Genetic and environmental control of foetal and placental growth in the mouse

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Abstract
SUMMARY: Birth weight was recorded in the first and second litters of six inbred strains of mice and of all possible crosses between them. The mean birth weight of F1young was significantly greater than that of inbred young in most cases; in no case were they significantly lighter than inbred young at birth.Foetal weight and placental weight were recorded on the 17th daypost coitumin the first litters of four strains and of two pairs of reciprocal crosses. F1foetuses were 13–16% heavier than inbred foetuses on crossing four strains. F1placentae were about 15% heavier than inbred placentae on crossing three strains. Crossbreeding did not affect placental weight in the fourth strain. The significance of these findings is discussed.Analyses of data from individual strains and crosses showed that foetal weight was affected by the number of implants in the same uterine horn and independently by the number in the whole litter. The two independent effects, local and systemic respectively, tended to decrease foetal weight as litter size increased. The independent local effect of litter size on placental weight was different and tended to increase placental weight slightly as the number of implants in the same uterine horn increased.Three different instances of maternal effects on foetal and placental growth were observed.The effects of the litter and the mother on foetal growth are discussed in relation to the physiological basis of environmental variation in pre-natal growth.

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