Stewart James R, Mendez de la Cruz Fausto R
Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee.
Departamento de Zoologia, Instituto de Biologia, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico.
J Morphol. 2019 Jan;280(1):35-49. doi: 10.1002/jmor.20912. Epub 2018 Nov 26.
The evolution of viviparity alters the physical relationship between mothers and offspring and the prevalence of viviparity among squamate reptiles presents an opportunity to uncover patterns in the evolution of placental structure. Understanding the breadth of this diversity is limited because studies of placental structure and function have emphasized a limited number of lineages. We studied placental ontogeny using light microscopy for an embryological series of the Mexican gerrhonotine lizard, Mesaspis viridiflava. This species develops an elaborate yolk sac placenta, an omphaloplacenta, which receives vascular support arising in a structure known only from other gerrhonotine lizards. A prominent feature of the omphaloplacenta is a zone of uterine and embryonic epithelial cell hyperplasia located at the upper shoulder of the yolk mass, often extending above the yolk mass. The omphaloplacenta covers more than one-half of the surface area of maternal-embryonic contact. The chorioallantoic placenta has a more restricted distribution because the allantois remains in the embryonic hemisphere of the egg throughout development and lies internal to the vascular support for the omphaloplacenta in areas where they overlap. The structural profile of the chorioallantoic placenta indicates a potential for respiratory exchange and/or hemotrophic nutritive transport, while that of the omphaloplacenta suggests that nutritive transfer is primarily via histotrophy. An eggshell is present in the earliest embryonic stages examined but regresses relatively early in development. Placental specializations of this species are consistent with a pattern of matrotrophic embryonic nutrition and have evolved in a unique lineage specific developmental pattern.
胎生的演化改变了母体与后代之间的生理关系，而有鳞目爬行动物中胎生的普遍存在为揭示胎盘结构演化模式提供了契机。由于对胎盘结构和功能的研究集中在少数几个谱系上，我们对胎盘个体发育的了解有限。我们利用光学显微镜对墨西哥吉氏石龙子（Mesaspis viridiflava）的胚胎系列进行研究，以探究胎盘个体发育。该物种发育出一种复杂的卵黄囊胎盘，即脐胎盘，它接受来自仅在其他吉氏石龙子中才有的一种结构所产生的血管支持。脐胎盘的一个显著特征是位于卵黄团上肩部的子宫和胚胎上皮细胞增生带，通常延伸至卵黄团上方。脐胎盘覆盖了母胎接触表面积的一半以上。尿囊绒膜胎盘的分布范围较窄，因为尿囊在整个发育过程中都留在卵的胚胎半球内，并且在与脐胎盘血管支持重叠的区域位于其内部。尿囊绒膜胎盘的结构特征表明其具有呼吸交换和/或血源性营养运输的潜力，而脐胎盘的结构特征表明营养转移主要通过组织营养。在所检查的最早胚胎阶段存在蛋壳，但在发育过程中相对较早退化。该物种的胎盘特化与母源性胚胎营养模式一致，并且是在独特的谱系特异性发育模式中演化而来的。
