Elucidating the role of apoptosis during cyclical body regeneration in Botryllus schlosseri

Apoptosis is the principal form of programmed cell death by which multicellular animals rid themselves of old cells in order to allow new cells to grow. The colonial ascidian Botryllus schlosseri is a model system for homeostatic cell death and regeneration. During its unique, weekly budding cycle known as blastogenesis, new buds are produced asexually and eventually replace the old adult zooid generation through resorption via circulating phagocytes. Using polyester wax‐embedded tissue sections, we have recently shown that TUNEL‐positive (terminal deoxynucleotidyl transferase nick end‐labeling), apoptotic cells were observed within circulating phagocytes during the colony's death phase (takeover). In contrast, the cells in regressing organs and tissues that had not yet been phagocytosed were TUNEL‐negative. The goal of the current study is to determine whether cells from regressing adults undergo internucleosomal DNA fragmentation characteristic of apoptosis or whether they utilize an alternative cell death mechanism not involving DNA fragmentation, such that they are undetected using TUNEL labeling. Using serial tissue sections from colonies isolated at various stages of takeover as well as budectomized colonies during which adults die but exhibit curtailed resorption, work is currently underway to address these questions to obtain a more holistic understanding of B. schlosseri blastogenesis.