Preimplantation human embryonic development from polypronuclear eggs after in vitro fertilization.

The occurrence of grossly normal-appearing morula stage human embryos (8- to 12-cell) that developed from eggs fertilized in vitro containing three pronuclei is described. Serial section analysis by transmission electron microscopy demonstrated that 25% of the blastomeres were multinucleate, with as many as five deoxyribonucleic acid-containing nuclei (determined by fluorescence microscopy) in a single cell. Light and electron microscopy indicated that normal cortical and zona reactions had taken place. Fine-structural development of the cytoplasm was characteristic of morula stage human embryos. The results suggest the need to determine pronuclear number prior to syngamy and are discussed with respect to the notion that human embryos may have the capacity to develop normally in spite of the presence of abnormal cells.