Signaling for germ cells

  1. Anne McLaren
  1. Wellcome/Cancer Research Campaign Institute, Cambridge, CB2 1QR, UK

This extract was created in the absence of an abstract.

Shakespeare advises us that some men are born great, some achieve greatness, and some have greatness thrust upon them. So it is with germ cells. In Drosophila, the very first cells to be formed in the embryo, the pole cells, are germ cells whose descendants have no fate other than to give rise to gametes. InCaenorhabditis and in zebra fish, germ cell determinants present in the egg are asymmetrically segregated at each subsequent cell division until a definitive germ cell lineage is achieved. But in mice, and by extrapolation in all mammals, it seems that cells have germ cell status thrust upon them.

Establishment of the germ cell lineage

What is the evidence? The earliest that primordial germ cells have been identified in the mouse is midway through gastrulation, ∼7.25 days after fertilization (Ginsburg et al. 1970). Recognizable by their high alkaline phosphatase activity, they are seen as a cluster of cells located in the extraembryonic region posterior to the primitive streak, and can subsequently be tracked from that location, along their migratory route and into the future gonads. All attempts to identify germ cells or germ cell determinants (e.g., germplasm, ‘nuage’) earlier in development have failed. Experiments in which a genetically marked cell was introduced into the embryo either at the 4-cell (Kelly 1977) or at the mid- or late-blastocyst stage (Gardner 1977) established that cells with primordial germ cells among their descendants also gave rise to somatic cells, thus no exclusive germ cell lineage existed in the preimplantation embryo.

More recently, this approach has been extended to the postimplantation period by Lawson and Hage (1994). Immediately before gastrulation and during the early stages (6.0 and 6.5 days after fertilization), the mouse embryo can be visualized as a thick-walled cup of tissue (the epiblast or embryonic ectoderm), which will give rise to …

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