Biology of human trophoblast

In order to elucidate the regulation of placental growth, we have characterized the expression of proliferating cell nuclear antigen (PCNA), apoptotic DNA fragmentation and bcl-2 protein in human placenta during pregnancy. PCNA and bcl-2 protein expression were examined by immunohistochemical techniques, while the occurrence of apoptotic DNA fragmentation was assessed by in situ analysis of DNA 3'-end labeling method. Both PCNA expression and apoptotic DNA fragmentation were found in cytotrophoblasts (C-cells), being most abundant in early placenta, less abundant in midterm placenta and least abundant in term placenta. In contrast, bcl-2 protein expression was found in syncytiotrophoblasts (S-cells), being least abundant in early placenta, less abundant in midterm placenta and most abundant in term placenta. These data indicate that early placenta is characterized by the highly proliferative activity of C-cells associated with the increased occurrence of apoptosis, whereas term placenta is characterized by the abundant expression of bcl-2 protein in S-cells. Furthermore, effects of EGF on the proliferative activity and differentiated function of trophoblasts were investigated using an organ culture system. Explants of trophoblastic tissues were cultured with or without EGF, in the presence or absence of 10(-8)M triiodo-L-thyronine (T3) in a serum-free condition. In 4-5 week placentas, EGF and EGF receptor were almost exclusively localized in C-cells, and EGF augmented the proliferative activity of C-cells without affecting the ability to secrete hCG and hPL. By contrast, in 6-12 week placentas, EGF and EGF receptor were predominantly localized in S-cells, and EGF stimulated hCG and hPL secretion without affecting the proliferative activity of C-cess. The addition of T3 (10(-8)M) resulted in an increased secretion of immunoreactive EGF by cultured placental explants. These findings suggest that EGF acts as a local factor in regulating early placental growth and function in synergy with thyroid hormone. On the other hand, progesterone selectively inhibited pleise hCG (alpha, beta) mRNAs expression and decreased hCG secretion in normal placental tissues, whereas choriocarcinoma did not respond to progesterone. This suggests that inhibitory regulation of hCG synthesis in choriocarcinoma is different from normal placenta. It was also found that in molar trophoblasts and choriocarcinoma cells PCNA expression was high, but both bcl-2 protein and apoptotic signal expression were low. Characterization of choriocarcinoma hCG revealed that there are striking differences in carbohydrate structures between normal hCG and choriocarcinoma hCG. Sialic acid content in choriocarcinoma hCG was extremely lower compared to that in normal hCG. The detection of the alteration in hCG sugar chains is useful for biochemical diagnosis of choriocarcinoma.