Stewart James R, Thompson Michael B
Department of Biological Sciences, Box 70703, East Tennessee State University, Johnson City, TN 37614-1710, USA.
J Morphol. 2004 Feb;259(2):214-37. doi: 10.1002/jmor.10179.
A prominent scenario for the evolution of reptilian placentation infers that placentotrophy arose by gradual modification of a simple vascular chorioallantoic placenta to a complex structure with a specialized region for nutrient transfer. The structure of the chorioallantoic placenta of Niveoscincus ocellatus, apparently described originally from a single embryonic stage, was interpreted as a transitional evolutionary type that provided support for the model. Recently, N. ocellatus has been found to be as placentotrophic as species with complex chorioallantoic placentae containing a specialized region called a placentome. We studied placental development in N. ocellatus and confirmed that the chorioallantoic placenta lacks specializations found in species with a placentome. We also found that this species has a specialized omphaloplacenta. The chorioallantoic placenta is confined to the region adjacent to the embryo by a membrane, similar to that found in some other viviparous skinks, that divides the egg into embryonic and abembryonic hemispheres. We term this structure the "inter-omphalopleuric" membrane. The position of this membrane in N. ocellatus is closer to the embryonic pole of the egg than to the abembryonic pole and thus the surface area of the omphaloplacenta is greater than that of the chorioallantoic placenta. In addition, the omphaloplacenta is regionally diversified and more complex histologically than the chorioallantoic placenta. An impressive and unusual feature of the omphaloplacenta of N. ocellatus is the development of extensive overlapping folds in the embryonic component of mid-gestation embryos. The histological complexity and extensive folding of the omphaloplacenta make this a likely site of placental transfer of nutrients in this species.
爬行动物胎盘形成进化的一个显著设想认为，胎盘营养是通过将简单的血管性尿囊绒毛膜胎盘逐渐改造为具有专门营养物质转移区域的复杂结构而产生的。眼斑新石龙子的尿囊绒毛膜胎盘结构最初显然是根据单个胚胎阶段描述的，被解释为一种过渡进化类型，为该模型提供了支持。最近，人们发现眼斑新石龙子与具有包含称为胎盘小叶的专门区域的复杂尿囊绒毛膜胎盘的物种一样具有胎盘营养功能。我们研究了眼斑新石龙子的胎盘发育情况，证实其尿囊绒毛膜胎盘缺乏在具有胎盘小叶的物种中发现的特化结构。我们还发现该物种具有专门的脐胎盘。尿囊绒毛膜胎盘通过一层膜被限制在与胚胎相邻的区域，这层膜类似于在其他一些胎生物种中发现的膜，它将卵分为胚胎半球和非胚胎半球。我们将这种结构称为“脐周膜”。在眼斑新石龙子中，这层膜的位置更靠近卵的胚胎极而非非胚胎极，因此脐胎盘的表面积大于尿囊绒毛膜胎盘。此外，脐胎盘在区域上具有多样性，组织学上比尿囊绒毛膜胎盘更复杂。眼斑新石龙子脐胎盘一个令人印象深刻且不同寻常的特征是，妊娠中期胚胎的胚胎部分会发育出广泛的重叠褶皱。脐胎盘的组织学复杂性和广泛褶皱使其成为该物种胎盘营养物质转移的可能部位。
