Powers Kathryn G, Blackburn Daniel G
Dept. of Biology, and Electron Microscopy Center, Trinity College, Hartford, Connecticut.
J Exp Zool B Mol Dev Evol. 2017 Jul;328(5):462-475. doi: 10.1002/jez.b.22750. Epub 2017 May 24.
Early amniotic vertebrates evolved large-yolked eggs that permitted production of well-developed, terrestrial hatchlings. This reproductive pattern required new mechanisms for cellularizing the yolk and mobilizing it for embryonic use. In birds, cells that line the yolk sac cavity phagocytose and digest the yolk material, a pattern that is commonly assumed to be universal among oviparous amniotes. However, recent evidence challenges the assumption that all squamate reptiles conform to the avian developmental pattern. In this paper, scanning electron microscopy and histology were used to study mechanisms of yolk processing in two colubrid snakes, the kingsnake Lampropeltis getula and the milksnake L. triangulum. Endodermal cells from the yolk sac splanchnopleure proliferate massively as they invade the yolk sac cavity, forming elaborate chains of interlinked cells. These cells grow in size as they phagocytose yolk material. Subsequently, vitelline capillaries invade the masses of yolk-laden cells and become coated with the endodermal cells, forming an elaborate meshwork of cell-coated strands. The close association of cells, yolk, and blood vessels allows yolk material to be cellularized, digested, and transported for embryonic use. The overall pattern is like that of the corn snake Pantherophis guttatus, but contrasts markedly with that of birds. Given recent evidence that this developmental pattern may also occur in certain lizards, we postulate that it is ancestral for squamates. Studies of lizards, crocodilians, and turtles are needed to clarify the evolutionary history of this pattern and its implications for the evolution of the amniotic (terrestrial) vertebrate egg.
早期羊膜动物进化出了富含大量卵黄的卵，这使得发育良好的陆生幼体得以产生。这种繁殖模式需要新的机制来使卵黄细胞化并将其用于胚胎发育。在鸟类中，卵黄囊腔的内衬细胞吞噬并消化卵黄物质，这种模式通常被认为在卵生羊膜动物中是普遍存在的。然而，最近的证据对所有有鳞目爬行动物都符合鸟类发育模式这一假设提出了挑战。在本文中，利用扫描电子显微镜和组织学方法研究了两种游蛇科蛇类——王蛇（Lampropeltis getula）和乳蛇（L. triangulum）的卵黄处理机制。来自卵黄囊脏壁中胚层的内胚层细胞在侵入卵黄囊腔时大量增殖，形成相互连接的复杂细胞链。这些细胞在吞噬卵黄物质时会长大。随后，卵黄毛细血管侵入充满卵黄的细胞团，并被内胚层细胞覆盖，形成由细胞覆盖的细丝组成的复杂网络。细胞、卵黄和血管的紧密联系使得卵黄物质能够被细胞化、消化并运输以供胚胎使用。总体模式与玉米蛇（Pantherophis guttatus）相似，但与鸟类明显不同。鉴于最近有证据表明这种发育模式也可能发生在某些蜥蜴中，我们推测它是有鳞目动物的祖先模式。需要对蜥蜴、鳄类和龟类进行研究，以阐明这种模式的进化历史及其对羊膜（陆生）脊椎动物卵进化的影响。
