ABSTRACT

The morphological and functional characteristics of rat yolk sac-endodermal  cells, with particular reference to fetal macrophages (Mø) differentiation, were studied in vivo and in vitro using the yolk sac of DA rat embryos from 6 to 20 days of gestation. By using three different monoclonal antibodies (mAbs) Mar 1, Mar 2 and Mar 3, which recognize preferentially fetal M ø populations, we demonstrated immunohistochemically that fetal M ø possessing a number of functional capability differentiate from the endodermal layer of yolk sac of the 12.5~13 days of gestation. The Mar 3 antigen which is one of phagocytosis-associated molecules was expressed first on these proximal endodermal cells on day 6 yolk sac. Moreover, the Mar 1 antigen which is one of M ø specific molecules was expressed on day 13.  Mar 1 and Mar 3 antigens continued to be presented afterward.

In vitro culture study revealed that the adhesive and phagocytic Mar 3⁺ · Mar 1⁺  M ø   differentiate from M ø precursors in the endodermal layer from the first 12.5~13 days of gestation, indicating that endodermal layer of early stage of yolk sac may possess M ø precursor potential in vivo.

Based on these findings , we will propose that the M ø differentiation from yolk sac-endodermal layer and the peripherization of the M ø into both the connective tissue and the liver of embryo could be almost fully developed during the prenatal period, and also two kinds of M ø subpools, low mobilizable and high mobilizable, are present under healthy steady-stage conditions and have a close relationship not only in the stage of the prenatal period, but also in the postnatal period.