Interspecific Pregnancy - Causes of Failure

Causes of Failure

Immunologically, an embryo or fetus of an interspecific pregnancy would be equivalent to xenografts rather than allografts, putting a higher demand on gestational immune tolerance in order to avoid an immune reaction toward the fetus. Some mice experiments indicate an imbalance between Th₁ and Th₂ helper cells with a predominance of Th₁ cytokines. However, other mice experiments indicate that an immune response towards xeno-fetuses does not belong to classical cytotoxic T lymphocyte or natural killer cell pathways.

Interspecies compatibility is related to the type of placentation, as mothers of species having the more invasive hemochorial placentation (such as humans) must create a stronger downregulation of maternal immune responses, and are thereby more receptive to fetuses of other species, compared to those with endotheliochorial (e.g. cats and dogs) or epitheliochorial placentation (e.g. pigs, ruminants, horses, whales), where there is no contact between the maternal blood and the fetal chorion.

Other potential hazards include incompatibility of nutrition or other support system. Notably, there is a risk of inappropriate interactions between the trophoblast of the fetus and the endometrium of the mother. For example, the placental glycosylation pattern at the fetomaternal interface should optimally be similar to that of the host species.

A gaur (left) embryo may develop to term in cattle (right), but with severe intrauterine growth restriction.

Yet, for some species, such as a Bactrian camel embryo inside a Dromedary, pregnancy can be carried to term with no other intervention than the embryo transfer. This is possible for gaur embryos inside cattle as well, but with severe intrauterine growth restriction, with uncertainty of how much is caused by the IVF procedure itself, and how much is caused by interspecies incompatibility.

The ability of one species to survive inside the uterus of another species is in many cases unidirectional; that is, pregnancy would not necessarily be successful in the inverse situation where a fetus of the other species would be transferred into the uterus of the first one. For example, horse embryos survive in the donkey uterus, but donkey embryos perish in the uterus of an untreated mare. Deer mouse embryos survive in the uterus of the white-footed Mouse, but the reciprocal transfer fails.