Adhesive papillae ofAscidia malacaswimming larvae: investigations on their sensory function

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Abstract
Swimming larvae of Ascidia malaca bear, in the anterior region of cephalenteron, three adhesive papillae through which they can attach to a substratum prior to metamorphosis. The ultrastructural investigations described in the present work have mainly revealed the presence of three cell types constituting the papillae: (a) collocytes, (b) axial columnar cells, (c) sensory cells. Collocytes, whose ultrastructure is typical of secretory cells, form the adhesive secretion. The ultrastructural observations of the axial columnar cells are insufficient to explain their function. Probably this cell type plays a supporting role though some sensory function cannot be excluded due to the presence of long microvilli in the apical region. Sensory cells are characterized by the presence of a single cilium at the cell apex and an axonal prolongation at its base. Based on their ultrastructure, these cells are primary sensory neurons playing a sensory role in papillae. In order to highlight papillar innervations we carried out laser confocal microscopy investigations on Ascidia malaca swimming larvae marked with anti-β-tubulin fluorescent antibody. In the larval cephalenteron we observed an extensive nervous network starting from the papillar base and converging into two papillar nerves extending to the visceral ganglion. In conclusion, besides confirming the adhesive role of Ascidia malaca papillae, our investigations have also revealed the sensory function played by these organs.