting the mononucleated pre-osteoclast to an active multinucleated osteoclast in presence of MCF(macrophage colony stimulating factor) and ODF(osteoclast differentiation factor).

What is the mechanism of bone resorption process?

As soon as the resorption process starts, the osteoclastic cell takes control of  the bone which is to be resorbed. Bone resorption promoters act on osteoblastic cells. This follows signalling to osteoclast precursors to differentiate into mature osteoclasts. The cell secrete acid in order to dissolve the bone mineral and enzymes to digest proteins in the bone matrix. The enzymes disintegrate the tissue surface into ions of calcium and phosphorous. These ions are then passed by the osteoclastic cell through the external membrane and further into the circulatory system. The duration of resorption lasts upto a  maximum of  three weeks. In the bloodstream this process is hormonally controlled. In the activated form, osteoclasts shift to areas of microfracture by a process called chemotaxis.First of all osteocytes secrete messages due to which stromal cells are activated and the pre-osteoblasts are produced. These differentiate into mature osteoclasts. The mature osteoclasts resorb bone by forming a space on the matrix surface and secreting hydrogen ions and cathepsin into the space. As the basic multicellular unit(which dissolves an area of the bone surface and then fills it with new You do not have access to view this node) wanders, new osteoclasts are continuously activated and then resorption process begins.

What is the cause of osteoporosis and how can it be prevented?

Osteoporosis that is closely associated with hormonal changes is an acute condition, which happens when the rate of bone loss exceeds the rebuilding rate of bone tissue. The hormone production rate is less than required with the result that osteoclasts resorb more tissue as compared to those rebuilt by the osteoblasts. This causes formation of very thin bones which are prone to fractures even if they are subjected to the weights or forces of least magnitude. Osteoporosis can be prevented by:

• intake of calcium rich food
• avoiding steroids, alcoholic drinks and tobacco
• lifting weights as physical exercise

Do osteoclasts deform all the  bone tissue?

Although the primary function of osteoclasts is bone resorption they do not deform all the bone You do not have access to view this node. Bones that have ossified or mineralised forming calcium and phosphorous are acted upon by the osteoclasts and the enzymes produced. Non-mineralised bone and cartilage formations like the nose are unaffected by osteoclasts.

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