Factors contributing to the variation in placental efficiency on days 70, 90, and 110 of gestation in gilts

Variations in placental efficiency (PE), a measure of grams of fetus produced per gram of placenta, were initially researched between swine breeds, where increased PE was associated with larger litters. Placental efficiency was also found to vary greatly within production herds and individual litters; however, the use of PE as a selection tool has been debated. Nonetheless, PE is an index of feto-placental adaptation and may help identify compensatory mechanisms that maintain fetal growth when placental size is reduced, potentially providing an opportunity to address production concerns like low birth weights and preweaning survival. Since the nutrient transport capacity of the placenta largely depends on vasculature and nutrient transporter abundance, the objectives of this experiment were to 1) determine the mRNA expression of genes encoding nutrient transporters in the placenta and adjacent endometrium, and 2) evaluate if a relationship existed between PE and vascular density and/or nutrient transporters. Gilts (n = 19) were ovario-hysterectomized on day 70, 90, or 110 of gestation to collect placental and adjacent endometrial samples. The mean litter size was 11.1. Placental efficiency increased (P < 0.0001) throughout the end of gestation, while the range of PE increased from day 70 to 90 and was reduced on day 110 (P < 0.0001). Placental efficiency and placental weight were negatively correlated throughout gestation (70 d, r = -0.83, P < 0.0001; 90 d, r = -0.81, P < 0.0001; 110 d, r = -0.44, P < 0.0007), but the negative correlation between PE and fetal weight was not maintained as gestation progressed (70 d, r = -0.58, P < 0.0001; 90 d, r = -0.36, P < 0.0005; 110 d, r = 0.09, P = 0.51). Based on conditional effects plots, variations in PE were associated with alterations in amino acid transporter expression in the placenta (SLC7A7, SLC3A1) and endometrium (SLC7A1) on day 70. On day 90, PE had a positive relationship with placental expression of a glucose transporter (SLC2A3), and on day 110 PE was positively related to placental vascular density. The results suggest utero-placental adaptations occur as a compensation for reduced placental size to meet the increasing nutrient demands of the growing fetus during late gestation in swine. Furthermore, nutrient requirements differ for individual feto-placental units on a given day; therefore, optimizing nutrient availability during late gestation may improve fetal growth and survival.