Apoptosis and recognition of senescent cells in a compound ascidian: a comparison with Vertebrates.

Programmed cell death by apoptosis is a fundamental process in the develop-ment and tissue homeostasis of Vertebrates. Although widespread among meta-zoans, few data are available on the role of apoptosis in invertebrates. Ascidians are tunicates belonging to the phylum Chordata and, therefore, closely related to Vertebrates. We studied the expression of markers related to apoptosis and recognition of senescent cells in this group of organisms. Adult zooids of the ascidian Botryllus schlosseri are cyclically resorbed and replaced by zooids of new blastogenic generations. Take-over lasts about 20 h and is characterised by massive apoptosis in zooid tissues. During this period, a significant increase in the amount of phagocytes engulfed with dying cells or cell debris, as compared to mid-cycle stages, occurs. In addition, a significant higher frequency of cells expressing Bax, Fas and FasL, and of Bcl-2-negative cells is observed. These observations are consistent with the occurrence of both mitochondria-independent and mitochondria-dependent pathways of cell death induction. The amount of cells expressing phosphatidylserine and anti-CD36 on their surface increased as colonies approach take-over. As both these markers are involved in the recognition of effete cells by phagocytes, results strongly suggest that fundamental recognition mechanisms are well conserved throughout Chordate evolution