Interdigitating cells and macrophages in the acute involuting rat thymus. An electron-microscopic study on phagocytic activity and population development.

Phagocytic activity and population development of medullary interdigitating cells (IDC) and cortical macrophages have been studied in the rat thymus after irradiation-induced thymocyte necrosis. IDC clearly demonstrate phagocytic activity particularly in the 16 h stage after irradiation. At this stage the number of necrotic thymocytes is maximal and the total number of phagocytic cells is insufficient to phagocytize all necrotic material. IDC increase in number slightly and are predominantly phagocytizing in the corticomedullary region (CMR), where they appear to develop from macrophage-like cells. The results indicate that medullary IDC can be phagocytic but have a different developmental pathway than cortical macrophages. Cortical macrophages greatly increase in number and acquire the appearance of tingible body macrophages by phagocytizing many necrotic thymocytes. they seem to develop from monocytes that normally enter the thymus at the CMR. During the acute involution macrophages probably also enter the cortex via the connective tissue capsule. It is suggested that thymus medullary IDC probably belong to the mononuclear phagocytes, as do the cortical macrophages. In the specific medullary environment IDC gradually develop their characteristic ultrastructure for an apparently other than phagocytic function. The similarities between IDC and epidermal Langerhans cells are discussed.