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ANNE McLAREN

The decidual reaction constitutes an important part of the response of the uterus to the embryo which it contains; yet in no species has it so far been possible to identify what feature of the embryo stimulates this response. The mouse is a suitable species in which to investigate this point, since the decidual reaction is intense and relatively precocious.

Having established that the distension of the uterus caused by the physical presence of the blastocyst was not responsible for inducing the decidual response (McLaren, 1968), an attempt was made to test the further hypothesis that the loss of the zona pellucida, and the consequent close contact between the vitelline surface and the uterine epithelium, constitutes the inducing stimulus. The time of loss of the zona in mice (Orsini & McLaren, 1967) coincides rather closely with the

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ANNE McLAREN

The first indication of a local decidual response to the presence of blastocysts in the mouse uterus is the appearance of Pontamine Blue reactivity at the end of the 4th day of pregnancy (Orsini & McLaren, 1967; Finn & McLaren, 1967). The factor inducing this response is unknown. The time relations make it unlikely that the appearance of W-bodies in the uterine endometrium is involved. Blandau (1949) claimed that in rats, though not in guinea-pigs, glass beads inserted into a pseudopregnant uterus during the sensitive period were sufficient to stimulate a decidual response, suggesting that, in pregnancy, the physical pressure of the blastocyst in the uterine lumen acts as the inducing factor. The present study was designed to show whether beads would stimulate a decidual response in mice.

Females of the randomly bred Q strain were mated to vasectomized Q

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Anne McLaren

Summary. XX germ cells in the fetal testes of XX Sex-reversed male mice were observed mostly to develop in the same manner as do XY germ cells in a normal fetal testis; but some, in the vicinity of the mesonephric rete region, entered the prophase of meiosis at the same time as do XX germ cells in a normal fetal ovary. No germ cells in meiosis were found in the fetal testes of XY males, nor of XO Sex-reversed males. It is suggested that a second X chromosome renders a germ cell more susceptible to the meiosis-inducing influence of the mesonephric rete.

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ANNE McLAREN

Summary.

In two strains of mice, mean placental weight was less on the 19th than on the 18th day of gestation.

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ANNE McLAREN

Summary.

Mouse blastocysts retain the zona pellucida longer in lactating females when implantation is delayed than during normal pregnancy. For most of the period of delay, however, they are free of the zona, but do not induce Pontamine Blue reactivity in the uterus. The shed zonae often remain in the uterine lumen during the delay period without undergoing lysis.

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ANNE McLAREN

Summary.

A combination of mouse inbred strains was studied, in which foetal but not placental weight is greater in the F1 hybrid than in the inbred litters. Gestation period was found to be shorter for the F1 hybrid than for the inbred litters. It is concluded that, insofar as the length of gestation is inversely related to the mass of the conceptuses, it is the foetus rather than the placenta which matters.

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Anne McLaren

The traditional definition of a maternal effect, which I shall adopt, involves any influence of a mother on her offspring exerted otherwise than by direct transmission of nuclear genes. The influence may be exerted through the cytoplasm of the egg, in which case it would be determined before ovulation, or through the reproductive tract, reflecting aspects of the maternal physiology during gestation, or after birth, through lactation or some other aspect of child rearing. Whether it is cytoplasmic, or exerted through the reproductive tract, or after birth, the source of the maternal effect may lie in the mother's genetic constitution or may involve some environmental factor, which could of course include the environment to which the mother herself was exposed.

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ANNE McLAREN

Summary.

Inbred (C57BL♀ x C57BL♂) and F1 hybrid (C57BL♀x C3H♂) litters were compared with respect to (a) birth weight, (b) foetal weight at 17½ days' gestation, (c) placental weight at the same stage. F1 hybrids were heavier than inbred young at birth, but not to any significant extent at 17½ days. Placentae in F1 litters were nearly 20% heavier than those in inbred litters.

Other factors examined were the number of young in the litter and in the uterine horn, and the position of implantation within the uterine horn. Birth weight and foetal and placental weight at 17½ days' gestation were all inversely related to the number of young in the litter. The effect on placental growth was systemic, i.e. it was not affected by the distribution of implantations between uterine horns. There was a suggestion of a systemic effect on foetal growth, in addition to a significant local effect exerted by the other implantations in the same horn. When allowance was made for the number in the litter, those litters distinguished by higher mean placental weight did not necessarily have large foetuses. But within a female, the horn containing the heavier placentae on average had heavier foetuses too, allowance again being made for the number of implantations. Within the uterine horn also, foetal and placental weight showed a positive association. Since there was no genetic heterogeneity among litter-mates, this association suggests a causal dependence of foetal growth on placental size.

The mean weight of foetuses and placentae decreased from the ovarian towards the cervical end of the uterine horn. This effect was most marked for placentae. Foetal growth was not affected directly by position in the uterine horn, but reflected the gradient in placental growth.

It is argued that haemodynamic factors are responsible (i) for the effect of position in the horn on placental growth, and hence on foetal growth, and (ii) for the systemic effect of number in the litter on foetal and placental growth. But the local effect of other foetuses in the same horn on foetal growth seems more likely to be a result of mechanical pressure arising from the distension of the uterus.

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ANNE McLAREN

Summary.

When blastocysts from pregnant mice 3½ days p.c. were transferred to the uteri of synchronous pseudopregnant recipients, the time of onset of implantation (judged by the appearance of pontamine blue reactivity in the uterus) was not altered by removal of the zona pellucida. The competence of zona-free blastocysts to elicit the pontamine blue reaction was similar to that of control blastocysts. The low implantation rate of eight-cell embryos from donors 2½ days p.c., transferred to recipients 3½ days p.c., was not increased by previous removal of the zona. It was concluded that, in mice, the decidualizing stimulus from the blastocyst to the uterus stems neither from the zona itself, nor from cellular contact between the vitelline surface and the uterine epithelium.

Removal of the zona by treatment with pronase reduced the viability of transferred blastocysts, so that by mid-gestation the proportion of zona-free blastocysts represented by live embryos was significantly below the control level.

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PATRICIA BOWMAN and ANNE McLAREN

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).