Powers Kathryn G, Blackburn Daniel G
Department of Biology, and Electron Microscopy Center, Trinity College, Hartford, Connecticut, 06106.
J Morphol. 2017 Jun;278(6):768-779. doi: 10.1002/jmor.20671. Epub 2017 Mar 23.
Non-avian reptiles commonly are assumed to be like birds in their overall patterns of development. However, colubrid corn snakes (Pantherophis guttatus) have mechanisms of yolk cellularization and processing that are entirely different from the avian pattern. In birds, a vascular "yolk sac" surrounds and digests the liquid yolk. In contrast, in corn snakes, the yolk material is converted into vascularized cords of yolk-filled cells. In this study, we used stereomicroscopy, histology, and scanning electron microscopy to analyze this unusual developmental pattern in corn snakes. Our observations reveal that the yolk sac cavity is invaded by endodermal cells that proliferate, absorb yolk spheres, and form aggregates of interconnected cells within the liquid yolk mass. As development proceeds, small blood vessels arise from the yolk sac omphalopleure, penetrate into the yolk mass, and become tightly encased in the endodermal cells. The entire vitellus ultimately becomes converted into a mass of vascularized, "spaghetti-like" strands of yolk-laden cells. The resulting arrangement allows yolk to be digested intracellularly and yolk products to be transported to the developing embryo. Indirect evidence for this pattern in other species raises the possibility that it is ancestral for squamates and quite possibly Reptilia in general.
非鸟类爬行动物通常被认为在整体发育模式上与鸟类相似。然而，黄颔蛇科的玉米蛇（豹纹玉米蛇）具有与鸟类模式完全不同的卵黄细胞化和处理机制。在鸟类中，一个血管化的“卵黄囊”包围并消化液态卵黄。相比之下，在玉米蛇中，卵黄物质被转化为充满卵黄的细胞的血管化索条。在本研究中，我们使用体视显微镜、组织学和扫描电子显微镜来分析玉米蛇这种不寻常的发育模式。我们的观察结果表明，卵黄囊腔被内胚层细胞侵入，这些细胞增殖、吸收卵黄球，并在液态卵黄团内形成相互连接的细胞聚集体。随着发育的进行，小血管从卵黄囊脐褶产生，穿透进入卵黄团，并被紧密包裹在内胚层细胞中。整个卵黄最终转化为一团血管化的、“意大利面条状”的充满卵黄的细胞索条。这种最终的排列方式使得卵黄能够在细胞内被消化，并且卵黄产物能够被运输到发育中的胚胎。在其他物种中这种模式的间接证据增加了这样一种可能性，即它是有鳞目动物的祖先模式，并且很可能也是整个爬行动物的祖先模式。
