CYCLIC APOPTOSIS IN THE DIGESTIVE TRACT OF A PROTOCHORDATE.

Tissue degeneration which occurs during development of organisms is often of morphogenetic importance as well as proliferation and differentiation. Cyclic apoptosis of organs was progressively lost in Chordates. Botryllus schlosseri is a colonial ascidian continuously forming new zooids by blastogenesis, through the recurrent formation of palleal buds, which grow and mature until an adult is formed. Three blastogenic generations are commonly co-present: adult, filtering zooids, their buds and budlets on buds. At a temperature of 19°C, adult zooids remain active for about one week (mid-cycle stages); then they contract, close their siphons and are gradually resorbed, being replaced by a new generation of adult zooids, represented by buds which reach functional maturity, open their siphons and begin their filtering activity (regression or take-over stage). This stage is characterised by the occurrence of diffuse programmed cell death by apoptosis in zooid tissues, as evidenced by TUNEL reaction for chromatin fragmentation and annexin V labelling for detection of exposed phosphatidylserine, whereas infiltration of circulating phagocytes, which appear engulfed with apoptotic cells, is observed. With these characteristics, colonial tunicates are suitable subjects for studies on cyclical involution and resorption of tissues. In residual zooids remaining for a long time in the centre of each colony, melanin and lipofuscins accumulate as detected with Masson-Fontana, Ziehl-Nielsen and H2O2 bleaching methods. Immunocytochemical assays to detect pro- and antiapoptotic factors reveal an opposite expression which progressively extends in tissues of adult zooids with an organ gradient starting from the branchial basket. Results support the idea that fundamental mechanisms for the induction of apoptosis are well conserved throughout Chordate evolution.