Morphometric analysis of yak placentomes during gestation.

To study the morphological characteristics of placentomes during gestation, thirty-one yaks were used to determine anatomical and histological data of placentomes by morphometric method. The results showed that there were no significant changes in the total number of yak placentomes during gestation, but remarkably fewer placentomes in the non-pregnant horn compared with the pregnant horn. The mean size of placentomes increased significantly from <61 days to 181-210 days and decreased in 211+ days. The development of placentomes in the pregnant horn were obviously different from that of the non-pregnant horn except <61 days, which were longer, wider, and thicker. The volume density of fetal villi increased during gestation, with a concomitant reduction in the volume density of caruncular endometrium. However, there was no significant change in the surface density of fetal villi. The volume density of binucleate cell increased in prophase and then decreased. No apparent changes in the volume densities of maternal and fetal pyknotic cell occurred throughout gestation. There was a significant increase in the average total volume of yak placentomes from 61-90 days to 181-210 days and a significant decrease in 211+ days. The same patterns were showed in the average total volumes of caruncular endometrium and binucleate cell as well as the mean total surface area of fetal villi. However, the mean total volumes of fetal villi, maternal and fetal pyknotic cell increased constantly with increasing gestation time. Three types of placentome shape were identified in yak. The results showed that the maternal-fetal contact area of yak was enlarged by increasing the placentome size as well as the total volume and surface area of placentome component during pregnancy, so placental function could be enhanced further to meet the needs of fetal development.