Angiogenic-like mechanism in the colonial circulatory system of B. schlosseri

The colonial circulatory system (CCS) of the ascidian Botryllus schlosseri runs in the common tunic and forms an anastomized network of vessels, defined by simple epithelium, connected to the open circulatory system of the zooids. The CCS originates from epidermal evagination, grows and increases its network accompanying colony propagation by means of mechanisms of tubular sprouting. The regeneration of experimentally ablated CCS shows that the regenerative process of CCS occurs by the same sprouting mechanism. In the two cases (normal growth and regeneration of CCS), the same histogenetic mechanisms and homologous factors and receptors are shared. Strong similarities in structure (e.g., cell filopodia) and organization (e.g., simple cubic epithelium) in the apexes of sprouting vessels during normal growth and regeneration are observed. Immunohistological responses to anti-PCNA as a proliferative marker and antibodies against vertebrate angiogenic growth factors VEGF, EGF, FGF-2 and receptors VEGFR-1, VEGFR-2, EGFR during asexual propagation and regeneration, reveal that proliferation and angiogenic signals are found in corresponding sprouting regions. Sprouting is the most common and best-known mechanism of vertebrate angiogenesis, but it is also found in other developing organs, such as nerve formation and in Drosophila tracheas. All our observations show that correspondences exist between the CCS sprouting modality of B. schlosseri and angiogenic sprouting in vertebrates, during both normal development and regeneration, and support the idea that this morphogenetic mechanism was co-opted during the evolution of various developmental processes in different taxa.