Preliminary data on senescence in haemocytes of the colonial ascidian Botryllus schlosseri.

Senescence is a cellular response to damage that limits the proliferation of aged or effete cells and plays physiological roles as it is required for tissue homeostasis. Colonies of the protochordate Botryllus schlosseri, undergo cyclical generation changes or takeovers (TOs) during which adult zooids are replaced by their buds reaching adulthood. The period of time between two TOs is referred to as blastogenetic cycle. During the TO, cells of adult zooid tissues die by apoptosis and are cleared by circulating phagocytes that, in turn, undergo phagocytosis-induced apoptosis and are cleared by new phagocytes in a recurrent, apparently endless, process. In the present work, we demonstrate that phagocytes, after the engulfment of effete phagocytes enter a senescence status and home, in the following mid-cycle, in the ventral islands, on both sides of the endostyle, where undifferentiated (stem) cells are also found. From these sites, senescent cells cross the peribranchial epithelium and are released in the peribranchial cavity where they will be expelled with the exhalant water.