Epithelioid cell
According to a common point of view epithelioid cells (also called epithelioid histiocytes) are derivatives of activated macrophages resembling epithelial cells. Structure and function. Structurally, epithelioid cells (when examined by light microscopy after stained with hematoxylin and eosin), are elongated, with finely granular, pale eosinophilic (pink) cytoplasm, and central, ovoid nuclei (oval or elongate), which are less dense than that of a lymphocyte. They have indistinct shape and often appear to merge into one another, forming aggregates known as giant cells. When examined by transmission electron microscopy in epithelioid cells in the field of Golgi lamellar complex are  taped not only zonated, but also sleek vesicles with dense center, and also great many (more than 100) large granulas with diameters up to 340 nm and with finegranular matrix more light than in macrophage granulas, sometimes with perigranular halo. “The most prominent feature of these cells is the enormous Golgi area; up to 6 individual stacks of Golgi cisternae may be present as well as a few bristle-coated and numerous smooth vesicles”. Epithelioid cells have tightly interdigitated cell membranes in zipper-like arrays that link adjacent cells. This cells are central in the formation of granulomas, which are  associated with many serious diseases. In granulomas, epithelioid cells perform the functions of delimiting. Peculiarities of the cytoskeleton. It is shown that the epithelioid cell cytoskeleton formed by filaments differs significantly from the macrophage cytoskeleton. A large increase in the number of filaments occurs in these cells, where filaments (90 to 100 A) surround the cytocentrum as a distinctive annular bundle often branching into the cytoplasm. Due to such cytoskeleton contiguous epithelioid cells display elaborate cytoplasmic interdigitation. By using the quick=freeze and freeze-substitution methods (prompt freezing, penetrating etching and freeze-substitution) it has been shown that the  organizations three-dimensional metastructure cytoskeleton of the epithelioid cells, formed in the focus of granulomatous inflammation, more compatible to cytoskeleton characteristic of typical epithelial cell than to cytoskeleton of active and movable macrophages. It is exhibited that the dense webs of intermediate filaments, bound with cores, mitochondrions and other organelles, are supervised everywhere in cytoplasm of epithelioid cells. Some fascicles of actinic filaments were posed in filopodiums below than membranes of the cells. Exact interdigital tripling of membranes of cells between interfacing epithelioid cells were clearly demonstrated. Interdigital filopodiums were identified. The characteristic indication of epithelioid cells is their aggregation  with formation tight interdigital triplings as a fastener "lightning", which, apparently, can have the important for the formation of a dense zone of delimitation of the body from the pathogen during the formation of epithelioid cell granulomas. Immunological phenotype and immunological markers. When using antibodies to the RFD9, RFD7 and HLA-DR antigens, it was found that all epithelioid cells have an immunological phenotype RFD9+/RFD7-/HLA-DR+. A series of monoclonal antibodies IHY-1, IHY-2, IHY-3 was obtained, which can be used to accurately identify epithelial cells formed in etiologically different forms of granulomatous inflammation. IHY-1 antibody reacts with epithelioid cells in  sarcoid granulomas as well as with epithelioid cells of various granulomatous diseases including tuberculosis. IHY-2 and IHY-3 l antibody, monoclonal antibody, react with epithelioid cells in sarcoidosis but not in tuberculosis. Clinical significance. Epithelioid cells are an essential characteristic of epithelioid cell granulomas. Epithelioid cell granuloma can be defined as specifically and structurally organized collection of epithelioid cells, macrophages, lymphocytes and dendritic cells. Foreign-body granulomas may be considered an organized collection of macrophages, including mere collections of giant cells surrounding inert substances like suture material – the so-called "non-immune granulomas." Granuloma formation is associated with pathogens that have  learned to evade the host immune system by various means like resisting phagocytosis and killing within the macrophages. Indigestibility of matter by macrophages is a common feature of granulomatous inflammation. Granulomas try to wall off these organisms and prevent their further growth and spread. Historically widespread and destructive diseases such as tuberculosis, leprosy and syphilis are granulomatous conditions. Granuloma formation is also the feature of many more contemporary conditions, like fungal infections, sarcoidosis and Crohn's disease. History of scientific research. The first mention of epithelioid cells as a specific cell form occurred in the 19th-century in works of  Koch R and Cornil J, who believed the leukocytes to be the originators of the epithelioid cells of the tuberculosis. In experiments on rabbits, Yersin A (1888) and Borrel A (1893) showed that epithelioid cells are formed from blood mononuclear leukocytes. The main patterns of epithelioid cells formation were first described in the first half of the 20th century by Lewis M (1925). This researcher showed that blood monocytes in cell cultures of mixed blood leukocytes of Avian (taken from the adult fowl as well as from embryos of various ages), mice and humans, when cultured in vitro, are transformed  into typical macrophages and epithelioid cells, followed by the formation of giant multinucleated cells. The formation of epithelioid type cells was noted by Lewis M on the 2nd-3rd day of the cultivation of leukocytes. Later in a study of a similar plan, Jerry S and Weiss L (1966), when using cultures of mixed blood leukocytes of chicken (separated from cardiac blood of Rhode Island Red) and electron microscopy, showed that the transformation of monocytes of chicken in epithelioid cells begins in culture on 3–4 days and ends on 5–6 days. Since all previous researchers have indicated that epithelioid cells are  formed from monocytes, and monocytes and macrophages were combined into a single mononuclear phagocyte system, Van Furth et al. (1972), referring to the work of Sutton J and Weiss L (1966), formally attributed epithelioid cells to the mononuclear phagocyte system. However, they did not specify exactly from which cells of the mononuclear phagocyte system epithelioid cells originate from. At the same time, they made a very cautious conclusion that «The epithelioid cells occurring in these lesions also arise from monocytes or macrophages». Adams D (1976), believing that epithelioid cells are the final stage of cell differentiation of the cells of  the mononuclear phagocyte system, formulated the concept of cytomorphogeesis of epithelioid cells according to which epithelioid cells are regarded as a derivative of an activated macrophages (which is still held by most researchers at the present time). Не based on the assumption that "stimulation of macrophages mature further into immature epithelioid cells and ultimatelv in Mature". De Vos et al. (1990) obtained the data that allowed them to suggest that in granulomatous inflammation foci, in granulomatous lymphadenitis, epithelioid cells be formed not from differentiated macrophages, but from so-called plasmacytoid monocytes (have similarities with plasmacytes). This is further supported by  the ultrastructural similarities between plasmacytoid monocytes and epithelioid cells. The present ultrastructural and immunoelectron microscopic study of epithelioid cell granulomas provided further arguments in favor of this hypothesis. Arkhipov S (1997, 2012)