PHAGOCYTE BEHAVIOUR DURING THE COLONIAL BLASTOGENETIC CYCLE IN THE COMPOUND ASCIDIAN BOTRYLLUS SCHLOSSERI.

Colonies of the ascidian Botryllus schlosseri experience a cyclical generation change (takeover) during which adult zooids stop their filtering activity, their tissue undergo cell death by apoptosis and are progressively resorbed. In the meantime, a new blastogenic generation reaches its functional maturity, opens siphons and starts filtering. During the blastogenetic cycle, the phagocytic differentiation pathway of the circulating immunocytes plays a key role: the presence of both hydrolytic enzymes and mediators of local inflammation supports the hypothesis that the phagocytic cell line maintains characteristics of a primordial system with high functional versatility. Phagocytes cyclically change their morphology and frequency. During the mid-cycle, hyaline amoebocytes are numerous in blood circulation, inside the tunic and free of moving on the surface of the tunic that internally covers the oral siphon, where they play an immunosurveillance role of the pharynx by recognising and phagocytising foreign particles, and, after exposure of colonies to bacterial spores, forming a transitory plug in the siphonal lumen by exocytosis of floccular and colloidal material rich of heparin, histamine and proteases. In the takeover, the frequency of hyaline amoebocytes falls abruptly since, by engulfing apoptotic cells, they change their shape becoming large and spherical macrophage-like cells. These scavenger phagocytes are massively recruited from the circulation to the dying tissues of old zooids, where they assure the clearance of senescent cells. This is fundamental for the progression of the takeover, as during this phase lasting 24-36 hs, colonies do not feed and rely uniquely on the recycling of nutrients deriving from the digestion of senescent cells. The massive ingestion of effete cells causes, in turn, an increase in reactive oxygen metabolite production and nitric ion release leading to the death of phagocytes and subsequent clearance by other phagocytes, so that a “Russian doll effect” can be observed. Finally, large macrophage-like cells accumulate in the pharyngeal lacunae and are continuously eliminated through the peribranchial chamber and then the cloacal siphon with a discharging mechanism never previously described which continues in the first phases of the mid-cycle. The consistent disappearance of large phagocytes from the circulation is counterbalanced by a population of new, undifferentiated cells (haemoblasts) already beginning from the late takeover. This work was supported by “Progetto di Ateneo” Grant CPDA082501 to F. Cima