Role of yolk sac endodermal cells with special reference to the fetal macrophage differentiation

The morphological and functional characteristics of rat yolk sac endodermal cells, with particular reference to fetal macrophage (M phi) differentiation, were studied in vivo and in vitro using DA rat embryos from 8 to 16 days of gestation. At a very early stage (day 5-6) of gestation, endodermal cells are derived from proximal endoderm as an essential and multipotent organ with various primitive functions. Based on toluidine blue staining properties and ultrastructures, we demonstrated that the endodermal cell layer of 8-16-day yolk sacs consists of two cell types, "clear" cells with clear cytoplasm (10%) and "dark" cells with dark cytoplasm (90%), and hypothesized that the endodermal cell layer is heterogeneous at both the morphological and functional levels. We produced three different monoclonal antibodies (mAbs), designated Mar 1, Mar 2, and Mar 3, that recognize rat M phi populations. Mar 1 binds specifically to the cells constituting the mononuclear phagocyte system (MPS). M phi Mar 3 antigen is a phagocytosis-associated molecule, and Mar 2 antigen is a differentiation antigen of the M phi subset. Application of these mAbs in both in vivo and in vitro studies allowed the functional capability and differentiation of fetal M phis to be assessed. The Mar 3 antigen was expressed first on proximal endodermal cells on day 6 yolk sac and continued to be presented afterward. In vitro culture study demonstrated that the adhesive, phagocytic Mar 3+-Mar1+ M phis differentiate from M phi precursors in the endodermal cell layer after the first 13 days of gestation. Based on these findings, we proposed that clear cells in the endodermal cell layer are derived from precursor dark cells, detach from the layer, move to the mesenchymal stromas, and subsequently migrate to the fetal liver, loose connective tissue, and other intraembryonic tissues, and consequently they differentiate free Mar 1+ M phis during gestation (day 13-15). Thus, the M phi differentiation and the peripherization of the M phis could be almost fully developed during the prenatal period.