Tripronuclear human zygotes: the first cell cycle and subsequent development.

About 6.6% of human oocytes fertilized in vitro were tripronuclear. The first cell cycle and developmental ability of these triploid zygotes were studied by autoradiographic analysis of pronuclear [3H]thymidine incorporation and by morphological observations of living zygotes at various times after insemination. Weak labelling over the pronuclei was first detected at 10 h after insemination and in many zygotes the pronuclei were heavily saturated with radioactive grains by 11-13 h. Thymidine incorporation was not apparent in the majority of tripronuclear zygotes by 21-22 h. At this time, 96% of living triploids (65/68) had visible pronuclei while the others entered the first mitosis and their pronuclei disappeared. By 25 h, 70% of triploid zygotes (37/58) still exhibited visible pronuclei and 26% were already in mitosis. The first divided embryos were seen by 25 h. The results suggest that the length of the complete cell cycle of human 1-cell stage triploids is approximately 25 h. Pronuclear DNA synthesis starts approximately 9-10 h after insemination and seems to last a minimum of 4 h (the precise time of S phase could not be determined). The minimal duration times of G2 phase and mitosis are estimated in the range of 4-6 h and 3.5-4 h, respectively. Tripronuclear zygotes were capable of substantial in-vitro development. About half of examined triploids attained at least the 4-6-cell stage of development (14/32; 44%); however, approximately 19% of these exhibited multinuclear cells. The remaining triploids had reached early morula (8 and 13 cells; 25 and 19%) or blastocyst (6%) stage.