Age-related changes of Δ⁵-3β-hydroxysteroid dehydrogenase (Δ⁵-3β-HSD) activity, using dehydroepiandrosterone as the substrate, were histochemically examined in the oocytes of 30-day-old, 60- to 90-day-old and 180- to 210-day-old mice. The activity was found in all the oocytes observed, and the percentage of the oocytes with a strong activity was significantly lower in 180-to 210-day-old mice than in 30-or60-to 90-day-old mice.In order to ascertain whether Δ⁵-3β-HSD activity in the oocytes of senescent animals is stimulated by gonadotrophins, as is inyoung animals, the activity in the oocytes of 180-to 210-day-old mice was detected after the oocytes were immersed in hCG-containing solutions. The percentage of the oocytes with a strong activity showed no significant differences between the percentage of the oocytes immersed in hCG-free solution. This result suggests that the oocytes of aged mice have lost sensitivity toward gonadotrophins.

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Using mice of the ICR strain, sudanophilic lipids, lipoids and neutral fats were histochemically detected in the oocytes immediately after ovulation, and the amounts were compared among 30-day-old, 60-to 90-day-old and 180- to 210-day-old ones. Sudan IV -positive lipids and Ciaccio-positive lipoids were demonstrated in all the oocytes as droplets of different sizes, showing no significant difference among different age groups. Concerning nile blue sulfate-positive neutral fats observed as droplets of varied sizes, none was detected in the oocytes of 30-day-old mice, but came to appear in 1.0 % of the oocytes from 60-to 90-day-old mice, and extended to 6.3 % of those from 180-to 210-day-old mice, even showing a significant difference between 180-to 210-day-old mice and 30-day-old or 60- to 90-day-old mice. These results show that neutral fats in mouse oocytes first come to exist when the animals become mature, and the percentages of neutral fat-containing oocytes tend to be higher with the advance of animals' ages; while the amounts of sudanophilic lipids and lipoids do not change with aging.

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The study was conducted to elucidate the interrelationship between the blastocyst and endometrium at early stage of implantation. Japanese White does were autopsied at 7 and 8 days post coitum. The samples were prepared with the method described by us eleswhere, and observed by use of Scanning electron microscopy. The results were as follows. 1. The inner part of the trophoblast was smoothly at 7 days post coitum, and the cellular bolders of the trophoblast was covered yet an abundance of microvilli. The endometrium richly provided round shaped cells with short microvilli. A small number of ciliated cells could be found at the antimesometrial surface of the uterus. 2. The blastocyst at 8 days post coitum contacted loosely with the endometrium antimesometrial portion. The surface of endometrium at this stage contained the characteristic microvilli which formed like a flower structure by the bundle of microvilli.

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Rat 2-cell embryos were preserved successfully in liquid nitrogen by vitrification. Embryos needed to be equilibrated in 90%VS1 in a stepwise manner, while protective agents could be removed directly from the vitrified embryos. The vitrified embryos developed to live youngs at high rates (59-67%) by transferring to pseudopregnant recipients.

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The development of mouse embryos formed by the aggregation of two 8-cell embryos was analysed in comparison with control single (normal) embryos. The analysis revealedthat:(1) No significant diffrence was observed in the weights of fetuse and placenta from postimplantation embryo at 18 days post coitum of recipient mice between aggregated and control embryos.(2) Postimplantation aggregated embryo was at about the same developmental stage as control embryo of the same chronological age, both before and after size regulation.(3) Size regulation of the aggregated embryos was brought about by 6 days and 5 hours post coitum of recipient mice.

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The copulation rate and the average number of normal embryos were examined after superovulatory treatment with 5iu PMSG and 5iu hCG given 48h apart in the adult femeals of 23 inbred and 3 mutant strains. Singnificant strain differences were observed in the copulation rate (15.8-84.2%) as well as in the rate of normal embryos among those recoverd (10.1-79.7%). Average number of normal embryos at the 8-16 cell stage, recoverd on Day 2 of pregnancy. varied from 3.5 to 31.3 among the strains.

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