Placentation and Placenta

Abstract

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