Apoptosis and pattern of Bcl-2 and Bax expression in the alimentary tract during the colonial blastogenetic cycle of Botryllus schlosseri (Urochordata, Ascidiacea).

Botryllus schlosseri is a colonial ascidian representing a suitable subject for studies on cyclical involution and resorption of tissues. At 19°, adult zooids remain active for one week, then they are gradually resorbed, being replaced by a new generation of adult zooids, represented by buds which reach functional maturity. This stage, known as regression or takeover, is characterised by diffuse programmed cell death by apoptosis in zooid tissues, as shown by annexin V labelling for detection of exposed phosphatidylserine and the TUNEL reaction for chromatin fragmentation. Immunocytochemical assays to detect proapoptotic and antiapoptotic factors reveal the opposite expression of Bcl-2 and Bax proteins, which extends in tissues of adult zooids with an antero-posterior progression starting from the pharynx. Bax is expressed in all tissues of the digestive tract in the stage preceding take-over: together with signs of DNA fragmentation in the pharynx and pyloric gland, it suggests that this is the crucial stage for triggering programmed cell death in old zooids. Residual zooids remain segregated for a long time as small dark vesicles rich in melanins and lipofuscins. Results support the idea that fundamental mechanisms for the induction of apoptosis are well conserved throughout chordate evolution.