Younossi-Hartenstein A, Hartenstein V
Department of Molecular Cell and Developmental Biology, University of California, Los Angeles 90095, USA.
Int J Dev Biol. 2000 Aug;44(5):499-506.
Dalyellida represents a taxon of small rhabdocoel flatworms that occur in freshwater habitats all over the world. Combining histology and electron microscopy we have analyzed the embryonic development of a new dalyellid species, Gieysztoria superba, in order to obtain more comparative data pertaining to morphogenesis and organogenesis in flatworms. We have used a morphological staging system that we recently introduced for another rhabdocoel, Mesostoma lingua (Younossi-Hartenstein et al., 2000). Our data show that in many fundamental respects, such as the irregular cleavage, mesenchymal embryonic primordium, and lack of gastrulation movements, Gieysztoria is highly similar to Mesostoma. During cleavage (stages 1 and 2) the embryo is located in the center of the egg where it is surrounded by a layer of yolk cells. Cleavage leads up to a solid, disc shaped cell cluster. During stage 3, the embryo migrates to the ventral side of the egg and acquires bilateral symmetry. Stages 4/5 sees the emergence of the first organ primordia, the brain, epidermis and pharynx. A peculiar invagination of the epidermal layer pushes the embryo back into the center of the yolk ("embryonic invagination"). Organogenesis takes place during stages 5 and 6 while the embryo is invaginated. A junctional complex, consisting initially of small septate junctions, followed later by a more apically located zonula adherens, is formed in all epithelial tissues, including epidermis, protonephridia, and pharynx. During late stages (6-8), Gieysztoria embryos evert back to the surface where the epidermal primordium expands and grows around the yolk to close dorsally. During this phase of development cytodifferentiation of the different organ systems takes place. Stage 7 is characterized by the appearance of eye pigmentation, brain condensation and spindle shaped myocytes. Stage 8 describes the fully dorsally closed and differentiated embryo. In comparison to other rhabdocoels, including Mesostoma, Gieysztoria exhibits a precocious differentiation of an intestinal epithelium and male genital apparatus. In Mesostoma, these structures are formed post hatching.
达耶利目代表了一类小型的杆吻涡虫，分布于世界各地的淡水生境中。我们结合组织学和电子显微镜技术，分析了一种新的达耶利目物种——华丽吉氏涡虫（Gieysztoria superba）的胚胎发育过程，以便获取更多有关涡虫形态发生和器官发生的比较数据。我们采用了一种形态学分期系统，该系统是我们最近为另一种杆吻涡虫——舌形中口涡虫（Mesostoma lingua）引入的（尤诺西 - 哈滕斯坦等人，2000年）。我们的数据表明，在许多基本方面，如不规则卵裂、间充质胚胎原基以及缺乏原肠胚形成运动等，吉氏涡虫与中口涡虫高度相似。在卵裂期（1期和2期），胚胎位于卵的中心，周围环绕着一层卵黄细胞。卵裂产生一个实心的盘状细胞团。在3期，胚胎迁移到卵的腹侧并获得双侧对称性。在4/5期，首次出现器官原基，即脑、表皮和咽。表皮层的一种特殊内陷将胚胎推回到卵黄的中心（“胚胎内陷”）。器官发生在5期和6期胚胎内陷时进行。在所有上皮组织，包括表皮、原肾管和咽中，形成了一种连接复合体，最初由小的分隔连接组成，随后在更顶端位置形成紧密连接。在后期（6 - 8期），吉氏涡虫胚胎外翻回到表面，此时表皮原基围绕卵黄扩展并生长，在背侧闭合。在这个发育阶段，不同器官系统发生细胞分化。7期的特征是出现眼色素沉着、脑浓缩和纺锤形肌细胞。8期描述的是完全背侧闭合且已分化的胚胎。与其他杆吻涡虫，包括中口涡虫相比，吉氏涡虫的肠上皮和雄性生殖器官表现出早熟分化。在中口涡虫中，这些结构是在孵化后形成的。
