The bovine placenta produces estrogens from the first trimester until the end of its life span. However, with the exception of the immediate prepartal and intrapartal phases, in which an involvement of placental estrogens has been suggested for the preparation of parturition, their function has not been elucidated yet. To test for a role of placental estrogens as local factors regulating placental growth and differentiation, placentomes from cows that were pregnant for 150, 220, 240, and 270 days, and parturient cows (3 animals per group) were screened immunohistochemically for the expression of estrogen receptor alpha (ERalpha). Indirect immunoperoxidase staining methods were applied using primary monoclonal antibodies (pmAbs) directed against the C-terminus (AER311, HT277) or the N-terminus (AER314, 1D5) of the ERalpha molecule. Both types of pmAbs identified ERalpha in stromal cells and capillary pericytes of the maternal caruncular septae. Using pmAb 1D5, the mean percentage of ERalpha-positive caruncular stromal cells decreased from 39.0% +/- 5.9% in pregnant cows to 17.5% +/- 8.3% at parturition (P = 0.011). Only pmAb recognizing the C-terminus identified ERalpha in the caruncular epithelium, in which positive reactions were found in all cells, with the exception of areas adjacent to the chorionic plate and to major chorionic villi, where the specific signal gradually faded and occasionally disappeared. No positive reactions were observed in the fetal part of the placentomes. The expression of ERalpha in bovine placentomes was further confirmed by the detection of ERalpha-specific mRNA by reverse transcriptase-polymerase chain reaction and by Western blot analysis. The results suggest a role for placental estrogens as paracrine factors involved in the regulation of placental growth and differentiation.

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The functions of placental estrogens in cattle are still poorly understood. In order to investigate a putative role as local regulators of placental growth, differentiation and functions via estrogen receptor beta (ERbeta), the expression of this ER isoform was determined in bovine placentomes obtained from cows at midgestation (days 110-150; n=7), late gestation (days 180-280; n=9), from prepartal cows (sampling immediately after the prepartal decline in maternal progesterone blood levels became obvious; n=3) and from cows at normal term (n=5) on the protein and mRNA level. By means of immunohistochemistry using a monoclonal antibody against human ERbeta (clone PPG5/10), nuclear signals were found in numerous cell types of the fetal (cotyledon) and maternal (caruncle) component of the placentomes with highest intensities in mature trophoblast giant cells (TGC), fetal and maternal vascular cells and caruncular stromal cells, in which the percentage of positive cells increased from 58.9+/-2.3 and 59.1+/-3.3 at mid- and late gestation to 65.1+/-4.9 and 69.4+/-4.2 in prepartal and parturient cows, resp. (p<0.0001). Clear staining of uninucleated trophoblast cells was only observed in prepartal and parturient animals. Conventional RT-PCR confirmed ERbeta expression in caruncles and cotyledons at all phases under investigation. With semi-quantitative real-time RT-PCR higher levels of ERbeta-specific mRNA were measured in cotyledons compared to caruncles (p<0.01). The results show that in contrast to ERalpha, which is only expressed in the caruncle, ERbeta is also extensively expressed in the cotyledon and suggest a role of ERbeta primarily in TGC differentiation and placental angiogenesis and/or vascular functions.