The Dynamic Equine Embryo from Postfixation (Day 17) to the End of the Embryo Stage (Day 40).

The research paper investigates the transformation of equine embryo from Day 17 to Day 40, detailing various processes such as rotation, orientation, expansion, and contraction within the uterus. It also describes a natural phenomenon in which one of the twins in a unilateral pregnancy is eliminated, a process referred to as natural embryo reduction.

Embryo’s Post-Fixation Transformation

• The study kicks off by discussing the observation that after the embryo fixation that occurs roughly on Day 16, the embryonic vesicle undergoes a rotation or orientation. This orientation ensures the proper embryo is positioned opposite the mesometrial attachment.
• During this process, the vesicle thickens, particularly at the pole that contains the proper embryo. This action is enabled by the rhythmic contractions of the turgid uterine horn which effectively ‘massage’ the vesicle.
• The research then notes that the thickening of the dorsal endometrium results in what could be described as a guitar-pick shape of the vesicle when viewed from a cross-section of the uterine horn.

Expansion and Interaction of the Allantoic and Yolk Sac

• From Days 21 to 40, the allantoic sac expands subsequently displaying a proportionally smaller yolk sac. This phase features the transposition of the embryo proper to the dorsal aspect of the embryonic vesicle.
• Further, the blood vessels originating from the embryonic vesicle converge at the mesometrial attachment, marking the initial stages of the formation of the umbilical cord.
• As Day 40 approaches, which signals the transition from the embryo to the fetal stage, the umbilical cord elongates in correlation with the fetus’s descent towards the base of the allantoic sac.

Natural Embryo Reduction

• The study also described the natural twins’ embryo reduction phenomenon, which commonly occurs in unilateral pregnancies. An estimated 85% of these cases result in the elimination of one of the embryos.
• During this process, the twins are subjected to uterine contractions. Involuntarily, a significant proportion of the doomed embryonic vesicle’s thicker wall is compelled into the thinner wall of its surviving twin.
• However, the natural embryo reduction process is not observed in the occurrence of bilateral twins, thus necessitating intervention from a theriogenologist to facilitate the reduction.