Ultrastructure of early cleavage stages and preimplantation in the rabbit

Summary

Fertilized rabbit ova were studied in the period from 25 to 144 hr after insemination. Eggs were recovered by flushing the uterine horns and oviducts with Hank's solution. The cells were morphologically alike in the 24 and 48 hr ova. At 661/2 hr blastomeres were differentiated into an inner mass of cells with dense cytoplasm and an outer trophoblastic layer with less dense cytoplasm. Otherwise no morphological differences were seen. Whether the 661/2 hr ova were morulas or blastocysts is discussed. The 96 hr ova were clearly blastocysts. Inner cells and trophoblastic cells at this stage bad the same cytoplasmic density. Mitochondria were increased in number and crystal-like figures were present for the first time. In the 120 and 144 hr ova the cytoplasm of the trophoblastic cells was denser than that of the inner cells. Trophoblastic cells were characterized by their density, crystal-like figures, elongated mitochondria with transverse cristae and many single ribosomes and they were interconnected with well developed junctional complexes. In a few cases a continuity seemed to exist between trophoblastic cells and inner cells. The latter were characterized by cytoplasm of less density than that of the trophoblastic cells, rounded mitochondria and fewer ribosomes. The fine structure of the crystal-like figures, their possible origin and differentiation of the mitochondria are discussed.