Laboratorio de Biología Tisular y Reproductora, Departamento de Biología Comparada, Facultad de Ciencias, Universidad Nacional Autónoma de México, México City, México.
J Morphol. 2024 Oct;285(10):e21783. doi: 10.1002/jmor.21783.
Extraembryonic membranes provide protection, oxygen, water, and nutrients to developing embryos, and their study generates information on the origin of the terrestrial egg and the evolution of viviparity. In this research, the morphology of the extraembryonic membranes and the types of placentation in the viviparous snake Conopsis lineata are described through optical microscopy during early and late gestation. When embryos develop inside the uterus, they become surrounded by a thin eggshell membrane. In early gestation, during stages 16 and 18, the embryo is already surrounded by the amnion and the chorion, and in a small region by the chorioallantois, which is product of the contact between the chorion and the growing allantois. A trilaminar omphalopleure covers the yolk sac from the embryonic hemisphere to the level of the equator where the sinus terminalis is located, and from there a bilaminar omphalopleure extends into the abembryonic hemisphere. Thus, according to the relationship of these membranes with the uterine wall, the chorioplacenta, the choriovitelline placenta, and the chorioallantoic placenta are structured at the embryonic pole, while the omphaloplacenta is formed at the abembryonic pole. During late gestation (stages 35, 36, and 37), the uterus and allantois are highly vascularized. The allantois occupies most of the extraembryonic coelom and at the abembryonic pole, it contacts the omphaloplacenta and form the omphalallantoic placenta. This is the first description of all known placenta types in Squamata for a snake species member of the subfamily Colubrinae; where an eggshell membrane with 2.9 μm in width present throughout development is also evident. The structure of extraembryonic membranes in C. lineata is similar to that of other oviparous and viviparous squamate species. The above indicates not only homology, but also that the functional characteristics have been maintained throughout the evolution of the reproductive type.
胚外膜为胚胎发育提供保护、氧气、水和营养物质，对其进行研究可以揭示陆生卵的起源和胎生的演化。本研究通过光学显微镜观察，描述了胎生蛇类黑线乌梢蛇（Conopsis lineata）胚胎发育早期和晚期胚外膜的形态和胎盘类型。胚胎在子宫内发育时，被一层薄薄的蛋壳膜所包围。在妊娠早期（16 期和 18 期），胚胎已被羊膜和绒毛膜所包围，在一小部分区域还有尿囊绒毛膜，这是绒毛膜与生长中的尿囊接触的产物。一层三胚层的卵黄囊被膜从胚胎半球覆盖到位于终窦的赤道位置，从那里一层二胚层的卵黄囊被膜延伸到无胚胎半球。因此，根据这些膜与子宫壁的关系，在胚胎极形成绒毛胎盘、绒毛卵黄胎盘和绒毛尿囊胎盘，而在无胚胎极形成卵黄胎盘。在妊娠晚期（35 期、36 期和 37 期），子宫和尿囊高度血管化。尿囊占据了大部分胚外体腔，在无胚胎极与卵黄胎盘接触并形成卵黄尿囊胎盘。这是首次在 Colubrinae 亚科的蛇类中描述所有已知的蜥蜴胎盘类型；在此过程中还明显存在宽度为 2.9 μm 的蛋壳膜。黑线乌梢蛇的胚外膜结构与其他卵生和胎生蜥蜴物种相似。以上结果不仅表明了同源性，还表明了在生殖类型的演化过程中，功能特征得到了保持。
