Regeneration of the tunic cuticle in the compound ascidian, Botrylloides simodensis

The tunic cuticle of Botrylloides simodensis, a colonial ascidian, is an electrondense sheet overlaying the tunic in which zooids are embedded. When the margins of colonies were cut off and the colonies were incubated in running seawater, the tunic cuticle regenerated at the cut surface of the tunic matrix by the following process: 1) Electron-dense fibers appeared at the cut surface within a few minutes. 2) Fiber formation proceeded during the next several hours, and the number of fibers greatly increased. 3) In 12 to 24 hours, the fibers aggregated and formed a continuous layer, which was the newly regenerated cuticle. 4) Minute protrusions appeared in the new cuticle after 3 to 5 days. Fiber formation occurred even in fragments of colonial margins that were incubated in a bottle of seawater. It was inhibited under some experimental conditions including low pH and the presence of some proteases or protease inhibitors.