Cytological studies of human zygotes exhibiting developmental arrest

Developmental arrest of 111 (approximately 5%) fertilized ova which had formed two pronuclei was observed during a 5-year period of an in-vitro fertilization programme. At least 30 zygotes demonstrated visible pronuclei at 44-66 h after insemination, and 42 zygotes fragmented. Of the 107 prepared zygotes, 97 were informative and revealed that developmental arrest occurred at different stages of the cell cycle: from interphase (n = 48), to transition interphase-prophase (n = 20), to prophase (n = 11), and to metaphase (n = 13). The latter 13 zygotes were characterized by chromosome sets as follows: haploid (n = 2), diploid (n = 6), triploid (n = 1) and tetraploid (n = 4). Another five zygotes demonstrated different numbers of metaphase chromosomes (between 10 and 40), as well as prematurely condensed chromosomes (PCC) as a result of marked asynchrony in pronuclear morphogenesis. A total of 18 zygotes exhibited asynchrony in the morphology of the two pronuclei. It is concluded that abnormal chromosome sets and pronuclear asynchrony might be causes for early developmental arrest.