COMPARATIVE ULTRASTRUCTURAL INVESTIGATION ON THE ADHESIVE PAPILLAE OF THE SWIMMING LARVAE OF THREE ASCIDIAE SPECIES

Ascidian swimming larvae bear three peculiar organs of ectodermic origin, named “palps” or adhesive papillae, located in the anterior region of cephalenteron. Term “adhesive” is correlated to one of the function of these structure based on secretion of an adhesive substance which enables swimming larvae to adhere to a substratum. Recently a sensory function has also been described in some Phlebobranchia papillae with a simple morpho-functional organization. There are few ultrastructural investigations in literature, sometimes disputed, able to make clear papillae cells functions. To clarify this problem, a comparative investigation has been carried out, just in this work, about ultrastructural and morpho-functional organizations of adhesive papillae of three ascidian swimming larvae, Ascidia malaca, Phallusia mammillata and Ciona intestinalis. The investigations has been carried out by transmission electronic microscopy and confocal microscopy. Papillae of above-mentionated ascidian swimming larvae, are three in number and they are located at the vertices of a triangular field. According to a recent classification scheme by Burighel and Cloney, they are coniform and non-eversible simple type papillae. In the apex of each papilla there is an hyaline cap with an electron-dense substance made of proteoglycan component. It’s quite certain that this substance is used by ascidian larvae to adhere to the substratum. Ultrastructural investigations carried out on the adhesive papillae of Ascidia malaca and Phallusia mammillata have not emphasize significant ultrastructural differences in the papillae cells body. Ascidia malaca and Phallusia mammillata papillae are made of three types of cells that characterize their functions: a) collocytes, b) axial columnar cells, c) sensory cells. Collocytes, whose ultrastructure is typical of cells with secretory activity, are certainly deputed to form adhesive secretion. They lie in a median or semilateral side of the papillar body. Axial columnar cells that show an elongated shape, lie in the mid-central papillae region and they are characterized by the presence, in their apical part of long microvilli that run along the whole of hyaline cap length. Above a structural supporting role, it is supposed a possible sensory function concerning these cells. Sensory cells presents an ovoidal and elongated shape holding on tight at their base like a funnel lying a lateral and marginal position in papillar body. These type of cells have been described in Ascidia malaca which can be classified as primary sensory neurons. These are characterized by the presence of a single cilium in the apical end and by an axonicprocess at their base extending over papilla. Ultrastructural investigations on the adhesive papillae of Ciona intestinalis confirmed, even if showing some differences, ultrastructural likeness the two above-mentioned species of adhesive papillae. Collocytes with secretory activity, sensory cells and columnar axial cells come into sight also in Ciona intestinalis papillae. The columnar axial cells ultrastructure is different from the Ascidia malaca and Phallusia mammillata papillae. In the apical part of these cells there are not microvilli but digitiform protusions with numerous microtubules inside running paralleling along longitudinal cell axis. The ultrastructure of these cells suggest their lengthening ability to allow digitiform protrusions to put into contact with the cuticular layer of hyaline cap apex, during step before the adhesion. Ultrastructural characteristics suggest that they can also perform a sensorial duty but their mechanism is still unclear. The observation was carried out by confocal microscopy on the ascidian larvae just before or at the beginning of adhesion to the substratum (6-7 hours from hatching). The use of anti-β-tubulin antibody, have shown an extensive nervous network in all three Ascidiae species starting from the papillar base and converging into only one nerve extending up to the cerebral vescicle. These data confirm previous observations carried out with confocal microscopy on Ascidia malaca and Phallusia mammillata. larvae. In Ciona intestinalis adhesive papillae marked with anti-β- tubulin antibody, the fluorescence is more bright than Phallusia mammillata and Ascidia malaca. Probably this result is correlated with apical protrusion rich in microtubule. Ultrastructure observations of some neurons being in the anterior region of cephalenteron, extending from the base of papilla to the cerebral vescicle, have been emphasized in this work. These neurons are in the same position of the ones described like RTEN (rostral trunk epidermal neuron) by Imai and Meinertzhagen and they are formed by cellular ovoidal body extending into a long axon. Ultrastructural of these neurons have pointed out likeness with vertebrate neurons. Concluding, ultrastructural investigations have emphasize only some difference between papillar cells of Ascidia malaca, Phallusia mammillata and Ciona intestinalis larvae, confirming adhesive and sensory functions.