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Investigation of Blood Cell Populations in the Development and Regeneration of a Colonial Ascidian

Botrylloides violaceus is a colonial marine invertebrate with the capability of regenerating its whole body when only blood cells and vessels are present. These tunicates are more closely related to vertebrates than other model organisms that undergo whole‐body regeneration. Regeneration is induced when the organism sustains an injury and is carried out by a circulating stem cell population. Botrylloides’ sister genus, Botryllus, also has the capability to regenerate; however, regeneration in Botrylloides occurs more readily. We investigated whether stem cell abundance in the blood differs across life stages of B. violaceus and between the two genera. Using brightfield microscopy, we quantified blood cell types from different life stages and from regenerating animals, emphasizing the hemoblasts which include the stem cell population. Preliminary data show a significant decrease in hemoblasts between adult and young Botrylloides, a decrease and then plateau during the regeneration process, and significantly fewer hemoblasts in adult Botryllus than adult Botrylloides. Our data suggest that a high number of stem cells within circulating blood may be important for inducing regeneration. The decrease in frequency in regenerating animals may imply that stem cells are partially depleted during regeneration.