Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/17/2007
Publication Date: 12/1/2007
Citation: Vallet, J.L., Freking, B.A. 2007. Differences in placental structure during gestation associated with large and small pig fetuses. Journal of Animal Science. 85(12):3267-3275. Interpretive Summary: Litter size, piglet birth weights, and neonatal survival of piglets are all influenced by the efficiency of nutrient transfer from the sow to the pig fetus during pregnancy. The pig placenta is responsible for nutrient transport, and structurally, is relatively simple. The sow blood supply is separated from the fetal blood supply essentially by two closely apposed epithelial cell layers, one of sow origin and one of fetal origin. Sow and fetal blood flows within the placenta are roughly in opposite directions. The space of interaction between the two blood supplies occurs within folds of the two epithelial cell layers, and the width of these folds governs the extent of the interaction between the two blood supplies. We measured the width of the folds, to determine if it differs during gestation and between placenta associated with large and small fetuses in a crowded intrauterine environment. We found that the width of the folds increases as pregnancy advances, and increases more in placenta of small fetuses, which are associated with small (and therefore nutrient deprived) placenta. We hypothesize that the increase in fold width is a compensatory mechanism for nutrient deprived small placenta. The folds are embedded within placental connective tissue, and the width of the connective tissue was less in small placenta. From this observation, we hypothesize that the folds develop more in placenta of small fetuses at the expense of placental connective tissue. In a few placenta of small fetuses, this appeared to be exceeded, such that the folds completely penetrated the placental stroma. Thus, we further hypothesize that when this mechanism is exceeded in placenta of small fetuses, it may explain the late gestation fetal deaths that occur in a crowded intrauterine environment. If these hypotheses are true, the genes controlling placental microscopic fold development and placental connective tissue thickness may be exploited to improve placental efficiency of nutrient transport, and therefore increase litter size and piglet birth weights, and reduce neonatal piglet losses.
Technical Abstract: The efficiency of nutrient transport from the pregnant female pig to the developing fetus depends on the size and function of the placenta. It has been reported that maternal and fetal blood vessels are arranged in a cross-countercurrent arrangement within placental microscopic folds. Thus, the blood supplies are in close apposition to each other within these microscopic folds, and maternal and fetal blood flows in approximately opposite directions perpendicular to the plane of the placenta. This arrangement indicates that the width of the microscopic folds influences placental efficiency. The objective of this study was to determine whether differences in pig placental microscopic fold development are associated with differences in fetal size, or are influenced by selection for ovulation rate or uterine capacity. Gilts from a randomly selected control line, a line selected for ovulation rate, or a line selected for uterine capacity were slaughtered and uterine wall samples were collected within the placentas associated with the largest and smallest fetuses in each litter on days 45, 65, 85 and 105 of gestation. The samples were fixed in buffered formalin, embedded in paraffin and sectioned. Sections were placed on slides and stained with hematoxylin and eosin. Using computer assisted morphometry, two sections from each slide were evaluated. Average width of the placental folds and average width of the placental stroma above the folds were measured. To measure fold complexity, the length of the epithelial bilayer for a given length of placenta was also measured. Results did not differ between selected lines so data were combined. The width of the folded bilayer increased significantly from day 65 to 105, and increased more rapidly in placentas associated with small fetuses compared to large fetuses. In contrast, the width of the placental stroma above the folded bilayer decreased with gestation, and decreased more rapidly in placenta associated with the smallest compared to the largest fetus. These results indicate that the width of the microscopic folds of the placental trophoblast/endometrial epithelial bilayer is increased in placenta associated with small fetuses, which we hypothesize may improve placental efficiency in response to reduced placental size.