Cellular and Molecular Events in Early Leech Development

魏斯布拉特9723114 舌形水蛭，如Helobdella robusta，是有用的，在深入分析的发展过程中有两个原因：第一，他们是相对简单的动物与大，可访问的胚胎，适合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，称为胚带（节肢动物如果蝇）或胚板（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，在早期水蛭胚胎中细胞命运是如何决定的问题是相当有趣的。本建议的第一部分包含的实验，旨在进一步以前的调查，如何不同的命运被分配给特定的细胞在发育中的胚胎，重点是分配中胚层和外胚层的命运，以个别identified细胞在第四卵裂。在这项工作中的重点是水蛭同源物的表达和发展的作用，从果蝇，如纳米，背，扭曲，蜗牛，和麻木的进化保守的调控基因的特征模式。 第二个感兴趣的领域是形态发生细胞运动导致germinal板形成Helobdella。分节的创始细胞柱在移过胚胎的大部分表面后合并成生殖板;这些移动伴随着鳞状上皮的扩展，称为表皮。本提案第二部分的实验旨在阐明这些运动所涉及的细胞过程，并比较水蛭和鱼类的外延机制。具体来说，我们试图测试的假设，即节段性创始人细胞和上皮细胞的“拖”植物内胚层前体细胞内起源的力量，如果是这样，我们进一步寻求区分可能的模式产生的牵引运动，如“皮质的旋转”和“皮质收缩波”的过程。

Weisblat 9723114 Glossiphoniid leeches such as Helobdella robusta, 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 mollusks) 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 melanogaster) 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 molluscan embryos. Thus, the question of how cell fates are determined in the early leech embryo 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 individually iden tified cells at fourth cleavage. The focus in this work is on characterizing the patterns of expression and development roles of leech homologs to evolutionarily conserved regulatory genes from Drosophila, such as nanos, dorsal, twist, snail, and numb. A second area of interest is the morphogenetic cell movements leading to germinal plate formation in Helobdella. Columns of segmental founder cells coalesce into the germinal plate having moved over most of the surface of the embryo; these movements are accompanied by the spreading of a squamous epithelium in a process called epiboly. The experiments in the second part of this proposal are designed to elucidate the cellular processes involved in these movements and to compare the mechanisms of epiboly in leech and fish. Specifically, we seek to test the hypothesis that the segmental founder cells and epithelium are "towed" vegetally by forces originating within the endodermal precursor cells; if so, we further seek to distinguish between possible modes of generating the towing movements, such as "cortical treadmilling" and "cortical contractile wave" processes.