Ziese Stefanie, Dorn August
Institut für Zoologie, Johannes Gutenberg-Universität, D-55099 Mainz, Germany.
J Morphol. 2003 Feb;255(2):146-61. doi: 10.1002/jmor.10056.
In Manduca sexta the germ band is formed 12 h post-oviposition (p.o.) (=10% development completed) and is located above the yolk at the egg surface. The cells show a polar organization. They are engaged in the uptake and degradation of yolk globules, pinched off from the yolk cells. This process can be observed in the integumental cells during the first growth phase of the embryo that lasts until "katatrepsis," an embryonic movement that takes place at 40% development completed. At 37% development completed, the ectoderm deposits a thin membrane at its apical surface, the first embryonic membrane, which detaches immediately before katatrepsis. The second period of embryonic growth--from katatrepsis to 84 h p.o. (70% development completed)--starts with the deposition of a second embryonic membrane that is somewhat thicker than the first one and shows a trilaminar, cuticulin-like structure. Whereas the apical cell surface is largely smooth during the deposition of the first embryonic membrane, it forms microvilli during deposition of the second one. At the same time, uptake of formed yolk material ceases and the epidermal cells now contain clusters of mitochondria below the apical surface. Rough endoplasmic reticulum (RER) increases in the perinuclear region. The second embryonic membrane detaches about 63 h p.o. At 69 h p.o., a new generation of microvilli forms and islands of a typical cuticulin layer indicate the onset of the deposition of the larval cuticle. The third growth phase is characterized by a steady increase in the embryo length, the deposition of the larval procuticle, and by cuticular tanning at about 100 h p.o. Beginning at that stage, electron-lucent vesicles aggregate below the epidermal surface and are apparently released below the larval cuticle. Manduca sexta is the first holometabolous insect in which the deposition of embryonic membranes and cuticles has been examined by electron microscopy. In correspondence with hemimetabolous insects, the embryo of M. sexta secretes three covers at approximately the same developmental stage. A marked difference: the second embryonic cover, which in Hemimetabola clearly exhibits a cuticular organization, has instead a membranous, cuticulin-like structure. We see the difference as the result of an evolutionary reductional process promoted by the redundancy of embryonic covers in the egg shell. Embryonic "molts" also occur in noninsect arthropods; their phylogenetical aspects are discussed.
在烟草天蛾中，胚带在产卵后12小时形成（=发育完成10%），位于卵表面卵黄上方。细胞呈现极性组织。它们参与从卵黄细胞分离出的卵黄球的摄取和降解。在胚胎的第一个生长阶段，直到“下迁”（一种在发育完成40%时发生的胚胎运动），这个过程可以在体壁细胞中观察到。在发育完成37%时，外胚层在其顶端表面沉积一层薄膜，即第一胚膜，在“下迁”前立即分离。胚胎生长的第二个阶段——从“下迁”到产卵后84小时（发育完成70%）——开始于第二胚膜的沉积，第二胚膜比第一胚膜稍厚，呈现出三层的、类角质素的结构。在第一胚膜沉积期间，顶端细胞表面基本光滑，而在第二胚膜沉积期间形成微绒毛。与此同时，已形成的卵黄物质的摄取停止，表皮细胞现在在顶端表面下方含有线粒体簇。粗面内质网（RER）在核周区域增加。第二胚膜在产卵后约63小时分离。在产卵后69小时，新一代微绒毛形成，典型角质素层的岛状结构表明幼虫表皮沉积开始。第三个生长阶段的特征是胚胎长度稳步增加、幼虫前表皮的沉积以及在产卵后约100小时的表皮鞣化。从那个阶段开始，电子透明小泡在表皮表面下方聚集，并明显在幼虫表皮下方释放。烟草天蛾是第一种通过电子显微镜检查胚膜和表皮沉积的全变态昆虫。与半变态昆虫一致，烟草天蛾的胚胎在大致相同的发育阶段分泌三层覆盖物。一个显著的区别是：在半变态昆虫中明显呈现角质组织的第二胚膜，在这里却具有膜状的、类角质素的结构。我们认为这种差异是由卵壳中胚膜的冗余所推动的进化简化过程的结果。胚胎“蜕皮”也发生在非昆虫节肢动物中；对它们的系统发育方面进行了讨论。
