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. Howev er, comparative vertebrate embryology suggests that the time point and mode of germline segregation may vary between mammalian species to a greater ex tent than hitherto suspected. Therefore, we started to make use of the mono clonal antibody PG-2 specific for primordial germ cells (PGCs) of the rabbi t 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 mitoch ondrial 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 labell ed 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 t he confines of the embryo as defined by the peripheral margin in the epibla st/ectoderm layer. Interestingly, cells expressing the PG-2 epitope in an a typical, finely granulated intracellular pattern were found in the hypoblas t layer, but not in the epiblast, at the primitive streak stage. This atypi cal expression pattern may be interpreted as a sign of cells gradually losi ng the PG-2 epitope and this, in turn, may indicate that PGC progenitors ar e allocated to the hypoblast layer before appearing in the mesoderm compart ment of the primitive streak. These results raise the question as to whethe r the germline in the rabbit is separated during early blastocyst stages, i .e. rather earlier than in the mouse.