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RESEARCH ARTICLE
Contents Vol 35(2)

84 Low plasma progestogen concentration during the early luteal phase delays endometrial development and the beginning of placentation in mares

C. Okada A , M. Kaps A , I. Walter B C , C. Gautier D , J. Aurich D and C. Aurich A
+ Author Affiliations
- Author Affiliations

A Platform Artificial Insemination and Embryo Transfer, Department for Small Animals and Horses, Vetmeduni Vienna, Vienna, Austria

B VetCore Facility for Research/VetBiobank, Vetmeduni Vienna, Vienna, Austria

C Institute of Morphology, Vetmeduni Vienna, Vienna, Austria

D Section for Obstetrics, Gynecology, and Andrology, Department for Small Animals and Horses, Vetmeduni Vienna, Vienna, Austria

Reproduction, Fertility and Development 35(2) 168-168 https://doi.org/10.1071/RDv35n2Ab84
Published: 5 December 2022

© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of the IETS

The concentration of plasma progestogens is determinantal for pregnancy establishment in several animal species—influencing endometrial function, histotroph production, and conceptus development during early gestation. Recent studies demonstrated that experimentally reduced progestogen concentration during the early postovulatory phase in mares caused delayed progesterone receptors’ downregulation, growth of the embryo proper, and organogenesis until Day 34 of pregnancy. The aim of the present study was to extend the investigation by focusing on the endometrial and conceptus membrane responses to the reduced progestogen concentration. Eleven mares, after insemination and spontaneous ovulation, were assigned to treatment (TREAT) or control (CON) in a Latin square design for two consecutive pregnancies. While mares were left untreated during the CON pregnancy, they received PGF2α analogue cloprostenol (125 µg once daily from ovulation until Day 3; Estrumate, MSD) during the TREAT pregnancy. After conceptus collection and endometrial biopsies on Day 34 of pregnancy, the chorionic girdle (CG), allantochorion (AC), and endometrium were assessed for histomorphology, epidermal growth factor (EGF), and EGF receptor (EGFR) mRNA abundance (quantitative PCR) and protein expression (immunohistochemistry). In addition, the abundance of regulatory T lymphocytes (Tregs) in the endometrium was analysed by immunohistochemistry for forkhead box P3. This approach was chosen because Tregs are involved in maternal acceptance of the allogeneic conceptus in response to CG cells during endometrial cup formation. Statistical analysis was performed by nonparametric tests for paired samples with the IBM SPSS statistics software (version 27). The histomorphometric evaluation revealed more developed luminal and glandular endometrial epithelium as well as a higher CG epithelium in CON compared with TREAT pregnancies (P < 0.05). The mRNA of EGF and EGFR was highly abundant in the endometrium, CG, and AC, in both CON and TREAT pregnancies (n.s.). In the endometrium of TREAT pregnancies, the number of Treg cells was smaller compared with CON pregnancies (P < 0.05). In conclusion, the present study indicates detrimental effects of low plasma progestogen concentration during the early postovulatory period on endometrial development and endometrial cup formation in mares. These results explain the delayed development of the equine conceptus in mares treated with PGF2α analogues after ovulation.