Reproduction in mules: Embryo transfer using sterile recipients

Abstract

SummaryNine intraspecies horse embryos and four intraspecies donkey embryos were recovered non‐surgically on Days 7 or 8 after ovulation and transferred surgically to recipient mules. Five of the horse embryos and three of the donkey embryos were transferred to cycling mules that had ovulated five to seven days before transfer as determined by serial plasma progesterone measurements. The remaining embryos, four horse and one donkey, were transferred to anoestrous mules that were given daily oral doses of the synthetic progestogen, allyl trenbolone, to simulate dioestrus.Three horse‐in‐mule and one donkey‐in‐mule pregnancies were established in the cycling recipients. One horse‐in‐mule conceptus was removed surgically on Day 73 for histological examination of the endometrial cups and other placental tissues, and the other three mules foaled normally after 366, 344 and 357 days of gestation respectively.One horse‐in‐mule pregnancy was established in an anoestrous recipient that received allyl trenbolone. Despite the presence of very high levels of equine chorionic gonadotrophin in the serum of this mule after Day 40, its endogenous plasma progesterone levels remained basal. The transferred foetus died spontaneously at about Day 55 and the conceptus was resorbed. A hysterotomy was performed on Day 84 for examination and recovery of the persisting endometrial cups.These experiments demonstrated that some female mules, although infertile, can ovulate and cycle regularly. Their reproductive tracts are capable of supporting the development of either horse or donkey cenceptuses to term. Furthermore, the development of a horse conceptus to Day 55 in one of the non‐cycling mules given allyl trenbolone indicated that progesterone is the only steroid hormone that is essential for the establishment of pregnancy in equids.