Post-implantation Embryo Development in the South American Plains Vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

The early embryo development after implantation varies among mammals. In muroid rodents, an egg cylinder is formed by folding during gastrulation; however, in most mammals, a flat disc develops after the embryo implants in the uterine wall. In contrast to what occurs in muroids, the embryo of the caviomorph Lagostomus maximus develops from a bilaminar disc by cavitation rather than folding. The flat disc morphology of the epiblast is shared with non-rodent mammals such as rabbits, pigs, cows, and humans and is considered the mammotypic embryonic morphology. Nevertheless, the flat embryo of L. maximus is a somewhat intermediate situation between the mammotypical bilaminar disc and the egg cylinder of muroids. In muroid rodents, the extraembryonic ectoderm is in close contact with the epiblast as a consequence of embryo folding during gastrulation. In contrast, the typical flat disc of mammals does not show extraembryonic ectoderm. L. maximus embryo develops extraembryonic ectoderm homologous to that found in muroids; however, it remains far apart from the epiblast by the visceral endoderm. Here we describe the embryonic and fetal development throughout gestation in L. maximus by discussing similarities and differences with other mammals from an evolutionary and phylogenetic perspective.