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SUMMARY
The effects of ovariectomy at different stages of early pregnancy, and the response to various amounts of exogenous progesterone and oestrogen, were studied in Q-strain mice. When the ovaries were removed on the third day of pregnancy, unimplanted blastocysts were retained in the uterus and no lysis of the zona pellucida occurred; removal on the following morning permitted lysis of the zona pellucida but again no implantation; by the afternoon the ovaries were no longer required for the initiation of implantation, but in the absence of progesterone no sites were maintained. A single dose of oestrogen eliminated all unimplanted blastocysts from the uteri of ovariectomized mice. Daily treatment with a low dose of progesterone after ovariectomy on the third day altered the appearance of the uterus but did not permit implantation unless oestrogen was also given. Under all the above conditions, some reduction in the number of embryos occurred. The maximum number of implantation sites in ovariectomized mice was achieved by daily injection of a high dose of progesterone, with no added oestrogen. An intermediate level of progesterone induced abortive implantation in mice ovariectomized on the third day, and abnormal development of implantation sites after ovariectomy on the fourth day. When the start of progesterone injections was postponed, either after ovariectomy or during lactational delay, it proved more difficult to induce implantation with the progesterone preparation alone, in the absence of added oestrogen.
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SUMMARY
The variation in the number of eggs shed by mice in response to treatment with gonadotrophic hormones depends on the sexual maturity and genetic constitution of the treated females, on the amount of pregnant mare serum used, and on the number of injections into which the total dose is divided. Dividing the dose into six administrations spread over 24 hr. nearly halved the variability of the response in one strain, and reduced the variance sixfold in another.
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SUMMARY
The variance of the distribution of eggs and embryos between left and right sides was analysed in mice. Little or no departure from the variance of a binomial distribution was detected for eggs, whether ovulation was spontaneous or induced, and whether the mean number of eggs shed was within normal limits or was raised well above the normal level. As pregnancy advanced, the distribution of embryos between sides became slightly more equal than the binomial, as a result of pre- and post-implantational mortality. In pigs, the tendency towards equalization was much stronger owing to migration of embryos between the two uterine horns. The right side tended to be reproductively superior to the left in mice, in respect both of ovulation rate and survival of embryos.
The correlation between left and right sides was negative after spontaneous ovulation, and after induced ovulation with a dose of hormone such that ovulation rate remained within normal limits. If, on the other hand, ovulation rate was raised above the normal level, a positive correlation between sides was found. These correlations arose not from any departures from binomiality in the distribution between sides, but from the level of variation between females in the total number of eggs or embryos. The condition for no correlation between sides is that the variance of total number should be equal to the mean. Positive correlations arise from inflation of this variance, a not unexpected consequence of increasing the mean ovulation rate above its physiological norm. Negative correlations imply that the variation between females is less than the variation between the two ovaries of a single female. In other words, the limit to the number of eggs shed operates on the female as a whole, and not on each ovary independently. The physiological implications of this finding are discussed.
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SUMMARY
Varying numbers (2–12) of fertilized eggs from donor mice 3½ days post coitum were transferred to the uteri of pseudopregnant recipients 2½ days post coitum. A higher proportion of blastocysts than of morulae implanted and developed into live foetuses. The number of eggs transferred did not affect the proportion which implanted. Litters of one or two young, resulting from the transfer of two eggs to one horn only, were successfully carried to late pregnancy and suffered less post-implantational mortality than did larger litters resulting from the transfer of larger numbers of eggs.
An empty uterine horn did not exert either a systemic or a local luteolytic effect. Eggs were transferred to one horn of a pseudopregnant female, and at the same time the ovary on that side was removed. Implantation occurred normally, pregnancies were maintained, and the corpora lutea in the remaining ovary did not appear to be affected by the adjacent unoccupied horn.
When litters of one or two young were allowed to go to term, the young suffered very heavy perinatal mortality. This was not a direct effect of the small number of young, but was entirely due to the significant prolongation of the gestation period with small litters, combined with the significant decrease in perinatal survival when parturition was delayed.
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SUMMARY
Blastocysts were studied on the 5th and 8th day of pregnancy in lactating mice, in the fresh state, flushed from the uterus, in squash preparations and in serial sections. At the earlier period some mitosis was observed. Tritiated thymidine incorporation studies gave some evidence of DNA synthesis on the 5th and 6th days of pregnancy. By the 8th day the blastocysts were longer, contained more cells, and mitosis had ceased. They were located at the anti-mesometrial end of the uterine lumen, closely apposed to the uterine epithelium, and with their long axes parallel to the long axis of the uterine horn. Implantation could be induced, either by the removal of the litter, or by the injection of an appropriate dose of oestrogen on the 5th or 7th (but not the 4th) day of pregnancy. Both treatments were followed by the appearance of W-bodies in the neighbourhood of the blastocysts, the disappearance of the shed zonae, and the appearance of Pontamine Blue reactivity, oedema of the uterine stroma and formation of the primary decidual zone, in that order.
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SUMMARY
A study was made of the response to induced ovulation of female mice from strains in which the spontaneous ovulation rate had increased as a result of artificial selection for litter size. In response to standard doses of follicle-stimulating hormone (FSH) within the physiological range, the number of eggs shed differed significantly among the three strains tested, in the direction expected on the basis of their spontaneous ovulation rates. This indicates that the principal effect of selection has been to increase the sensitivity of the ovary to FSH. In one strain, the possibility remains open that there may also have been some increase in the amount of FSH available.
In response to larger doses of FSH, females from the strain with the highest spontaneous ovulation rate shed significantly fewer eggs than the other two groups. Selection must therefore have altered the shape of the dose-response curve, increasing the ovarian response at low levels of hormone and decreasing it at high levels.
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SUMMARY
The rate of CO2 production by 8-cell embryos (2½ days post coitum) and blastocysts (3½ days post coitum) was measured in mice of the Q strain. The values for blastocysts resembled those obtained by Brinster (1967) in the Swiss strain, both in mean and variance; the values for 8-cell eggs differed in both respects from those obtained by Brinster.
Blastocysts in delay of implantation during lactation or after ovariectomy gave off less CO2 per hr. than either 3½-day blastocysts from normal pregnancies or blastocysts from post-partum matings when the litters were removed at birth. This indicates that blastocysts enter a state of metabolic dormancy during delay of implantation.
When ovariectomized pregnant mice were maintained on 0·5 mg. progesterone per day, their blastocysts produced even less CO2/hr. than those from ovariectomized animals not given exogenous hormone. The implications of this finding are considered with respect to the role of progesterone in delay of implantation.
