Department of Biology, and Electron Microscopy Center, Trinity College, Hartford, Connecticut.
J Exp Zool B Mol Dev Evol. 2019 Sep;332(6):187-197. doi: 10.1002/jez.b.22894. Epub 2019 Jul 22.
Evolution of the large-yolked, amniotic egg required mechanisms by which extracellular yolk could be made available for embryonic development. In birds, the endodermal lining of the yolk sac absorbs and digests the yolk. In contrast, recent studies on lizards and snakes (squamates) have revealed that yolk is processed by means of a proliferating mass of "spaghetti-like" strands formed by endodermal cells attached to anastomosing blood vessels. To clarify the method of yolk processing in chelonians, we applied electron microscopy to an extensive series of embryos of the pond slider turtle, Trachemys scripta. Our findings demonstrate that proliferating endodermal cells phagocytose yolk spheres. These cells remain attached to one another following mitosis, thereby forming clumps that progressively occupy the yolk sac cavity. Upon invasion of blood vessels, the cells become organized into elongated, vascularized "spaghetti-like" strands of cells like those found in squamates. Residual yolk found in the body cavity of new hatchlings chiefly consists of these vascularized strands. Such strands of cells also develop in the false map turtle, Graptemys pseudographica (Emydidae). We infer that the developmental pattern by which yolk is processed is ancestral for both Chelonia and Reptilia, and therefore must have been modified or abandoned in birds or their archosaur ancestors.
大型羊膜卵的演化需要一些机制，使细胞外卵黄能够为胚胎发育提供营养。在鸟类中，卵黄囊的内胚层衬里吸收和消化卵黄。相比之下，最近对蜥蜴和蛇（有鳞目）的研究表明，卵黄是通过内胚层细胞附着在吻合的血管上形成的增殖的“意大利面条状”链来处理的。为了阐明龟鳖类的卵黄处理方法，我们应用电子显微镜对滑龟（Trachemys scripta）的大量胚胎进行了研究。我们的发现表明，增殖的内胚层细胞吞噬卵黄球。这些细胞在有丝分裂后仍然相互连接，从而形成逐渐占据卵黄囊腔的团块。当血管侵入时，细胞会组织成类似于有鳞目动物中发现的长而血管化的“意大利面条状”细胞链。在新孵化的幼龟的体腔中发现的残余卵黄主要由这些血管化的链组成。在拟地图龟（Graptemys pseudographica）（鳖科）中也会形成这种细胞链。我们推断，这种卵黄处理的发育模式是龟鳖类和爬行动物的祖先进化而来的，因此在鸟类或它们的恐龙祖先中，这种模式一定是被修改或放弃了。
