Fixation and orientation of the early equine conceptus.

The research looked into how the early equine conceptus (embryo and its attachments) secures itself and orientates within the uterus of non-pregnant horses. Forty mares were considered in this study. Ultrasound images revealed that the conceptus underwent significant changes in shape, size, and position before stabilizing.

Conceptus Growth and Shape

• The study observed the conceptus daily using ultrasound from day 11 to day 21, and then every three days until day 48.
• The growth of the conceptus was found to have a steady plateau phase between days 17 and 24, where its enlargement seemed to temporarily halt.
• The size of the conceptus was measured by its width and area depicted in ultrasound images, growing at an average of 3-4mm daily before the plateau, and 2-3mm per day after the plateau.
• The conceptus was initially spherical in shape between days 11-16. On day 17, the shape changed to an oblong. It further altered to a triangular shape from days 18-21 and became irregular from days 24-48.

Embryo Detection and Position

• The embryo was first identified in a few mares on day 20 and in a majority of them by day 21.
• When first detected, the embryo always positioned itself on the ventral aspect of the vesicle, opposite the side connected to the mesometrial attachment.
• The embryo gradually moved towards the mesometrial attachment, occurring simultaneously with the growth of the allantois (a part of a mammalian placenta) located beneath the embryo from days 21 to 36.

Hypotheses for Vesicle Fixation and Orientation

• The investigators hypothesized that the cessation of mobility of the vesicle was due to the increasing uterine tone, leading to compression of the endometrial folds that envelops the vesicle.
• The orientation of the vesicle was possibly due to the comparatively robust yolk sac wall at the embryonal pole, and the greater pressure exerted on the vesicle by the dorsal uterine wall compared to the ventral wall.