Sea cucumbers (a class of echinoderms) exhibit a high capacity for regeneration, such that, following ejection of inner organs in a process called evisceration, the lost organs regenerate. There are two ways by which evisceration occurs in sea cucmber species: from the mouth (anterior) or the anus (posterior). Intriguingly, regenerating tissues are formed at both the anterior and posterior regions and extend toward the opposite ends, and merge to form a complete digestive tract. From the posterior side, the digestive tube regenerates extending a continuous tube from the cloaca, which remains at evisceration. In posteriorly-eviscerating species, the esophagus remains in the body, and a new tube regenerates continuously from it. However, in anterior-eviscerating species, no tubular tissue remains in the anterior region, raising the question of how the new digestive tube forms in the anterior regenerate. We addressed this question by detailed histological observations of the regenerating anterior digestive tract in a small sea cucumber, Eupentacta quinquesemita (“ishiko” in Japanese) after induced-evisceration. We found that an initial rudiment consisting of mesenchymal cells is formed along the edge of the anterior mesentery from the anterior end, and then, among the mesenchymal cells, multiple clusters of epithelial-like cells appears simultaneously and repeatedly in the extending region by mesenchymal–epithelial transition (MET) as visulalized using toluidine blue staining. Subsequently, multiple cavities were formed surrounded with these epithelial cells, and appeared to coalesce with each other to form into multiple lumens, and to eventually become a single tube. This anterior tube then fused to the tube regenerated from the posterior rudiment. Thus, we elucidated the process of regeneration of the anterior portion of the gut in an anteriorly eviscerating species, and suggest the involvement of MET and fusion of cavities/lumens in regeneration of the digestive tube.

中文翻译：

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前内脏海参Eupentacta quinquesemita消化道的再生，以及间充质-上皮转化参与消化管形成

海参（一类棘皮动物）具有很高的再生能力，因此，在称为“内脏”的过程中内脏器官弹出后，丢失的器官会再生。在海豚物种中，有两种清除内脏的方法：从口腔（前部）或肛门（后部）。有趣的是，再生组织既在前区又在后区形成，并朝着相反的两端延伸，并融合形成完整的消化道。消化管从后侧再生，从泄殖腔延伸出一根连续的管，该管一直处于内脏。在后部切除的物种中，食道保留在体内，并且新管不断从中再生。但是，在前内脏物种中，前区没有管状组织残留，提出了新的消化管如何在前部再生的问题。我们通过详细的组织学观察来解决这个问题，该观察是在对小刺参诱除后的小海参Eupentacta quinquesemita（日语中的“ ishiko”）中的再生前消化道进行的。我们发现，由间质细胞组成的初始残基从前端开始沿前肠系膜边缘形成，然后，在间质细胞中，上皮样细胞的多个簇在间质细胞的延伸区域同时并反复出现。上皮化转变（MET），用甲苯胺蓝染色观察。随后，在这些上皮细胞的周围形成了多个腔，并相互融合形成多个腔，并最终成为单管。然后，该前管融合到从后骨再生的管上。因此，我们阐明了在前内脏物种中肠道前部的再生过程，并提出了在消化管再生中MET的参与和腔/管腔的融合。