Date of Award


Document Type

Open Access

Degree Name

Bachelor of Science



First Advisor

Robert J. Lauzon




cells, generation, adult, cell, dying


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 shown that TUNEL-positive (terminal deoxynucleotidyl transferase nick end-labeling), apoptotic cells are observed within circulating phagocytes in all stages of the colony’s death phase (takeover), including the final stage during which the primary bud undergoes a growth surge and becomes a functional adult zooid. Suggested explanations of these intriguing results yield three potential theories: (1) the growing primary bud is a site of cell corpse elimination for the dying generation, (2) the phagocytosed materials of the dying generation are recycled and reincorporated into the new adult generation, or (3) the apoptotic cells of the dying adult zooid generation release mitogenic factors and stimulate proliferation in neighboring cell populations. Our findings are consistent with the idea that dying cells are trophic in nature, promoting growth and differentiation during whole body regeneration.