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In the mouse zygote and in two-cell stage embryos the inner leaflet of the nuclear envelope of pronuclei and that of blastomere and polar body II nuclei evaginate, forming multiple blebs within the perinuclear space, which contains a granular material. Blebbing exists only in oocytes activated by sperm in vivo or in vitro, or parthenogenetically by treatment with ethanol or puromycin. The germinal vesicle and an interphase nucleus formed after treatment of the oocyte at metaphase I by puromycin do not form blebs. Formation of blebs is specifically located in the cell cycle. The burst of the blebbing activity occurs during the first half of the cell cycle in one-cell embryos and in the earliest interphase period in the second cell cycle. Blebbing ceases from the beginning of the third cell cycle. The occurrence in the cytoplasm of ‘double-layered’ vesicles containing granular material resembling bleb contents and the disappearance of blebs from the nuclear envelope by the end of the cell cycle provide evidence that blebs represent a step in the transport of some material from the nucleus to the cytoplasm. Ethanolic phosphotungstic acid does not stain blebs, suggesting the absence of basic protein in their contents. Blebbing can be induced in somatic (thymocyte) and embryonic (blastomere of 8-cell stage embryo) nuclei following cell hybridization with activated oocytes. Their response to the oocyte cytoplasm by initiating blebbing depends on: (1) the position of the host cell in its cell cycle at the moment of hybridization, and (2) the time spent by the foreign nuclei in the host cytoplasm following cell fusion. If donor nuclei are introduced close to the time of activation, they start to produce blebs at the time corresponding to the initiation of blebbing by the female pronucleus in the first cell cycle. If foreign nuclei are introduced a few hours after activation they must be incubated in the host cytoplasm for some time before initiation of bleb formation, provided that the host pronucleus has initiated blebbing by that time. The existence of blebbing in nuclei formed only after oocyte activation, and the timing and the general occurrence of this event during the earliest cleavage stages of almost every mammalian embryo, suggest that this special nucleocytoplasmic transport plays a specific role at the beginning of development.

‘Blebbing’ of the nuclear envelope of mouse zygotes, early embryos and hybrid cells