The placentation of eulipotyphla-reconstructing a morphotype of the Mammalian placenta.

Placentation determines the developmental status of the neonate, which can be considered as the most vulnerable stage in the mammalian life cycle. In this respect, the different evolutionary and ecological adaptations of marsupial and placental mammals have most likely been associated with the different reproductive strategies of the two therian clades. The morphotypes of marsupial and placental neonates, as well as the placental stem species pattern of Marsupialia, have already been reconstructed. To contribute to a better understanding of the evolution of Placentalia, a histological and ultrastructural investigation of the placenta in three representatives of Eulipotyphla, that is, core insectivores, has been carried out in this study. We studied the Musk shrew (Suncus murinus), the four-toed hedgehog (Atelerix albiventris), and the Iberian mole (Talpa occidentalis). As a result, a eulipotyphlan placental morphotype consisting of a compact and invasive placenta was reconstructed. This supports the widely accepted hypothesis that the stem lineage of Placentalia is characterized by an invasive, either endothelio- or hemochorial placenta. Evolutionary transformations toward a diffuse, noninvasive placenta occurred in the stem lineages of lower primates and cetartiodactyles and were associated with prolonged gestation and the production of few and highly precocial neonates. Compared to the choriovitelline placenta of Marsupialia, the chorioallantoic placenta of Placentalia allows for a more intimate contact and is associated with more advanced neonates.

Morphology and ultrastructure of the chorioallantoic placenta of the Iberian mole (Talpa occidentalis) with special reference to heterophagous areolas and the nature of interhaemal barrier.

This study provides a contribution to the reconstruction of the eulipotyphlan placental morphotype and also may help resolving a long-standing conflict about the interhaemal barrier in moles. As detailed descriptions of talpid placentation, only available for Talpa europaea and Scalopus aquaticus, led to a controversial debate on the nature of interhaemal barrier, the collection of more placental data of further mole species was strongly desired. Hence, the placentas of six gestational stages of Talpa occidentalis have been studied concerning their morphogenesis and ultrastructure with special focus on the structure of the interhaemal barrier and heterophagous regions. Generally, the mode of placentation in T. occidentalis resembles that of T. europaea, including a broad, discoid, antimesometrial, definitive chorioallantoic placenta of labyrinthine type being still villous in earlier stages. Within the labyrinth, the zona intima shows an endotheliochorial interhaemal barrier with a two-layered trophoblast. This clearly contradicts former statements on the S. aquaticus placenta made by Prasad et al. (1979), although their findings cannot exclude a totally different interpretation. Regardless, the placenta of moles represents the least invasive mode of placentation among Eulipotyphla, which otherwise have highly invasive placentas. Although the phagocytic areolas situated above uterine gland openings are heterophagous, they mainly seem to serve fetal histiotrophic nutrition, at least early in pregnancy. In later stages the number of glands and areolas decreases. This special type of additional phagocytic region is usually most common in species with noninvasive, epitheliochorial placentation, which suggests a correlation between placental invasiveness and the occurrence and type of phagocytic placental structures. The compact and invasive mode of placentation of Talpidae and all other Eulipotyphla seems to be plesiomorphic within Laurasiatheria and is always correlated with an altricial neonate.