Primordial germ cells in the mouse embryo during gastrulation

TLDR

The cluster of cells lies just posterior to the definitive primitive streak in the extraembryonic mesoderm, separated from the embryo by the amniotic fold. The authors contend that these cells are primordial germ cells, identified at a stage earlier than previously reported. Whole‑mount analysis shows a small cluster of ALP‑positive cells appearing at 7–7¼ dpc, expanding to 50–80 cells by the end of gastrulation, then migrating into the mesoderm and endoderm, with removal of the cluster eliminating PGCs after 48 h culture, confirming the cluster as the earliest primordial germ cell lineage.

Abstract

With the aid of a whole-mount technique, we have detected a small cluster of alkaline phosphatase (ALP)-positive cells in whole mounts of mid-primitive-streak-stage embryos, 7-7 1/4 days post coitum (dpc). Within the cluster, about 8 cells contain a small cytoplasmic spot, intensely stained for ALP activity and possibly associated with an active Golgi complex. The cluster lies just posterior to the definitive primitive streak in the extraembryonic mesoderm, separated from the embryo by the amniotic fold. Towards the end of gastrulation, the number of cells containing the ALP-positive spot rises to between 50 and 80. Thereafter the number of cells in the extraembryonic cluster declines, and similar cells start to be seen in the mesoderm of the primitive streak and then in the endoderm. At 8 dpc, about 125 ALP-stained cells are found, mainly in the hindgut endoderm and also at the base of the allantois, their appearance and location at this stage agreeing closely with previous reports on primordial germ cells (PGCs). Embryos from which the cluster area has been removed at the 7-day stage are devoid of PGCs after culture for 48 h, whereas the excised tissue is rich in PGCs. We argue that the cells in the cluster are indeed primordial germ cells, at a stage significantly earlier than any reported previously. This would indicate that the PGC lineage in the mouse is set aside at least as early as 7 dpc, possibly as one of the first 'mesodermal' cell types to emerge, and that its differentiation, as expressed by ALP activity, is gradual.