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Twin embryos in mares I: from ovulation to fixation
Author(s) -
GINTHER O. J.
Publication year - 1989
Publication title -
equine veterinary journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.82
H-Index - 87
eISSN - 2042-3306
pISSN - 0425-1644
DOI - 10.1111/j.2042-3306.1989.tb02132.x
Subject(s) - ovulation , andrology , fixation (population genetics) , embryo , horse , biology , gynecology , medicine , pregnancy , genetics , paleontology , gene
Summary Recent findings on the origin and development of twins from ovulation (Day 0) to fixation (mean: Day 16) are reviewed. Available data show that almost all twins originate from multiple ovulations. Results of recent ultrasound studies indicate that the number of days between double ovulations does not affect the conception rate per ovum or embryo survival during the first 16 days after each ovulation. Embryo reduction is the natural elimination of excess embryos so that only one embryo enters the foetal stage. In two studies, embryo reduction before or on the day of fixation was not considered an important aspect of the natural correction of twins. Diameters and growth rates on Days 11 to 16 were similar between singletons and twins and the presence of two vesicles did not have a direct effect on their diameter other than that attributable to their age. Twin and singleton embryonic vesicles were mobile within the uterine lumen from the first day of detection (Days 9 to 11) to the day of fixation (mean: Day 16). In one study, the embryonic vesicles were in the uterine body for over 50 per cent of the time during Days 9 to 12 and thereafter were most often in the uterine horns. For twins of dissimilar size, the preference for uterine body versus horns appeared to be an independent function of each vesicle based on its age or size. After Day 12, the number of entries from the uterine body into the horns increased and the vesicles began a maximum mobility phase which continued until fixation. Both members of twin sets were in a given uterine segment more often than would be expected by chance. Results of ultrasound studies indicated that uterine contractions are the propulsive force for conceptus mobility, apparently in response to the production of a stimulant by the conceptuses. Greatest uterine contractility coincided with the period of maximum conceptus mobility. The increase in contractility began four days earlier in pregnant than in nonpregnant mares. Fixation probably occurs when an embryonic vesicle becomes so large and uterine tone so great that the vesicle can no longer move despite the continuation of uterine contractions. Combined data showed that fixation of twins occurred unilaterally more often than would be expected by chance. The incidence of unilateral fixation increased greatly when vesicles were dissimilar in diameter or originated from asynchronous ovulations.