T-zone histiocytes with S100 protein. Development and distribution in human fetuses.

Histiocytic cells with S100 protein compose a cell lineage independent of the monocyte-macrophage system. Langerhans cells and indeterminate cells in the skin and oral mucosa, interdigitating cells in the T-zone of the lymph node, and other lymphoid tissues belong to this cell lineage. In addition to these cells, small S100+ cells showed morphological transition to large histiocytes. In human fetuses, a large number of S100+ lysozyme- NCA- cells first appeared in the thymic medulla by the end of the third month of gestation, and rapidly disseminated to the various lymphoid organs in accordance with the spread of T-lymphocytes during the fourth month of gestation. S100+ small cells were more frequent than large cells and showed more rapid dissemination in the early stage. S100- lysozyme+ NCA+ immature macrophages appeared in the liver, spleen, lymph node anlage, and other tissues at the second month of gestation, and their distribution was completely different from S100+ histiocytes. Fetal development of T-zone histiocytes with S100 protein supported the hypothesis that there are two histiocytic cell lines; one is the monocyte-macrophage system, another is the S100+ T-zone histiocyte system.