Morula cells, phenoloxidase and DOPA-containing proteins in the compound ascidian Botryllus schlosseri.

Morula cells (MCs) represent the most abundant circulating haemocyte type in the compound ascidian Botryllus schlosseri. They are involved in defence reactions as they: i) recognize alien substances and cells and induce cytotoxicity; ii) are the effectors of the cytotoxicity rejection reaction which occurs between contacting, genetically incompatible colonies. A main role in MC-related cytotoxicity is exerted by the enzyme phenoloxidase (PO) which converts polyphenol substrata to quinones; the latter, in turn, polymerise to form melanins. In the present research, we carried out new spectrophotometrical and cytochemical analysis to investigate further the behaviour of PO and the nature of its substrates. Results confirm that PO is located inside MC vacuoles. In addition, immunocytochemical analysis indicate that MCs contain quinones which probably represent ready-to-use cytotoxic molecules, likely deriving from the oxidation, by PO, of DOPA-containing proteins. In addition, small DOPA-containing peptides, called tunichromes, are likely present inside MCs.