Stage-specific tissue and cell interactions play key roles in mouse germ cell specification

Primordial germ cells (PGCs) in mice have been recognized histologically as alkaline phosphatase (AP) activity-positive cells at 7.2 days post coitum (dpc) in the extra-embryonic mesoderm, However, mechanisms regulating PGC f ormation are unknown, and an appropriate in vitro system to study the mecha nisms has not been established. Therefore, we have developed a primary cult ure of explanted embryos at pre- and early-streak stages, and have studied roles of cell and/or tissue interactions in PGC formation. The emergence of PGCs from 5.5 dpc epiblasts was observed only when they were co-cultured w ith extra-embryonic ectoderm, which may induce the conditions required for PGC formation within epiblasts, From 6.0 dpc onwards, PGCs emerged from who le epiblasts as did a fragment of proximal epiblast that corresponds to the area containing presumptive PGC precursors without neighboring extra-embry onic ectoderm and visceral endoderm, Dissociated epiblasts at these stages, however, did not give rise to PGCs, indicating that interactions among a c luster of a specific number of proximal epiblast cells is needed for PGC di fferentiation. In contrast, we observed that dissociated epiblast cells fro m a 6.5-b (6.5+15-16 hours) to 6.75 dpc embryo that had undergone gastrulat ion gave rise to PGCs, Our results demonstrate that stage-dependent tissue and cell interactions play key roles in PGC determination.