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THE REACTION OF THE MOUSE BLASTOCYST AND ITS ZONA PELLUCIDA TO pH CHANGES IN VITRO

Once the implantation reaction has been initiated, the zona pellucida of the mouse undergoes rapid lysis. This occurs irrespective of whether the zona has been shed several days earlier, as in lactational delay of implantation (McLaren, 1967, 1968a), or whether it is in the process of being shed, as in the case of normal implantation, or whether it surrounds a passive vitellus, such as that of the unfertilized egg (McLaren, 1968b). The zona lysin is probably produced by the uterus rather than by the blastocyst.

Lysis of the zona pellucida of the mouse and rat has been reported to occur in solutions of pH 4·0 to 5·0 (Hall, 1935; Gwatkin, 1964). In contrast, a study of the pH tolerance of mouse eggs in vitro showed that the development of 2-cell stages was inhibited by a pH of 6·08, while optimal development occurred in the pH 6·40 to 7·21 (Brinster, 1965a).