Evidence for substrate guidance of primordial germ cells

Primordial germ cells (PGCs) have been removed from their normal migratory route in early embryos of Xenopus laevis, and their behaviour studied in vitro. They adhere to, and move over the upper surface of, layers of outgrowing cells from expiants of adult Xenopus mesentery. They move by the extrusion of single filopodia, elongation, forward streaming of the yolky cytoplasm and retraction of their trailing ends. When the underlying cells are polarized in one direction only, PGCs always elongate and move along the same direction. Furthermore, when PGCs elongate and move over less obviously polarized cells, they always do so in the direction of ‘stress fibres’ (actin bundles) in the underlying cells. A substrate-guidance hypothesis for PGC migration is only tenable if there is some orientation in their natural substrate in vivo. Using the scanning electron microscope, we demonstrate that the coelpmic lining cells, beneath which PGCs migrate up the dorsal mesentery of the gut, are orientated in the direction of travel. Furthermore, this orientation changes at the time of gonadal ridge formation. This raises the intriguing possibility that PGCs are guided for at least part of their migration in Xenopus laevis embryos by a substrate-guidance mechanism.