Metamorphosis is the dramatic conversion of an animal body from larva to adult. In ascidians, tadpole-shaped, swimming larvae become sessile juveniles by losing their tail during metamorphosis. This study investigated the cellular and molecular mechanisms underlying this metamorphic event called tail regression, in the model ascidian Ciona. The ascidian tail consists of internal organs such as muscle, notochord, nerve cord, and the outer epidermal layer surrounding them. We found that the epidermis and internal organs show different regression strategies. Epidermal cells are shortened along the anterior-posterior axis and gather at the posterior region. The epidermal mass is then invaginated into the trunk by apical constriction. The internal tissues, by contrast, enter into the trunk by forming coils. During coiling, notches are introduced into the muscle cells, which likely reduces their rigidness to promote coiling. Actin filament is the major component necessary for the regression events in both the epidermis and internal tissues. The shortening and invagination of the epidermis depend on the phosphorylation of the myosin regulatory light chain (mrlc) regulated by rho-kinase (ROCK). The coiling of internal tissues does not require ROCK-dependent phosphorylation of mrlc, and they can complete coiling without epidermis, although epidermis can facilitate the coiling of internal tissues. We conclude that tail regression in ascidians consists of active morphogenetic movements in which each tissue’s independent mechanism is orchestrated with the others to complete this event within the available time window.

变态是动物身体从幼虫到成年的急剧转变。在海象中，t状的游泳幼虫在变态过程中会失去尾巴，成为无柄的幼体。这项研究在模型海鞘Ciona中研究了这种称为尾部回归的变态事件背后的细胞和分子机制。。海鞘尾巴由内部器官（例如肌肉，脊索，神经索）和围绕它们的外表皮层组成。我们发现表皮和内部器官显示出不同的回归策略。表皮细胞沿前后轴缩短，并聚集在后部区域。然后，表皮块通过顶端收缩而被卷入躯干。相反，内部组织通过形成线圈进入躯干。在盘绕过程中，将切口引入肌肉细胞，这可能会降低其刚度，从而促进盘绕。肌动蛋白丝是表皮和内部组织中消退事件所必需的主要成分。表皮的缩短和内陷取决于由Rho激酶（ROCK）调节的肌球蛋白调节轻链（mrlc）的磷酸化。内部组织的缠绕不需要mrlc的ROCK依赖性磷酸化，尽管表皮可以促进内部组织的缠绕，但是它们可以完成没有表皮的缠绕。我们得出的结论是，海鞘中的尾巴消退由活跃的形态发生运动组成，在这种运动中，每个组织的独立机制与其他组织协调，以在可用的时间范围内完成此事件。