FURTHER INSIGHTS ON SIPHONAL GUARD CELLS OF ASCIDIANS

In the oral siphon of the colonial ascidian Botryllus schlosseri, hyaline amoebocytes directly exposed to the sea-water flow entering into the pharynx have been recently observed and described. These cells, named “siphonal guard cells” (SGC), are free of moving on the surface of the tunic that internally covers the siphons. Our previous observations by means of histochemical, histoenzymatic and immunohistochemical techniques showed that they share many morpho-functional characteristics with the phagocytic blood cell line, from which probably they originate, and are able to recognise and phagocytise various foreign particles. After exposure of colonies to bacterial spores, the observations at both light and electronic microscope revealed that these cells are involved in a complex and unusual series of local and systemic immune events. Already after 5 min, the SGC showed bacteria inside their heterophagic vacuoles. After 10-15 min, as a transitory plug of floccular and colloidal material formed in the lumen of the siphon by exocytosis of some SGC, other ones with engulfed bacteria crossed the epidermis of the siphon reaching the siphonal sinus; cells of the cytotoxic blood cell line (morula cells) were drawn and crowded into the siphonal sinus, where most of them were positive to anti-TNF-α and anti-CD57 antibodies and degranulated stimulating, after this time and until 12 h, large scavenger phagocytes. The latter showed bacteria engulfed in their large phagosomes, increased in number in the blood circulation and were continuously eliminated through the peribranchial chamber with a mechanism which was never previously described. As regards the ability to transfer an alert signal, the role of SGC appears important as regards the immunosurveillance of the opening of the alimentary canal, similarly to what occurs in the vertebrate oropharyngeal lymphatic tissues.