The morphology and timing of fertilization and early cleavage in the Mongolian gerbil and Deer mouse

Ovulation, fertilization and early cleavage have been investigated in few myomorph rodents and in only two members of the family Cricetidae. The Golden hamster (subfamily Cricetinae: Mesocricetus auratus) was studied by Graves (1945), Venable (1946a, b), Ward (1948), Strauss (1956) and Harvey, Yanagimachi & Chang (1961), using fixed and stained materials; and Austin (1956a), Samuel & Hamilton (1942) and Hamilton & Samuel (1956) examined the living eggs. The effects of delayed fertilization were investigated by Chang & Fernandez-Cano (1958) and Yanagimachi & Chang (1961). The Field vole (subfamily Microtinae: Microtus agrestis) was studied by Austin (1957).

The present paper deals with the processes of fertilization and cleavage in the Mongolian gerbil (subfamily Gerbillinae: Meriones unguiculatus) and the Deer mouse (subfamily Cricetinae: Peromyscus maniculatus bairdii).

Mongolian gerbils were bred in a controlled-light environment (day from 7 a.m. to 7 p.m.) under constant conditions of management (Marston & Chang, 1965).

Histochemical demonstration of succinic dehydrogenase in hamster and rabbit eggs

Succinic dehydrogenase activity, using Nitro-blue tetrazolium salt as electron acceptor, has been demonstrated histochemically in hamster and rabbit eggs before implantation. The formazans formed as blue granules are spread throughout the cytoplasm and among vitelline granules, corresponding to the distribution of mitochondria. The intensity and distribution of these granules vary according to the developmental stage of the eggs. Incubation of hamster eggs for 1 hour produced a weekly and evenly distributed reaction in most (92%) of the unfertilized eggs, while 57% of fertilized 1-cell eggs showed a moderate reaction. Although 84% of the 2-cell eggs, 95% of the 4-cell eggs, and 98% of the 8-cell eggs showed moderate reaction, 78% of blastocysts showed strong activity. In the rabbit egg the time required to develop the reaction was about 4-6 hours for unfertilized and pronuclear eggs, 3-5 hours for 2-cell eggs, 2 hours for 32-cell eggs, 1-2 hours for morulae, and only 0.5-1 hour for blastocysts. The development of succinic dehydrogenase in mammalian eggs at the first cleavage, its gradual increase during cleavage, and its tremendous increase at the blastocyst stage have been clearly demonstrated. The importance of this enzyme activity in relation to oxygen uptake, pathways of carbohydrate metabolism, and the degeneration of eggs are discussed.