Histological and haematological analysis of the ascidian Botrylloides leachii (Savigny, 1816) during whole-body regeneration

Abstract

Abstract (250 words)Whole-body regeneration, the formation of an entire adult from only a small fragment of its own tissue, is extremely rare among chordates. Exceptionally, in the colonial ascidian Botrylloides leachii, a fully functional adult is formed from their common vascular system, upon ablation of all adults from the colony, in just 10 days thanks to their high blastogenetic potential. While previous studies have identified key genetic markers and morphological changes, no study has yet focused on the haematological aspects of regeneration despite the major involvement of the remaining vascular system and the contained haemocytes in this process. To dissect this process, we analysed colony blood flow patterns using time-lapse microscopy to obtain a quantitative description of the velocity, reversal pattern, and average distance travelled by haemocytes. We also observed that flows present during regeneration are powered by temporally and spatially synchronized contractions of the terminal ampullae. In addition, we revised previous studies on B. leachii haematology as well as asexual development using histological sectioning, and compared the role of haemocytes during whole-body regeneration. We found that regeneration starts with a rapid healing response characterized by blood clotting and infiltration of immunocytes, followed by increased activity of haemoblasts, recruitment of macrophage-like cells for clearing the tissues of debris, and their subsequent disappearance from the circulation concomitant with the maturation of a single regenerated adult. Overall, we provide a uniquely detailed account of the haematological properties of regenerating B. leachii colonies, providing novel lines of inquiry towards the decipherment of regeneration in chordates.