Interspecific pregnancy: barriers and prospects.

Investigations on the mechanisms that allow survival of the fetal allograft have been extended by pregnancies in which the fetus and pregnant female are from different species. Such interspecific pregnancies are useful models for the study of maternal/fetal interactions and also may assist in the preservation of endangered species. Results of experiments with three different interspecific model systems are discussed: a murine model using Mus musculus and M. caroli; an equine model involving primarily the domestic horse and donkey, but including several wild Equidae; and a bovid model that crosses genera--pregnancy between the domestic sheep and goat. Species differences are reflected in results of experiments involving the various models. An immunological barrier appears to restrict interspecific pregnancy, but how the barrier is manifested appears to differ with species. Evidence for inappropriate interaction between trophoblast and endometrium is also presented. Results of experiments aimed at overcoming barriers to interspecific reproduction are discussed.