ABSTRACT
A microsurgical grafting technique has been used to introduce primordial germ cell (PGC) precursors into intact primitive-streak-stage mouse embryos in vitro. Operated embryos were cultured for 36–40 h and then analysed by a combined histochemical and autoradiographic method. PGC chimaerism occurred in embryos that received grafts of caudal primitive streak cells but not adjacent embryonic endoderm or anterolateral ectoderm/mesoderm cells. Graft-derived PGCs were found to be migrating through the gut endoderm alongside host-derived PGCs in approximately half of the chimaeric embryos whereas in the other 50 % of cases PGCs remained at the site of grafting in association with graft-derived somatic cells. A similar pattern of somatic chimaerism was produced by primitive streak and anterolateral ectoderm/mesoderm grafts: the allantois was colonized predominantly, with, in addition, formation of amnion, surface ectoderm and caudal mesoderm in a few embryos. The majority of embryonic endoderm grafts failed to incorporate into host embryos and formed yolk-sac-like vesicles.
The findings of this study indicate that (a) PGCs originate from the embryonic ectoderm via the primitive streak during development of the mouse embryo, and (b) anterolateral ectoderm and mesoderm cells are unable to form PGCs after heterotopic grafting to the posterior primitive streak site. The combined microsurgical and embryo culture methods provide an experimental system for the analysis of PGC development in intact mouse embryos.
Cell lineage relationships in the endoderm of the primitive-streak-stage embryo are complex, possibly involving three distinct cell types: future yolk sac endoderm, future foetal endoderm, and cells that are destined to die during the next 24h of development (Lawson, Meneses & Pedersen, 1985). For this reason, the term ‘embryonic endoderm’ will be used in referring to the portion of the endodermal cell layer that overlies the caudal end of the primitive streak.
