Evolution of Xenopus endodermal cells cultured on different extracellular matrix components. Identification of primordial germ cells.

Plated on untreated glass substrate, Xenopus endodermal cells are unable to undergo any morphological or cytological differentiation. Culture on artificial substrates prepared with components of the extracellular matrix, the endodermal cell behavior is entirely different. To identify the primordial germ cells (PGC), we use three coated substrate types: fibronectin, collagen and collagen plus fibronectin. These substrates allow us to distinguish three cell types shortly after explantation. Using fibronectin-coated substrate, most of the cells, after attachment and spreading, form cellular islets which tend to fuse, leading to the formation of a polyhedric cell monolayer. Such fusing is notably reduced on composite substrate (Coll + FN) or on collagen substrate only. Thus it is possible to distinguish the special morphological features exhibited by the rest of the cells. Some of them retain the aspect of endodermal gastrula cells in vitro. Others, elongated or spindle-shaped, possess the characteristics of PGC. Nevertheless, the identification and sampling of the presumed germ cells is easier on COLL + FN-coated substrate. The morphological and cytological characteristics of the elongated cells are similar to those observed during PGC migration through the endodermal mass. According to these results, there is little doubt that these elongated cells are primordial germ cells.