Shimoda Marine Research Center, University of Tsukuba, Shimoda, Shizuoka, 415-0025, Japan.
Shimoda Marine Research Center, University of Tsukuba, Shimoda, Shizuoka, 415-0025, Japan.
Dev Biol. 2020 Sep 1;465(1):66-78. doi: 10.1016/j.ydbio.2020.07.009. Epub 2020 Jul 19.
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.
变态是动物体从幼虫到成体的戏剧性转化。在海鞘中，蝌蚪状的游泳幼虫在变态过程中通过失去尾巴而变成固着的幼体。本研究以模式海鞘 Ciona 为研究对象，探讨了这种称为尾巴退化的变态事件的细胞和分子机制。海鞘的尾巴由内部器官如肌肉、脊索、神经索和环绕它们的外表皮层组成。我们发现表皮和内部器官表现出不同的退化策略。表皮细胞沿前后轴缩短并聚集在后区域。然后表皮质量通过顶端收缩向内胚层凹陷。相比之下，内部组织通过形成线圈进入体腔。在卷曲过程中，肌肉细胞中出现凹口，这可能降低了它们的刚性，以促进卷曲。肌动蛋白丝是表皮和内部组织退化事件所必需的主要成分。表皮的缩短和内陷依赖于 Rho 激酶 (ROCK) 调节的肌球蛋白调节轻链 (mrlc) 的磷酸化。内部组织的卷曲不需要 ROCK 依赖性 mrlc 磷酸化，它们可以在没有表皮的情况下完成卷曲，尽管表皮可以促进内部组织的卷曲。我们的结论是，海鞘的尾巴退化包括主动的形态发生运动，其中每个组织的独立机制与其他组织协调，以在可用的时间窗口内完成该事件。
