Development of Meishan and Yorkshire Littermate Conceptuses in Either a Meishan or Yorkshire Uterine Environment to Day 90 of Gestation and to Term1

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Abstract
To investigate the impacts of uterine type and conceptus genotype on development through late gestation, Meishan and Yorkshire embryos were co-transferred into the uteri of either Meishan or Yorkshire recipients that were subsequently slaughtered on Day 90 of gestation. At slaughter, regardless of conceptus genotype, fetuses and placentae were markedly smaller when recovered from Meishan than from Yorkshire recipients. Whereas Meishan and Yorkshire fetuses recovered from Meishan uteri were similar in weight, Meishan fetuses were markedly lighter than littermate Yorkshire fetuses when recovered from Yorkshire uteri. Because of the marked differences between fetal weights observed in Yorkshire recipients on Day 90 of gestation, Meishan and Yorkshire embryos were co-transferred to Yorkshire recipients that were allowed to farrow. Surprisingly, Meishan and Yorkshire fetuses cogestated in Yorkshire recipients were born at similar weights, whereas Meishan placentae were markedly smaller. The weight of Meishan placentae were similar on Day 90 and at term, whereas the weight of Yorkshire placentae were markedly larger (approximately 70%) at term than on Day 90. The constant weight of Meishan placentae from Day 90 to farrowing appears to result from an ability to increase their vascularity during this interval. In contrast, Yorkshire placentae may be forced to increase their weight to keep pace with fetal growth during this period. Because uterine capacity sets the upper limit on litter size, the decreased endometrial surface area required per conceptus in the Meishan pig seems to explain its greater potential for increased litter size.

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