Summary. The embryonic ectoderm of the pig differentiated and became part of the outer barrier of the blastocyst (earlier formed by the trophectoderm alone) before shedding of the overlying polar trophectoderm around Day 10, thus securing the integrity of the rapidly expanding blastocyst. Ferritin, added to the medium of the blastocyst, was taken up rapidly by trophectoderm cells, but did not reach the blastocoele, and consequently no tracer was found within hypoblast cells. Embryonic ectoderm cells did not absorb the macromolecule, before or after loss of the polar trophectoderm. When ferritin was injected into the blastocoele, trophectoderm, hypoblast and embryoblast cells all absorbed the tracer. At Day 11, blastocyst diameter and embryoblast cell number varied widely and were hardly correlated. We suggest that embryoblast development may be a more reliable indicator for the developmental stage of a blastocyst than its diameter, which may merely be an indication of the viability of the trophoblast.
Keywords: pig; blastocyst; ultrastructure; developmental variation