The role of the zona pellucida in the development of mouse eggs in vivo

Up to the present time the function and significance of the zona pellucida in the development of mammalian eggs has not been fully explained. Zona-free mouse eggs will develop in vitro from the 2-cell stage, or later, up to the blastocyst stage (Tarkowski, 1961; Mintz, 1962; Gwatkin, 1963). Single blastomeres isolated at the 2-cell (Mulnard, 1965), 4and 8-cell stage (Tarkowski & Wróblewska, 1967) will also develop in vitro up to the blastocyst stage. Similar experiments on development in vitro of 1and 2-cell rabbit eggs (Edwards, 1964) showed that in this species also cleavage can occur when the zona pellucida is absent, although the blastomeres exhibit a tendency to fall away from each other. Tarkowski's observations (unpublished) would appear to show, however, that naked 1-, 2and 4-cell mouse eggs do not develop when transferred to the oviduct. A few hours after transplanting the naked eggs none could be recovered by flushing the oviduct, whereas eggs surrounded by zonae which were transplanted simultaneously were recovered. However, naked morulae and blastocysts transplanted to the oviduct develop and may become implanted in the uterus (Tarkowski, 1961; Mystkowska & Tarkowski, 1968). Moore, Adams & Rowson (1968) also transplanted single naked blastomeres from 2and 4-cell rabbit eggs to the oviduct and found that no further development occurred. Contrary to the above findings, Nicholas & Hall (1942) claimed that isolated 'half' blastomeres and pairs of naked 1-cell rat eggs transferred to the uterus were able to continue development beyond implantation, suggesting that the presence of zona pellucida is not essential for the development of early cleavage stages in vivo. The purpose of the present study was to follow the fate of naked mouse eggs in vivo after their transplantation to the oviduct.