| Abstract: | This paper describes the relationship between primordial germ cells (p.g.cs) and the substrate over which they migrate in early embryos of the anuran amphibian Xenopus laevis. P.g.cs migrate from the embryonic gut to the dorsal body wall along the dorsal mesentery at the earliest swimming stage. Our earlier papers have described the way in which p.g.cs move in vitro. In this work we have studied the shape and cytoarchitecture of both p.g.cs and the coelomic epithelial cells (c.e.cs) over which they migrate. We have concentrated on three aspects of the morphology of these cells: first the shapes of the c.e.cs and the way that they affect the shapes of the p.g.cs; secondly the presence of adhesion plaques between the two types of cell; and thirdly the arrangement of cytoskeleton elements. The results show that c.e.cs in the dorsal mesentery are orientated cranio-caudally while those on the dorsal body wall and at the junction with the mesentery are arranged transversely, at 90 degrees to the cranio-caudal plane. P.g.cs are found in both elongated and rounded state. Where elongated, they are always in the same plane as the c.e.cs with which they are associated. The implications of this are discussed. Adhesion plaques between p.g.cs and c.e.cs are shown both by disaggregation studies and transmission electron microscope studies. Plaques are associated with the well defined microfilamentous cytoskeleton of c.e.cs, but only with a sparse array of filaments in p.g.cs. The only parts of p.g.cs where filaments are regularly found are their filopodia, which are generally seen on elongated p.g.cs in longitudinal section. We suggest on the basis of this work that p.g.cs have a dispersed cytoskeleton except during filopod extension, that they move by forming direct adhesion plaques with c.e.cs, and that c.e.cs provide a firm orientated support and possible guide to p.g.c. movement. |
