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Immunolocalization of Progesterone Receptors in Bovine Placentomes Throughout Mid and Late Gestation and at Parturition¹

^a Klinik für Geburtshilfe, Gynäkologie und Andrologie der Groß- und Kleintiere mit Tierärztlicher Ambulanz, and ^b Institut für Veterinär-Anatomie, -Histologie und -Embryologie, Justus-Liebig-Universität, D-35392 Giessen, Germany

The corpus luteum is the main source of progesterone (P₄) responsible for maintenance of gestation in cattle. So far it has not been possible to assign any biological role to placental P₄, which contributes only marginally and temporarily to peripheral maternal blood levels. In order to identify possible P₄ 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 P₄ of placental rather than luteal origin. Thus, whereas luteal P₄ may regulate "coarse" systemic progestational functions in the maternal compartment in the classical hormonal manner, placental P₄ may act as a paracrine factor involved in the local regulation of caruncular growth, differentiation, and functions.

¹ Supported by the German Research Foundation (DFG) grant SCHU 1195/1–1 and the Hilde und Ewald Berge-Stiftung.

² Correspondence: Gerhard Schuler, Klinik für Geburtshilfe, Gynäkologie und Andrologie der Groß- und Kleintiere mit Tierärztlicher Ambulanz, Justus-Liebig-Universität Giessen, Frankfurter Strasse 106, D-35392 Giessen, Germany. FAX: 49 641 29328; gerhard.schuler{at}vetmed.uni-giessen.de

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