Morula cells and histocompatibility in the colonial ascidian Botryllus schlosseri

The role of morula cells (MC) in the formation of necrotic regions characterizing the rejection reaction between incompatible (i.e. nonfusible) colonies was investigated in the colonial ascidian Botryllus schlosseri. These blood cells share several chemical and histochemical properties with the pigment of the necrotic masses: both of them show strong reducing activities and contain high quantities of sulphur and iron. Sulphur is present mainly as sulphates and thiols in MC and as sulphates and disulphides in necrotic regions; iron is in the form of ferrous iron in MC and as ferric iron in the necrotic regions. Inside MC vacuoles phenoloxidase (PO) activity and polyphenol substrata are present: the latter are oxidized by PO to quinones, which then polymerize to form the melanin-like substances of dark-brown colour of the necrotic regions. When hemocytes are incubated with heterologous incompatible blood plasma (BP), MC change their morphology, and a significant increase in PO activity is found in the recovered medium as compared with the activity of untreated plasma. No increase in PO activity is observed after incubation with autologous or heterologous compatible blood plasma. These results are consistent with the hypothesis of a humoral factor diffusing from incompatible colonies which is recognized by MC and leads to their degranulation, with the consequent release of the content of their vacuoles, mainly oxidative enzymes, responsible for the localized cell death in necrotic regions.