Epithelioid cells are cells of the mononuclear phagocyte system found in certain
granulomas mainly associated with intense immunological activity. These cells show little phagocytic activity. In certain experimental
granulomas such as those produced in guinea pigs sensitive to
zirconium, and at sites of intense inflammatory reaction in man, they may contain varying amounts of rough endoplasmic reticulum ("secretory" epithelioid cells). In other situations such as
tuberculoid leprosy and in some cases of
sarcoidosis they may have the appearance of activated macrophages or take on a multivesicular appearance ("vesicular" epithelioid cells). It is suggested that "vesicular epithelioid cells could develop from "secretory" epithelioid cells by a process of degeneration. In studies comparing
granulomas induced in lymph nodes draining the site of
intradermal injection of mycobacteria, epithelioid cell
granulomas were produced with
BCG vaccine, whereas, the
granulomas induced by Mycobacterium leprae contained undifferentiated macrophages that contained phagocytosed organisms. The BCG
granulomas were in addition characterised by fibroblast infiltration, the presence of
collagen and resolution by
fibrosis. M. leprae
granulomas showed little evidence of fibroblastic activity. Biochemical studies confirmed that BCG
granulomas formed new
collagen in vitro, whereas this did not take place with M. leprae
granulomas. It is suggested that epithelioid cells could play an important role in
fibrosis possibly by the secretion of a fibroblast activating factor.