Steroid synthesis by equine conceptuses between days 7 and 14 and endometrial steroid metabolism.

The research study investigates whether the equine conceptus (early stage embryo) undergoes changes in its steroid synthesis around the period of maternal recognition of pregnancy, with a specific focus on the production of 17 alpha-hydroxyprogesterone and its metabolism in the endometrium.

Study Objective and Methodology

• The primary purpose of the study is to discern whether the horse blastocyst, which is an early-stage embryo, undergoes changes in its steroid synthesis around the time the mother horse starts recognizing the pregnancy.
• To evaluate this, embryos collected between day 7.5 and 14.5 were incubated for an 8-hour period in vitro in a medium known as HAM’s F10, a mixture that was further enriched with radio-labelled pregnenolone, which acts as a precursor for the production of all steroid hormones.
• The steroid metabolites that were released into the incubation medium were isolated via reverse phase High-Performance Liquid Chromatography (HPLC), and the main peaks displayed as a percentage of total metabolites.

Findings

• The study found that no significant changes were noted in the profile of metabolites throughout the period of the study, although an increased conversion was observed as the conceptuses (early stage embryos) developed.
• Contrary to their expectation of finding estradiol as the main metabolite, researchers discovered that the major metabolite produced was 17 alpha-hydroxyprogesterone.
• A subsequent experiment was conducted to verify if 17 alpha-hydroxyprogesterone was metabolized by the endometrial tissue- a layer of cells that line the uterus. During this experiment, endometrial biopsies from anestrous (non-reproductive) mares, and pregnant and non-pregnant mares on day 11 were incubated with radio-labelled 17 alpha-hydroxyprogesterone, progesterone, or pregnenolone.
• The study found that 17 alpha-hydroxyprogesterone, but neither progesterone nor pregnenolone, was converted to a more polar metabolite in all the tested groups. Further, the production of this metabolite was notably higher in the anestrous mares.
• The identity of the metabolite derived from 17 alpha-hydroxyprogesterone, however, remains unknown.

Conclusions

• The study concluded that 17 alpha-hydroxyprogesterone is the major steroid synthesized by the equine blastocyst.
• It was also observed that this steroid is further metabolized to an unidentified steroid by the endometrium.
• These synthesized and subsequently metabolised steroids could potentially play a crucial role in conceptus development and in the mare’s recognition of pregnancy.