Uterine Crowding in the Sow Affects Litter Sex Ratio, Placental Development, and Embryonic Myogenin mRNA Expression in Early Gestation.

Uterine crowding in the pig results in intra-uterine growth restriction (IUGR), and permanently affects fetal muscle fiber development, representing production losses for the commercial pig herd. Muscle fiber hyperplasia is complete by late gestation; however the determination of the total number of muscle fibers occurs much earlier and is sensitive to the uterine environment. Previous studies from our group have shown lower placental and fetal weights, and fewer secondary muscle fibers in day 90 fetuses recovered from relatively crowded uterine environments than from the less crowded uteri of oviduct-ligated sows. Thus, this study sought to understand how different levels of uterine crowding in sows would affect the mRNA expression of myogenic regulatory factors (MRF), specifically myogenin (MYOG) and myogenic differentiation 1 (MYOD1), in day 30 embryos at the time of muscle fiber differentiation and proliferation. Sows underwent surgical, unilateral oviduct-ligation (LIG; n=10) to reduce the number of embryos in the uterus or remained as intact, relatively crowded controls (CTR; n=10). A total of 251 embryos were collected at day 30 of gestation, and ovulation rate, viable embryo numbers and embryo and placental weights were recorded. Genomic DNA was extracted for each embryo and subjected to sex-typing PCR. Total RNA was extracted, reverse-transcribed and MRF transcript abundance was determined using real-time PCR for both MYOG and MYOD1. The ΔCT values were calculated using cyclophilin as the endogenous control for each sample. All data was subjected to MIXED models analysis with sow as a random effect, and treatment and sex by treatment interactions were tested as the main effects in the model. Unilateral tubal ligation resulted in lower numbers of embryos in utero, higher placental weights and a higher male:female sex ratio (P<0.05). Embryo survival rate (P=0.14) and embryo weight (P=0.28) were not different in the limited numbers of sows used in this study. Male embryos may be more sensitive to crowding than female embryos and our evidence supports this concept as the more crowded sows exhibited a lower male:female ratio. Relative MYOD1 expression was not different, but MYOG expression was higher (P<0.05) in the LIG group embryos and this effect was predominantly due to the increase in MYOG expression in male embryos across treatments. It is possible that the gender-specific differences observed in MYOG expression may be due to mechanisms altered by uterine crowding to which male embryos are more sensitive. Therefore, the observed differences in secondary muscle fibers in day 90 fetuses from LIG and CTR sows in our previous study may be due to the differences in myogenin gene expression at day 30 and its impact on the differentiation of primary fiber numbers. Relatively modest uterine crowding therefore affects MRF expression, even at very early stages of embryonic development, and could contribute to reported differences in fetal muscle fiber development, birth weight and thus post-natal growth performance in swine.

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