The continuum of events leading to maternal recognition of pregnancy in mares.

This research investigates how certain substances within horses’ bodies influence the early stages of pregnancy recognition. It outlines that a factor present in the horse, with a molecular weight between 1000 and 6000, puts a stop to the production of a particular pregnancy-influencing substance (PGF), with the effectiveness being tie to the mobility of the conceptus.

Experiments and Results

• The study was conducted on the endometria of pregnant mares (female horses). The endometria were able to produce PGF (Prostaglandin F2 Alpha), which is a substance that influences pregnancy progression, in laboratory conditions.
• When the endometria were co-incubated with conceptus membranes (tissue including or surrounding the embryo), the quantity and rate of PGF production were markedly decreased.
• To determine the molecular weight of the compounds that influenced PGF production, the conceptus membranes from mares at 14 days into pregnancy were placed inside bags made from dialysis tubing and co-incubated with pregnant Day-14 mare endometria.

Molecular Weight of Inhibitors

• The production of PGF was notably less when the membranes were in bags with molecular weight exclusion limits of 12,000, 6000, and 3500. However, it was unchanged when the limit was 1000, suggesting that the inhibitory factor(s) has a molecular weight larger than 1000 but smaller than 6000.
• Prostaglandin production at a molecular weight of 3500 was only slightly different from the control endometria, which indicates that this molecular weight is close to the threshold.

The Transience and Need for Mobility

• As the inhibitory factors seemed to be short-lived, the importance of conceptus mobility (movement of embryo and its related tissues) in preventing PGF production was tested.
• On the 4th day of pregnancy, constricting structures were placed around the uterine horns at the division point that was either on the same side or the opposite side as the ovulation site.
• When the embryo and its surrounding tissue’s mobility was drastically restricted (ligatures on the same side as ovulation), pregnancy failed in 88% (7/8) of the mares.
• When the ligatures were on the opposite side to the ovulation, pregnancy failed in 50% (2/4) of the mares.
• However, when the ligatures were placed around the upper tip of the uterine horn on the side opposite to ovulation, all 5 of the mares established pregnancy.

Implications of the Research

• These findings suggest that the inhibitory factor has a molecular weight between 1000 and 6000.
• The short-lived nature of these factors highlights the need for the embryo and its surrounding tissues to move extensively around the majority of the uterus to inhibit the production of PGF.