GENETIC AND ENVIRONMENTAL EFFECTS ON FOETAL AND PLACENTAL GROWTH IN MICE

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Abstract
Inbred (C57BL female XC57BL male) and F1 hybrid (C57BL female X C3H male) litters were compared with respect to (a) birth weight, (b) fetal weight at 17 1/2 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 17 1/2 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 fetal and placental weight at 17 1/2 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 fetal 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 fetuses. But within a female, the horn containing the heavier placentae on average had heavier fetuses too, allowance again being made for the number of implantations. Within the uterine horn also, fetal and placental weight showed a positive association. Since there was no genetic heterogeneity among litter-mates, this association suggests a causal dependence of fetal growth on placental size. The mean weight of fetuses and placentae decreased from the ovarian towards the cervical end of the uterine horn. This effect was most marked for placentae. Fetal growth was not affected directly by position in the uterine horn, but reflected the gradient in placental growth. It is argued that hemodynamic factors are responsible (i) for the effect of position in the horn on placental growth, and hence on fetal growth, and (ii) for the systemic effect of number in the litter on fetal and placental growth. But the local effect of other fetuses in the same horn on fetal growth seems more likely to be a result of mechanical pressure arising from the distension of the uterus.

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