Loss of test cells leads to the formation of new tunic surface cells and abnormal metamorphosis in larvae of Ciona intestinalis (Chordata, Ascidiacea)

The larvae of the ascidian Ciona intestinalis from which the chorion with t he test cells and follicle cells were removed developed normally without th e test cells until the early tailbud stage. A number of round-shaped cells morphologically similar to the test cells but with different lectin affinit ies and autofluorescence, then appeared on the neck region of the demembran ated embryos. The new cells had three different types: round, particulate, and granular, and these cells increased in number after the late tailbud st age. The morphology of the adhesive papillae, tunic layers and epidermis of the demembranated larvae was similar to that of control larvae; however, t he affinity to lectins was different in the swimming period. Control larvae attached to the substratum after the swimming period, resorbed the tail co mpletely and underwent rotation of the visceral organs. Conversely, rotatio n occurred before completion of tail resorption in the demembranated larvae . Furthermore, the metamorphic events progressed more slowly in the demembr anated larvae. These results suggest that the test cells play important rol es in normal development and morphogenesis of ascidian larvae.