Demonstration of oestrogens in developing pig trophectoderm and yolk sac endoderm between days 10 and 16.

Segments of individual blastocysts collected on Days 10, 12, 14 and 16 were examined microscopically to observe yolk-sac development and treated immunocytochemically to localize oestrogens in specific membranes. Mesoderm was present beneath the embryonic disc of ovoid blastocysts on Day 12. The mesoderm spread beyond 1 cm from the disc on Day 14, producing a splanchnic yolk-sac membrane extending across the blastocoelomic cavity, but no mesodermal cells had yet reached 5 cm. By Day 16, proliferation of mesoderm and development of the yolk sac had progressed beyond 20 cm from the disc in most of the specimens examined. Incubation of ultrathin sections with sheep antiserum to oestrone or oestradiol-17 beta followed by rabbit anti-ovine IgG-gold complex and subsequent counting of gold particles retained over the tissues gave a weakly positive reaction for oestrone in trophectodermal cells on Day 10. The most intense reaction for oestradiol-17 beta was also present in the trophectoderm and yolk-sac endoderm on Days 12, 14 and 16.