Between‐species transfer of horse and donkey embryos: A valuable research tool

Abstract

SummaryDuring the six year period between 1979 and 1984 attempts to recover embryos non‐surgically at Days 6 to 8 after ovulation (Day 0) were successful on 15 of 30 occasions (50 per cent) from mated pony mares and on 112 of 210 occasions (53 per cent) from mated or artificially inseminated jenny donkeys. Surgical transfer of nine horse embryos to jenny donkeys and 95 donkey embryos to pony mares resulted in five horse‐in‐donkey and 64 donkey‐in‐horse pregnancies (56 and 67 per cent, respectively).The striking differences in placentation and hormone secretion patterns that exist between these two types of extraspecific equine pregnancy make them ideal models to study foetomaternal interactions during pregnancy and immunological aspects of early foetal death. Donkeys appear able to carry horse foetuses to term, perhaps because of normal endometrial cup development and the resulting high secretion rates of equine chorionic gonadotrophin (eCG) and progesterone. Conversely, endometrial cups fail to develop in horses carrying donkey foetuses, the great majority of which grow normally to Day 70 of gestation but thereafter succumb to a vigorous maternal cell‐mediated reaction directed against the donkey allantochorion.Administration to mares carrying donkey conceptuses of: (a) Synthetic progestogen; (b) a partially purified extract of eCG; (c) large volumes of pregnant and non‐pregnant mares' serum; and (d) repeated injections of lymphocytes from the genetic donkey sire and dam of the embryo, resulted in a significant improvement in pregnancy survival only in those mares treated with pregnant mares' serum and parental donkey lymphocytes. The hypothesis is therefore advanced that failure of the untreated donkey‐in‐horse pregnancy results from the absence of the normal antigenic stimulation of the maternal immune system to provide immunoprotection for the foreign conceptus. In all other types of intra‐, inter‐ and extraspecies equine pregnancy this occurs during invasion of the maternal endometrium by the specialised foetal trophoblast cells to form endometrial cups.