Osmolality of equine blastocyst fluid from day 11 to day 25 of pregnancy.

This study aims to understand why horse embryos collected from day 11 to day 18 of pregnancy float in a liquid similar to, or equally concentrated as, body fluids. The research involved the analysis of equine blastocyst fluids from various pregnancy stages for osmolality and concentrations of different substances. The results indicate that while certain substance concentrations increase over time, the osmolality never exceeds a certain level. However, the reason for the relatively low concentration of substances in equine blastocyst fluid, which may contribute to the floating phenomenon, is still unclear.

Study Overview and Methodology

• The research examined equine (horse) conceptuses, or embryos, collected between day 11 and day 18 of pregnancy.
• The study aimed to investigate why these early-stage embryos float in isotonic media, which is a liquid with a concentration of substances equal to that in bodily fluids.
• To this end, the research analyzed the blastocyst fluids from 30 conceptuses derived from 13 mares for osmolality – a measure of the total concentration of solute particles per kilogram of water – and the concentrations of substances including sodium (Na+), chloride (Cl-), potassium (K+), glucose, urea, and creatinine.

Findings

• The average osmolality found from Day 11 to Day 16 of pregnancy was 121.4 mOsm kg-1, while the concentrations of Na+, Cl-, K+, glucose, urea, and creatinine were 11.0 mM, 29.3 mM, 26.2 mM, 0.6 mM, 6.0 mM, and 9.6 µM respectively.
• From day 16 to day 25, the concentrations of osmolality and sodium increased gradually. However, the concentration of osmolality never exceeded 255 mOsm kg-1.

Experimentation with Different Osmolalities

• The researchers used eight conceptuses exposed to saline solutions of varying osmolality levels over different periods. They noted that a 1-3mm increase in the perivitelline space, located at the bottom of the floating conceptus, became evident after 45 minutes of exposure to solutions with osmolalities equal to or greater than 300 mOsm kg-1.
• This finding suggests that sodium and chloride ions are able to freely diffuse through the capsule enveloping the blastocyst but not through the trophoblast, the outer layer of the blastocyst which forms the placenta.

Conclusion and Further Questions

• The research reveals that the hypo-osmolality – condition of having lower concentration of substances – of equine blastocyst fluid plays a role in why the conceptus floats in isotonic media. However, the exact significance of this finding, and how it pertains to the processes of equine embryogenesis and implantation, remains unclear. This provides an avenue for future research.