The term cell epitope PG-2 is expressed in primordial germ cells and in hypoblast cells of the gastrulating rabbit embryo.

Rapid progress in the functional analysis of germline segregation has been made recently using the mouse as an experimental and molecular model. However, comparative vertebrate embryology suggests that the time point and mode of germline segregation may vary between mammalian species to a greater extent than hitherto suspected. Therefore, we started to make use of the monoclonal antibody PG-2 specific for primordial germ cells (PGCs) of the rabbit as an opportunity to investigate the early phases of germ cell formation in a mammalian species other than the mouse. Using immunohistochemistry on whole mount preparations and frozen sections we describe the typical mitochondrial labelling of PGCs in the posterior part of the primitive streak at 7.0 days post conception (d.p.c.) and the subsequent distribution of labelled PGCs at early somite stages (8 d.p.c.) within a bilobed area that flanks the posterior margin of the embryo. At these later stages, PGCs were found close to, and within, the yolk sac epithelium but they were still within the confines of the embryo as defined by the peripheral margin in the epiblast/ectoderm layer. Interestingly, cells expressing the PG-2 epitope in an atypical, finely granulated intracellular pattern were found in the hypoblast layer, but not in the epiblast, at the primitive streak stage. This atypical expression pattern may be interpreted as a sign of cells gradually losing the PG-2 epitope and this, in turn, may indicate that PGC progenitors are allocated to the hypoblast layer before appearing in the mesoderm compartment of the primitive streak. These results raise the question as to whether the germline in the rabbit is separated during early blastocyst stages, i.e. rather earlier than in the mouse.