Primordial germ cells (PGCs) are the founder cell population of the gametes which form during the sexually mature stage of the life cycle. In the mouse, they arise early in embryogenesis, first becoming visible in the extraembryonic mesoderm, posterior to the primitive streak, at 7.5 days post coitum (d.p.c.). They subsequently become incorporated into the epithelium of the hind gut, from which they emigrate (9.5 d.p.c.) and move first into the dorsal mesentery (10.5 d.p.c.), and then into the genital ridges that lie on the dorsal body wall (11.5 d.p.c.). We have used confocal microscopy to study PGCs stained with an antibody that reacts with a carbohydrate antigen (Stage-Specific Embryonic Antigen-1, SSEA-1) carried on the PGC surface. This allows the study of the whole PGC surface, at different stages of their migration.

The appearance of PGCs in tissue sections has given rise to the conventional view that they migrate as individuals, each arriving in turn at the genital ridge. In this paper, we show that PGCs leave the hind gut independently, but then extend long (up to 40 μm) processes, with which they link up to each other to form extensive networks. During the 10.5 – 11.5 d.p.c. period, these networks of PGCs aggregate into groups of tightly apposed cells in the genital ridges. As this occurs, their processes are lost, and their appearance suggests they are now non-motile. Furthermore, we find that PGCs taken from the dorsal mesentery at 10.5 d.p.c. perform the same sequence of movements in culture. At first they are actively locomotory. They become linked to other PGCs via long processes and form clusters of non-motile cells. This aggregation together into closely apposed masses may be an important component of PGC migration from the gut into the genital ridges and would also allow signalling interactions between PGCs.

We show also that the first PGCs to emigrate from the hind gut, between 9 and 9.5 d.p.c., do so directly into the area where the genital ridges will form. This suggests that an adhesive interaction between these ‘pioneer germ cells’ and the target tissue may play a role in the localisation of PGCs into the genital ridges as they aggregate.