CYTOLOGICAL EVENTS LEADING TO FORMATION OF 2-CELL STAGE IN RABBIT - ASSOCIATION OF MATERNALLY AND PATERNALLY DERIVED GENOMES

The cytological transformations leading to the establishment of the two-cell stage in the rabbit have been studied with techniques of light and electron microscopy. Subsequent to their migration the male and female pronuclei become centrally located and juxtaposed. Each pronucleas appears to be nearly identical in size and in structure and contains a fibrous nucleoplasm in which fine-textured and granular aggregations are embedded. Later the pronuclei become closely apposed and their proximal surfaces flatten and become highly convoluted. Along the proximal margin of the pronuclear envelopes small vesicular structures are produced. They are filled with a flocculent material and appear to be budded-off into the cytoplasm. During further morphogenesis, the pronuclei produce an extensive array of projections which interdigitate with one another to form complex configurations. Concomitantly, the blebbing activity of the pronuclear envelopes ceases and the chromatin condenses into large reticular aggregations, primarily in the region where the pronuclei are intimately associated. Association of the maternal and paternal genomes is effected when the two pronuclear envelopes break down at widely spaced intervals leaving regions of intact double-membrane structures (pronuclear envelope). Microtubules (spindle) are associated with the condensed chromatin and course through the disrupted regions of the pronuclear envelope. The two groups of chromosomes move together and assume positions on the first cleavage spindle. Thus, in the rabbit, there is an absence of a zygote nucleus. © 1969 Academic Press, Inc.