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.

The corpus luteum is the main source of progesterone (P(4)) responsible for maintenance of gestation in cattle. So far it has not been possible to assign any biological role to placental P(4), which contributes only marginally and temporarily to peripheral maternal blood levels. In order to identify possible P(4) target cells within the placenta, placentomes from 150-, 220-, 240-, and 270-day-pregnant cows and from parturient cows (3 animals per group) were screened immunohistochemically for expression of the progesterone receptor (PR). During gestation, PR-positive staining was found exclusively in the nuclei of caruncular stromal cells (CSC; maternal part of the placentome) and of caruncular vascular pericytes. In placentomes from parturient cows, occasional positive nuclear staining was also observed in the walls of small caruncular arteries. The percentage of PR-positive CSC increased slightly from 51.8 +/- 2.6% on Day 150 to 56.2 +/- 5.6% at Day 270 (p < 0.05) and was 58.9 +/- 1.8% at parturition. These results suggest that in pregnant cattle, CSC are under the control of P(4) of placental rather than luteal origin. Thus, whereas luteal P(4) may regulate "coarse" systemic progestational functions in the maternal compartment in the classical hormonal manner, placental P(4) may act as a paracrine factor involved in the local regulation of caruncular growth, differentiation, and functions.

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Contents In order to characterize proliferative activity in bovineplacentomes, the expression of the proliferation marker Ki67‐antigen was investigated immunohistochemically at days 150, 220, 240, 270 of gestation and at parturition (n = 3 animals/group). The percentage of Ki67‐antigen‐positive cells ([%Ki67+]) was determined for the caruncular stroma cells (CS), caruncular epithelium (CE), trophoblast (T) and stromal cells of chorionic villi (fetal stroma, FS). The proliferation pattern as indicated by [%Ki67+] was substantially different (p < 0.0001) between the four cell categories with the highest proliferation rates in CE (58.0 ± 6.9 to 68.3 ± 5.7), followed by CS (10.6 ± 3.4 to 45.3 ± 5.4), T (23.3 ± 3.4 to 25.4 ± 4.7) and FS (2.9 ± 0.4 to 10.5 ± 1.7). Influence of gestational age between days 150 and 270 was significant for FS (p < 0.01) with a linear trend (regression coefficient: ‐0.056%/day; p < 0.01) and for CS (p < 0.02) with a nonlinear trend (p < 0.05) which was described by a polynomial model: y = 220–1.96x + 0.0046x2 with y = [%Ki67+] and x = day of gestation (p < 0.05). The results suggest that the continuous high proliferation of CE is partly independent from total caruncular growth and may therefore also serve the permanent tissue remodeling and the compensation of the destructive activity of weakly invasive trophoblast giant cells, and that proliferation of CS is transiently suppressed between days 150 and 270 of gestation. An involvement of placental oestrogens in the control of proliferation in CE and CS is suggested.