Poly-implantation and Embryonic Death

Embryo implantation in the vizcacha begins at 18 days post-coitum (dpc). It occurs in the caudocranial direction, that is, from the caudal to the cranial uterine segments. For each uterine horn, between 5 and 6 implantation sites (IS) are found. In each IS, an interstitial and antimesometrial implantation is observed, with an elongated blastocyst due to the growth of the visceral endoderm. Between 26 and 70 dpc, the death and resorption of the embryos begins in the cranial and middle IS; on the other hand, the caudal IS keep their growth and development rate, giving rise to two precocial pups after 5 months of gestation. Former studies concluded that death was not due to infectious agents, superfetation, or macro and microscopic variations in the uterine horns. However, later studies revealed that the uterine horns are heterogeneous in their anatomy and physiology along their craniocaudal extension. It is proposed that the uterine glands and blood vessels would have a differential role between the IS that are resorbed and those that will survive. It was observed that embryonic death in this species follows a morphological pattern like that registered in murine rats and mice, considered classic physiological and experimental models. That pattern is characterized by the presence of hemorrhage, neutrophilic infiltration, necrosis, and fibrosis. Morphological and physiological similarities observed between related and phylogenetically distant species allow us to propose the vizcacha as an unconventional model for studying spontaneous embryonic death.