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

Gross and ultrasonic morphology of the equine conceptus on days 10 to 40

The gross and ultrasonic equine embryo morphology are described with emphasis on specific days after ovulation. Included are labeled colored photographs and detailed descriptions of the embryo proper (future fetus and foal) and of the entire embryonic vesicle on Days 21, 24, 30, 35/36, and 40. A few related aspects are included for the early fetus on Days 45 and 50. Regression lines for growth in the length of the embryo proper and diameter of the embryonic vesicle along with the mean days of the morphological event are included. Ultrasonograms of the embryonic vesicle are shown and discussed from Days 10 to 45. Major morphological changes in the embryo proper include: (1) appearance of forelimb and hindlimb buds, (2) appearance of the pontine flexure, (3) appearance of the genital tubercle, (4) closure of the pontine flexure, and (5) tapering of limbs toward the midline with hoof-shaped tips. Major changes in the embryonic vesicle are: (1) vascularization of mesoderm, (2) appearance of sinus terminalis, (3) emergence of allantoic sac, (4) formation of embryonic circulatory system, (5) formation and maturation of chorionic girdle, and (6) transition from yolk sac to allantoic sac.

Physical activities and morphologic aspects of the equine fetus during Days 40 to 150

Intrauterine mobility of the fetal-amniotic unit is unique in equids among domestic species. Intrinsic activity begins as head nods on ∼ Day 40 (Day 0 = ovulation) and by Day 60 has progressed into intermittent subtle to vigorous head, neck, limb, and body movements. On Days 60-100, fetal mobility is maximal with traveling of the fetal-amniotic unit throughout an allantoic pool that encompasses the uterine horns and uterine body. The fetus may be entirely within one uterine horn with the horn entrance closed behind it, and then may work through the entrance into the uterine body. Mobility gradually decreases after Day 100 with a decrease in relative quantity of allantoic fluid but intrinsic activity continues. Changes in each of fetal intrauterine location, presentation, and recumbency can occur frequently (e.g., 5-min intervals). About 80% of fetal mobility is from the propulsive effects of intrinsic fetal activities, and the remainder is from currents and shifts in the allantoic pool. The fluid currents are attributable to transient uterine constrictions that vary from 10 mm in width to the length of a uterine horn and to extrinsic mare and adjacent visceral activity. The fetus is tethered by a long umbilical cord attached at the mid-uterus allowing travel throughout an allantoic sac that involves the entire uterus. The mobile fetus seemingly practices the neuromuscular coordination that it will need during independent life. The theriogenologist can educate and fascinate onlookers by ultrasound demonstrations of bursts of fetal activity and mobility. An online video is included.