Cellular and Molecular Events in Early Leech Development

舌管形水蛭，包括Helobdella的各种物种，可用于深入分析发育过程，原因有两个：首先，它们是相对简单的动物，具有大的、可接近的胚胎，适合于结合细胞和分子技术。 其次，由于水蛭属于环节动物门，否则研究不足的群体，所获得的结果是有用的，使系间比较。 这样的比较是必不可少的，以区分一般的发展过程，从门特定的，也要了解如何发展过程的进化修改导致不同的动物类型的外观。 例如，环节动物和节肢动物彼此相似（与软体动物不同），因为它们都有分段的身体计划。 在环节动物和节肢动物中，节段组织都是由纵向的、位于腹侧的细胞阵列产生的，称为生殖带（节肢动物如果蝇）或生殖板（环节动物）。 在水蛭中，生殖板是由称为端细胞的特化胚胎干细胞重复分裂而严格地从头到尾（吻尾）发展而来的。 果蝇的喙尾模式中涉及的几个基因的同源物似乎在水蛭的喙尾模式中以同源模式表达。 这表明，分化和形态发生的分子遗传过程在以后的发展将是非常相似的两门。 但是早期的Helobdella胚胎与早期的果蝇胚胎有着惊人的不同，而与软体动物胚胎有着惊人的相似。 因此，在早期水蛭中细胞命运是如何决定的问题是相当有趣的。 该提案的第一部分包含旨在进一步研究发育胚胎中特定细胞的不同命运的实验，重点是将中胚层和外胚层命运分配给胚胎仅包含10个细胞时出生的单个识别细胞，并寻找Helobdella和果蝇之间在这方面的相似性。 第二个有趣的问题是，早期水蛭发育的“软体动物”模式如何与后期发育的“节肢动物”模式相吻合。 这个过程的一个关键方面是在Helobdella的生殖板形成期间，一层细胞在胚胎的整个表面上扩散。 本建议第二部分的实验旨在阐明这些形态发生运动中所涉及的细胞过程。

Glossiphoniid leeches, including various species of Helobdella, are useful for in depth analyses of developmental processes for two reasons: first, they are relatively simple animals with large, accessible embryos that are suited for combined cellular and molecular techniques. Second, since leeches belong to the phylum Annelida, an otherwise poorly studied group, the results obtained are useful for making interphyletic comparisons. Such comparisons are essential to distinguish general developmental processes from phylum-specific ones, and also to learn how evolutionary modification of developmental processes leads to the appearance of diverse animal types. For example, annelids and arthropods resemble each other (and differ from molluscs) in that they both have segmented body plans. In both annelids and arthropods, segmental tissues arise from longitudinally oriented, ventrally situated arrays of cells, termed the germ band (in arthropods such as the fruitfly Drosophila) or germinal plate (in annelids). In leech, the germinal plate arises in strict head-to-tail (rostrocaudal) progression by repeated divisions of specialized embryonic stem cells called teloblasts. Homologs of several genes involved in rostrocaudal patterning of Drosophila seem to be expressed in homologous patterns during rostrocaudal patterning of leech. This suggests that the molecular genetic processes of differentiation and morphogenesis in later development will be quite similar in the two phyla. But the early Helobdella embryo is strikingly different from the early Drosophila embryo and strikingly similar to the molluscan embryos. Thus, the question of how cell fates are determined in the early leech is of considerable interest. The first part of this proposal contains experiments designed to further previous investigations of how distinct fates are assigned to particular cells in the developing embryo, with emphasis on the assignment of mesodermal and ectodermal fates to individual ly identified cells that are born when the embryo contains only 10 cells, and to look for similarities between Helobdella and Drosophila in this regard. A second question of interest is that of how the "molluscan" mode of early leech development meshes with the "arthropodan" mode of later development. A critical aspect of this process is the spreading of one layer of cells over the entire surface of the embryo during the formation of the germinal plate in Helobdella. The experiments in the second part of this proposal are designed to elucidate the cellular processes involved in these morphogenetic movements.