Placentation and embryo death in the plains viscacha (Lagostomus maximus).

Caviomorpha are an exceptional group among rodents due to their extended gestational period and the delivery of precocial offspring. Among them, Lagostomus maximus is characterized by its polyovulation, polyembryony, and the highest embryonic death known in mammals. Its chorioallantoic placenta is hemomonochorial, an ancestral character among rodents. It resembles more the human placenta than the murine models. As in all caviomophs, the chorioallantoic placenta is divided in a main placenta and a subplacenta. The former is organized in labyrinth lobes surrounded by trophospongium, as in most caviomorphs. The giant cells (more numerous than in other caviomorphs) near the decidua could be related to invasiveness. During placentation of L. maximus, uterine natural killer cells are found. These cells have been related to invasiveness and remodeling of blood vessels in Mus musculus and Homo sapiens, although in other caviomorphs are not frequently found. In L. maximus, the placenta develops in all conceptuses (5-6 per uterine horn). Necrosis was observed in each implantation site at day 70 post-coitum, except in that closest to the vagina in each horn. This process of embryo death followed by resorption begins at day 26-30 post-coitum. Recently, we found variations in the percentage of blood vessel and uterine gland areas that could explain the regional differences in embryo survival. The characteristics of the placenta and implantation of L. maximus are important to stablish a unique model for studying placentation as well as early embryonic death, of interest for human and veterinary medicine.