Angiogenic-like sprouting mechanism during normal and regenerative development in a colonial ascidian

Tunicates are useful models for comparing differing developmental processes such as embryogenesis, asexual reproduction, and regeneration, because they are the closest relatives to vertebrates and are the only chordates to reproduce both sexually and asexually. Among them, the ascidian Botryllus schlosseri forms colonies of numerous individuals embedded in the common tunic, that is the peculiar extracellular matrix (ECM) of tunicates. In B. schlosseri colonies this ECM holds also the colonial circulatory system (CCS): an anastomized network of vessels defined by simple epithelia and connected to the open circulatory system of the zooids. Recently, we have demonstrated that, during asexual propagation, new vessels form by means of a tubular sprouting mechanism, resembling that occurring in other metazoans, particularly during vertebrate angiogenesis. Moreover the immunohistology of CCS structures using antibodies against vertebrate angiogenic factors (VEGF, FGF-2, EGF) and receptors (VEGFR-1, VEGFR-2, EGFR), have shown that CCS sprouting occurs with their participation (1). In B. schlosseri, the CCS possesses a remarkable regenerative potentiality, as shown by its ability to reform tunic and peripheral vascular network. We have also studied the regeneration of experimentally ablated CCS by analyzing the general dynamics of reorganization of vessels and tunic, their ultrastructure, cell proliferation, and the immunohistology of regenerating structures (2). Results show that the regenerative process of CCS occurs, similarly to normal growth, by the sprouting mechanism, with participation of same angiogenic factors. Statistical analyses on recent experiments are now indicating a significant increase of tunic and vessels regeneration in groups of colonies injected with angiogenic factors (EGF, VEGF) with respect to injected with PBS. This demonstrates that same factors have an angiogenic impact in B. schlosseri and vertebrates circulatory systems. Despite the different embryonic origin of their tissues (the former has an ectodermic origin, the latter mesodermic), all these data indicate that an homologous morphodynamic mechanism, controlled by homologous signal pathways, is involved during normal growth and regeneration of both systems. As a conclusion 1) we can hypothesize that tubular sprouting had a parallel evolution in these two structures, 2) our data support the idea that this morphogenetic mechanism was co-opted during the evolution of various developmental processes in several taxa.