What has embryo recovery and transfer taught us about equine reproductive physiology?

Equine embryo transfer has been the foundation stone on which modern reproductive technologies and novel research in this species has been built. Nonsurgical embryo recovery and transfer methods developed from the original surgical methods, and the ease with which these procedures can be undertaken has enabled these techniques to play a continuing role in original research. We document how embryo recovery and/or transfer have increased our knowledge about many of the events involved in equine reproductive physiology. These include, when the equine embryo enters the uterus, the importance and development of the blastocyst capsule, secretory products of early embryos, early maternal-embryo interactions, effects of uterine asynchrony on early embryo development, the role of endometrial cups and maternal immunological tolerance of extraspecies embryo transfers, and the influence of uterine environmental on fetal growth and development.