The Dynamic Equine Embryo

What a wonderful description of the embryo stage of life. Also is a reminder of the effects of twinning and proper management.
DrO

The Dynamic Equine Embryo from Postfixation (Day 17) to the End of the Embryo Stage (Day 40)
J Equine Vet Sci. 2022 Jan;108:103808. doi: 10.1016/j.jevs.2021.103808. Epub 2021 Nov 13.
Author
O J Ginther 1
Affiliation

1 Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, Madison, WI. Electronic address: oj.ginther@wisc.edu.

PMID: 34883360
DOI: 10.1016/j.jevs.2021.103808

Free article
Abstract

After the cessation of equine embryo mobility (fixation) on mean Day 16, the embryonic vesicle is rotated or oriented so that the pole with the embryo proper is opposite to the mesometrial attachment. Orientation involves massage of the vesicle by contractions of the turgid uterine horn and greater thickening of the vesicle at the pole with the embryo proper. Thickening of the dorsal endometrium (encroachment) especially on each side of the mesometrial attachment accounts for a guitar-pick shape of the vesicle when viewed in cross section of the uterine horn. On Days 21-40, the allantoic sac expands, and the relative size of the yolk sac diminishes highlighted by carrying of the embryo proper to the dorsal aspect of the embryonic vesicle. There, the blood vessels from the embryonic vesicle meet at the mesometrial attachment to become the beginning of the umbilical cord. At the end of the embryo stage and beginning of the fetal stage (Day 40), the umbilical cord lengthens in association with the descent of the fetus to the bottom of the allantoic sac. After unilateral fixation of twins, a natural embryo reduction process frequently (∼85%) eliminates one of the embryos. The twins are massaged by the uterine contractions, and a critical proportion of the thicker wall of the doomed embryonic vesicle is forced into the thinner wall of the survivor. Natural embryo reduction does not occur for bilateral twins and reduction requires intervention from the theriogenologist.

Keywords: Allantoic sac; Encroachment; Orientation; Twin reduction; Yolk sac.