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Trophoblastic cell proliferation and differentiation are essential to develop the expansive exchange surface of the chorionic placenta. The purpose of these studies was to assess direct and indirect effects of endometrial epithelial cells and direct effects of fetal serum, amniotic fluid, and allantoic fluid on proliferation and differentiation of bovine trophoblastic cells.
For this purpose, primary bovine trophoblastic cell lines were developed. Ultrastructural features of cultured cells were typical of native cells. Fewer binucleate cells were present in subcultures than in primary dispersions. Epidermal growth factor and fetal serum were essential to maintain these lines in long term culture.
Direct and indirect effects of endometrial epithelial cells were assessed using bovine endometrial epithelial primary cell lines. Polarization of endometrial epithelial cells was achieved by subculturing cells on collagen or amniotic membranes and maintaining cultures at the air/liquid interface. Polarized cultures had ultrastructural and lectin histochemical similarities to endometrial epithelial cells in vivo. Attachment of trophoblast vesicles from day 14 embryos to the polarized cultures had ultrastructural characteristics of early implantation.
Conditioned medium from either polarized or non-polarized endometrial epithelial cultures had mitogenic effects on bovine trophoblastic cell cultures, but did not enhance binucleate cell differentiation in vitro. The proliferative response was greater to medium from polarized than non-polarized endometrial cell cultures.
Proliferating and differentiating effects on cultured bovine trophoblastic cells by fetal bovine serum, allantoic fluid, amniotic fluid, and fetal urine also were assessed. Amniotic and allantoic fluids were highly mitogenic to trophoblastic cell cultures. Proliferative responses to allantoic fluid were three times those of serum-supplemented cultures and twice those of amniotic fluid supplemented cultures. Proliferative responses to ovine serum and amniotic fluid were similar. Binucleate cell differentiation was greater in cultures containing fetal bovine serum or allantoic fluid than in serum free cultures; highest percentages were obtained in cultures with fetal serum and without epidermal growth factor or insulin.
It is concluded that factors from fetal fluids and endometrial epithelial cells are mitogenic to trophoblastic cells in vitro, but that factors affecting binucleate trophoblastic cell differentiation are possibly from fetal sources.