Compaction and surface polarity in the human embryo in vitro.

The surface morphology of the human ovum fertilized and cultured in vitro to the morula stage was studied by scanning electron microscopy with the specific aim of investigating embryo compaction and polarity. Unfertilized oocytes examined one day after attempted insemination (Day 0) were evenly and densely covered by long microvilli. The length and density of microvilli appeared to decrease in fertilized polypronuclear oocytes; a further decrease was observed in Day 2 and Day 3 embryos with 2-12 cells. No evidence of compaction or surface polarity was observed in any of these stages. On Day 4, compaction was evident in the majority of embryos with 10 or more cells, and the microvilli appeared dense again with a polarized distribution over the free surface of the compacted blastomeres. This study provides ultrastructural evidence that the human conceptus undergoes a relatively marked compaction at the morula stage during Day 4 postinsemination development in vitro.