The expression of Fas/FasL and apoptosis in yak placentomes

To clarify the status and distribution of Fas and Fas-Ligand (FasL) in yak's placentomes, immunohistochemistry (IHC) was carried out to analyze the expression and location of Fas and FasL in paraffin embedded sections. The area of positive stained sites was selected and measured using image analyses software (Image Pro-Plus 6.0). So the positive index (PI) was calculated to estimate the intensity of protein expression according to the percentage of positive area in corresponding compartment of the placentomes. In cotyledonary villi, Fas mainly presented on the villous trophoblast cells in early pregnancy. The positive index reached a maximum of 20.7 ± 8.8 at the third month of pregnancy. Then Fas was declined rapidly along with the progress of gestation and the value was 2.8 ± 1.3 after the 7th month of pregnancy. However, in caruncular crypts, Fas was mainly localized to isolated cells or clustered cells of the uterine stroma underlying the caruncular epithelium. The intensity was lower and the positive index was changed between 4.7 ± 0.9 and 8.5 ± 1.6 throughout gestation. For FasL, it gave a distinct immunostained distribution. In cotyledonary villi, FasL was localized dominantly and strongly in the cytoplasm of binuclear, mononuclear and trinuclear trophoblast giant cells (TGC). The positive index of FasL maintained a moderate level all through the gestation. In caruncular crypts, the expression of FasL was weak and the positive index was declined. Only in the first two months, maternal uterine epithelial cells intensely expressed FasL and the index reached to the maximum of 19.8 ± 5.2. The result of subcellular localization of Fas ligand using immunoelectron microscopy technology indicated that FasL was subcellular located in some intracellular vesicles of TGC. This means the vesicles of trophoblast giant cells itself can express FasL. By the TUNEL method, apoptosis was detected in yak placentomes. The amount of apoptotic cells was rare. The fetal chorionic trophoblast cells and caruncular crypt epithelium cells demonstrated higher percentage of apoptosis in middle pregnancy, which suggested that apoptosis plays an important role in placental cellular regeneration. In addition, the apoptosis of maternal caruncular stromal cells provides a local mechanism for maternal immunotolerance to the fetus and this mechanism was mediated by Fas–FasL pathway.