Fine structure of human two-cell ova fertilized and cleaved in vitro.

The human two-cell ovum consists of two equally sized blastomeres, surrounded by the zona pellucida. Cumulus cells may be apposed to the exterior of the zona. The blastomere nuclei contain spherical nucleoli, made up of fibrillar matrix, and randomly dispersed chromatin. The cytoplasmic structures include mitochondria, smooth endoplasmic reticulum, annulate lamellas, the Golgi complex, free ribosomes, secondary lysosomes, cortical granules, and microtubules. The cell membrane bristles with microvilli and displays signs of pinocytosis except at the site of apposition of the two blastomeres. The fine structure of the two-cell ova fertilized and cleaved in vitro corresponds with the presumptive pattern with respect to the current knowledge of mammalian embryo preimplantation development. The ova seem to have been capable of further growth.