The occurrence, recognition and developmental fate of pseudo-multipronuclear eggs after in-vitro fertilization of human oocytes.

The presence of three or more presumed pronuclei in 27 of 884 (3%) in-vitro fertilized human eggs suggested the occurrence of polyspermic fertilization. Detailed examination by differential interference contrast microscopy indicated that approximately 40% of presumed multipronuclear eggs were normally fertilized but contained a cytoplasmic vacuole(s) (pseudo-pronucleus) of pronuclear dimension and gross morphology. Closely timed observations of presumed multipronuclear eggs revealed that pseudo-pronuclei (PPN) can co-migrate and become juxtaposed with true pronuclei, thus presenting the impression of an aberrant fertilization. Fluorescent probe and electron microscopic analyses demonstrated that PPN are enclosed by a plasma membrane and contain neither DNA nor nucleoli. PPN can develop either in the mature oocyte or the newly fertilized egg. Pseudo-multipronuclear eggs progress through the preimplantation stages in an apparently normal fashion and, as demonstrated by one birth and two ongoing pregnancies, are developmentally viable. Criteria for the unambiguous designation of pseudo-multipronuclear eggs are based on: the absence of normal morphodynamic changes associated with pronuclear development; and the failure of nucleoli to appear during the perisyngamic stage. The findings strongly suggest that pseudo-multipronuclear human eggs may represent a class of morphological variants of normally fertilized and developmentally competent eggs.