Differentiation and maturation of macrophages into interdigitating cells and their multicellular complex formation in the fetal and postnatal rat thymus.

Three mouse anti-rat macrophage monoclonal antibodies, TRPM-1, TRPM-2, and TRPM-3, as well as anti-rat Ia monoclonal antibody, were used to study the emergence, differentiation, and maturation of macrophages in the fetal and postnatal rat thymus immunohistochemically and immunoelectron microscopically. At 14 days of gestation, primitive/fetal macrophages entered the thymic primordium and showed Ia expression, where afterwards the epithelial cells also expressed Ia antigens prominently at 15 days of gestation. After 16 days of gestation, differentiation of a subpopulation of primitive/fetal macrophages into interdigitating cells (IDCs) is suggested. From 19 days of gestation, TRPM-1-positive dendritic cells including IDCs started forming multicellular complexes with thymocytes and the epithelial cells also formed similar complexes with thymocytes. One day after birth, TRPM-1 positive IDC-thymocyte complexes distributed throughout the thymic medulla. The number of TRPM-1- and Ia-positive IDCs increased by day, and Langerhans cells (LCs) appeared in the thymic medulla within a few days after birth. By two weeks after birth, the distribution pattern of Ia- and TRPM-1-positive cells became similar to that of adult rats. In ontogeny, intimate cell membrane appositions were frequently observed between thymocytes and Ia-positive epithelial cells or IDCs in the thymic multicellular complexes. These complexes were discriminated into two types; epithelial cell-thymocyte complexes and IDC- or LC-thymocyte ones. In vitro, two types of the thymic nurse cells (TNCs) were identified: epithelial cells and IDCs or LCs. Besides epithelial cells, IDCs or macrophages formed rosettes with thymocytes. These TNCs and rosettes in vitro seem to correspond to the thymic multicellular complexes in vivo.