Heart Morphogenesis in the Ascidian, Ciona Intestinalis

科学价值。对乔纳胚胎心脏形成最早阶段的详细可视化，有可能揭示更复杂的脊椎动物系统中错过的洞察力。莱文最近发现，最终的心脏来自一对细胞，即B7.5卵裂球，在胚胎发育的110个细胞阶段，就在原肠形成开始之前。这些细胞特异性地表达两个调控基因Tbx6和mesp，分别编码T-box和bHLH转录因子。B7.6分裂产生两个子代细胞。前子细胞只表达MESP调节基因，从而形成心脏。相反，后子细胞表达MESP和Tbx6，因此，这个细胞形成最前面的尾部肌肉。这些“A细胞”具有许多与脊椎动物心脏的室壁区相关的特性，尽管它们不构成最终心脏的组成部分。前子细胞迁移到头部并经历不对称分裂，然后走上通往心脏的分化途径。相反，后子体细胞留在尾部，形成A型肌细胞。他将对心脏祖细胞的前向迁移和不对称分裂所需的基因进行详细分析。他最近培育出了突变的蝌蚪，这应该有助于识别心脏迁移和分裂所需的基因。例如，Tbx6的过表达导致B7.5卵裂球的前子细胞停止并形成多余的A细胞，而MESP的过表达导致相反的表型：假定的A细胞异位迁移和分裂，从而形成第二个心脏。所有迹象表明，脊椎动物心脏形成的初始阶段依赖于与乔纳蝌蚪相同类型的定向细胞迁移和不对称分裂。心脏病是人类死亡和痛苦的主要原因。为了深入了解心脏病的基础，这项研究将调查一种简单的生物体--海鞘或海鞘--心脏发育的早期阶段。海鞘动物代表最简单的脊索动物--包括我们人类这样的脊椎动物的动物门。海鞘蝌蚪代表了脊椎动物蝌蚪的简化版本。早期的蝌蚪只有1000个细胞。海鞘类心脏很简单，但却代表了多腔脊椎动物心脏进化的先驱。尽管它很简单，但海鞘心脏拥有一种在脊椎动物中看不到的耐人寻味的特性：它有可逆的心跳。单腔心脏在一个方向上有节奏地跳动几分钟，然后停下来并逆转方向几分钟，然后再次改变方向。在斑马鱼、青蛙、鸡和老鼠等不同脊椎动物的胚胎发育过程中，人们已经投入了数百万美元来了解心脏发育的各个方面。然而，这些生物体的复杂性(发育中的心脏由数百甚至数千个细胞组成)阻碍了对心脏形成的早期阶段的详细了解。乔纳蝌蚪的简单性使人们能够看到最早的事件。没有其他脊椎动物的近亲拥有如此简单的心脏。它最初只由8个细胞组成，很容易用简单的显微镜观察到。

Scientific merit. The detailed visualization of the earliest stages of heart formation in the Ciona embryo has the potential to reveal insights that have been missed in the more complex vertebrate systems. Dr. Levin has recently found that the definitive heart arises from a single pair of cells, the B7.5 blastomeres, at the 110-cell stage of embryogenesis, just before the onset of gastrulation. These cells specifically express two regulatory genes, Tbx6 and Mesp, which encode T-box and bHLH transcription factors, respectively. B7.6 divides to produce two daughter cells. The anterior daughter cell expresses only the Mesp regulatory gene and thereby forms the heart. In contrast, the posterior daughter cell expresses both Mesp and Tbx6, and consequently, this cell forms the anterior-most tail muscles. These "A-cells" have a number of properties related to the ventricular regions of vertebrate hearts, although they do not form components of the definitive heart. The anterior daughter cell migrates into the head and undergoes an asymmetric division before embarking on the differentiation pathway leading to the heart. In contrast, the posterior daughter cell remains in the tail and forms A-type muscle cells. He will perform a detailed analysis of the genes required for the anterior migration and asymmetric division of the heart progenitors. He has recently produced mutant tadpoles that should facilitate the identification of genes required for heart migration and division. For example, the overexpression of Tbx6 causes the anterior daughter cells of the B7.5 blastomeres to arrest and form supernumerary A-cells, while overexpression of Mesp causes the opposite phenotype: ectopic migration and division of the presumptive A-cells, which form a second heart. There is every indication that the initial stages of heart formation in vertebrates depends on the same type of directed cell migration and asymmetric division seen in the Ciona tadpole.Broader impact. Heart disease is a major cause of death and suffering in human populations. To gain insights into the basis of heart disease this study will investigate the earliest phases of heart development in a simple organism, the sea squirt or Ascidian, Ciona intestinalis. Ascidians represent the simplest chordates--the animal phylum that includes vertebrates such as ourselves. The Ascidian tadpole represents a simplified version of vertebrate tadpoles. The early tadpole is composed of just 1,000 cells. The ascidian heart is simple, but nonetheless represents the evolutionary forerunner of multi-chambered vertebrate hearts. Despite its simplicity, the ascidian heart possesses an intriguing property not seen in vertebrates: it has a reversible beat. The single-chambered heart exhibits rhythmic beating in one direction for several minutes, then stops and reverses direction for a few minutes before once again changing direction. Many millions of dollars have been invested in understanding various aspects of heart development during the embryonic development of different vertebrates, including zebrafish, frogs, chickens, and mice. However, the complexity of these organisms (the developing heart is composed of many hundreds-even thousands-of cells) precludes a detailed understanding of the earliest stages of heart formation. The simplicity of the Ciona tadpole permits the visualization of the earliest events. There is no other relative of vertebrates that possesses such a simple heart. It is initially composed of just 8 cells that are easily visualized using simple microscopy.