Phagocyte dynamics in the blastogenetic cycle of the colonial ascidian Botryllus schlosseri: Cell senescence, segregation and clearance after efferocytosis.

In the colonial ascidian Botryllus schlosseri, phagocytes are involved in the clearance of apoptotic cells and corpses during the periodical generation changes or takeovers (TOs) that assure the renewal of the colonial zooids. The persistent respiratory burst associated with efferocytosis, leads to the induction of senescence. Indeed, giant, senescent phagocytes are abundant in the colonial circulation at TO, whereas, in the other phases of the colonial blastogenetic cycle, they colonise the ventral islands (VIs), a series of mesenchymal niches located in the lateral lacunae of the mantle, on both sides of the subendostylar sinus. VI phagocytes produce reactive oxygen species probably as a consequence of the massive phagocytosis of effete cells. VIs are progressively dismantled with the progress of the blastogenetic phases and phagocytes are released in the peribranchial chamber via transepithelial expulsion to be definitely expelled with the outflowing water through the cloacal siphon.