Placentation and Placenta

Gestation in the vizcacha lasts approximately 153 days. At 26–32 days post-coitus (dpc), extensions of the trophoblast invading the decidua are observed at caudal implantation sites (IS), marking the beginning of placentation. During later gestational stages (46–53 dpc), both the main placenta and a subplacenta are observed. The formed placenta is hemochorial and chorioallantoic. Its shape, size, and organization have been related to the development and delivery of precocial offspring. The subplacenta, unique to hystricomorphs, varies in size during gestation and serves as a source of trophoblasts. In advanced stages, the subplacenta exhibits necrotic areas with dystrophic calcification. Based on the positive response to the PAS histochemical technique and comparisons with findings in other hystricomorphs, it is hypothesized that this organ may have an endocrine function. The main placenta is covered by an avascular parietal sac. As occurs in other hystricomorphs, the visceral yolk sac forms an inverted vitelline placenta that persists throughout gestation. In early and intermediate stages of vizcacha gestation, two cell types are observed: giant trophoblastic cells (GTC) located at the trophospongium edges and infiltrating the decidua, and uterine natural killer cells (uNK), found near the uterine arteries. These uNK cells may be associated with the invasion and remodeling of uterine arteries and, consequently, to placentation. However, both cells show inconsistencies among different hystricomorph species. The division in placenta and subplacenta and the fibrovascular ring of the visceral yolk sac are characters common to all hystricomophs studied and found in L. maximus too, these characters reforze the hypothesis of the monophyletic origin of the group.