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

The research paper explores the impact of low plasma progesterone concentration during the early luteal phase in pregnant horses on the development of the endometrium, allantochorion (AC), and chorionic girdle (CG), hypothesizing that low progesterone levels may underlie delayed conceptus development.

Objective of the Study

• The study aims to understand the impacts of low plasma progesterone levels after ovulation on the development of the endometrium and on embryonic membranes. The researchers hypothesize that such reduction may be an underlying factor in delayed conceptus development in horses.

Methodology

• The researchers assigned eleven mares to either a treatment or control group across two pregnancies. In the treatment group, the mares received a PGF analogue for four consecutive days, starting from the day of ovulation to lower progesterone secretion.
• In the control group, the mares didn’t receive any treatment and served as their own controls.
• Data was collected on day 34 of pregnancy, including endometrial biopsies for the analysis of histomorphology, epidermal growth factor (EGF), and EGF receptor mRNA and protein expression in the endometrium, the AC, and the CG. The researchers also observed the number of regulatory T lymphocytes (Tregs).

Findings

• A comparative analysis of the histomorphometry of the luminal endometrium and CG epithelium in treated versus control pregnancies revealed higher numbers in the control group.
• The expression levels of EGF and EGFR mRNA were largely present in the endometrium, AC, and CG, however, there was no significant difference between the treatment and control groups.
• The researchers found a reduced number of endometrial regulatory T lymphocytes in the treated group compared to the control group, suggesting that reduced progesterone concentrations may have other detrimental effects on equine pregnancy.