Contact relations and guidance of primordial germ cells on their migratory route in embryos ofXenopus laevis

Abstract

The relationship between primordial germ cells (PGC) and the substrate over which they migrate in early embryos of the anuran X. laevis is described. PGC migrate from the embryonic gut to the dorsal body wall along the dorsal mesentery at the earliest swimming state. Earlier papers have described the way in which PGC move in vitro. The shape and cytoarchitecture of PGC and the coelomic epithelial cells (CEC) over which they migrate were studied. The shapes of the CEC and the way that they affect the shapes of the PGC; the presence of adhesion plaques between the 2 types of cell; and the arrangement of cytoskeletal elements were dealt with in particular. The results show that CEC 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.degree. to the cranio-caudal plane. PGC are found in both elongated and rounded state. Where elongated, they are always in the same plane as the CEC with which they are associated. The implications of this are discussed. Adhesion plaques between PGC and CEC are shown both by disaggregation studies and transmission electron microscope studies. Plaques are associated with the well defined microfilamentous cytoskeleton of CEC, but only with a sparse array of filaments in PGC. The only PGC areas where filaments are regularly found are their filopodia, which are generally seen on elongated PGC in longitudinal section. Evidently, PGC have a dispersed cytoskeleton except during filopod extension, they move by forming direct adhesion plaques with CEC and the CEC provide a firm support and possibly guide PGC movement.