A giant cell (also known as a multinucleated giant cell, or multinucleate giant cell) is a mass formed by the union of several distinct cells (usually histiocytes), often forming a granuloma. Although there is typically a focus on the pathological aspects of multinucleate giant cells (MGCs), they also play many important physiological roles. Osteoclasts are a type of MGC that are critical for the maintenance, repair, and remodeling of bone and are present normally in a healthy human body. Osteoclasts are frequently classified and discussed separately from other MGCs which are more closely linked with disease.
Non-osteoclast MGCs can arise in response to an infection, such as tuberculosis, herpes, or HIV, or as part of a foreign body reaction. These MGCs are cells of monocyte or macrophage lineage fused together. Similar to their monocyte precursors, they can phagocytose foreign materials. However, their large size and extensive membrane ruffling make them better equipped to clear up larger particles. They utilize activated CR3s to ingest complement-opsonized targets. Non-osteoclast MGCs are also responsible for the clearance of cell debris, which is necessary for tissue remodeling after injuries.
Types include foreign-body giant cells, Langhans giant cells, Touton giant cells, Giant-cell arteritis, and Reed–Sternberg cells.
History
Osteoclasts were discovered in 1873. However, it wasn't until the development of the organ culture in the 1970s that their origin and function could be deduced. Although there was a consensus early on about the physiological function of osteoclasts, theories on their origins were heavily debated. Many believed osteoclasts and osteoblasts came from the same progenitor cell. Because of this, osteoclasts were thought to be derived from cells in connective tissue. Studies that observed that bone resorption could be restored by bone marrow and spleen transplants helped prove osteoclasts' hematopoietic origin.
Other multinucleated giant cell formations can arise from numerous types of bacteria, diseases, and cell formations. Giant cells are also known to develop when infections are present. They were first observed as early as the middle of the last century, but it is not fully understood why these reactions occur. In the process of giant cell formation, monocytes or macrophages fuse together, which could cause multiple problems for the immune system.
Osteoclast
Osteoclasts are the most prominent examples of MGCs and are responsible for the resorption of bones in the body. Like other MGCs, they are formed from the fusion of monocyte/macrophage precursors. However, unlike other MGCs, the fusion pathway they originate from is well elucidated. They also do not ingest foreign materials and instead absorb bone matrix and minerals.
Osteoclasts are typically associated more with healthy physiological functions than they are with pathological states. They function alongside osteoblasts to remodel and maintain the integrity of bones in the body. They also contribute to the creation of the niche necessary for hematopoiesis and negatively regulate T cells. However, while the primary functions of osteoclasts are integral to maintaining a healthy physiological state, they have also been linked to osteoporosis and the formation of bone tumors.
Giant cell arteritis
Giant cell arteritis, also known as temporal arteritis or cranial arteritis, is the most common MGC-linked disease. This type of arteritis causes the arteries in the head, neck, and arm area to swell to abnormal sizes. Although the cause of this disease is not currently known, it appears to be related to polymyalgia rheumatica.
Giant cell arteritis is most prevalent in older individuals, with the rate of disease being seen to increase from age 50. Women are 2-3 times more likely to develop the disease than men.
Northern Europeans have been observed to have a higher incidence of giant cell arteritis compared to southern European, Hispanic, and Asian populations. It has been suggested that this difference may lie in the criteria used to diagnose giant cell arteritis rather than actual disease incidence, in addition to genetic and geographic factors.
Symptoms
Symptoms may include a mild fever, loss of appetite, fatigue, vision loss, and severe headaches. These symptoms are often misinterpreted leading to a delay in treatment. If left untreated, this disease can result in permanent blindness.