Searching for the Origin and the Differentiation of Haemocytes before and after Larval Settlement of the Colonial Ascidian Botryllus schlosseri: An Ultrastructural Viewpoint

The colonial ascidian Botryllus schlosseri possesses an innate immunity, which plays fundamental roles in its survival, adaptability, worldwide spread and ecological success. Three lines of differentiation pathways circulating haemocytes are known to be present the haemolymph, starting from undifferentiated haemoblasts: (i) phagocytic line (hyaline amoebocytes macrophage-like cells), (ii) cytotoxic (granular morula cells) (iii) storage cell (pigment cells nephrocytes). Many questions remain about their origin, thus, observations during various stages development were undertaken this study. Haemocytes detected beginning early tailbud embryo stage. Haemoblasts always first differentiated detected. In both next stage, just before hatching, swimming tadpole larva hyaline pigment also recognisable. Some containing active phenoloxidase migrated haemolymph into tunic after having crossed epidermis, behaviour could related preparation a defensive function for spatial competition. During larval metamorphosis, appeared with phagosomes positive acid phosphatase activity apoptotic tail tissue degeneration. After filter-feeding oozoid nephrocytes involved nitrogen catabolism finally appeared. subendostylar sinus peripheral blind-sac vessels (ampullae), clusters haemoblasts recognisable, some showed incipient specialisations, considering hypothesis presence putative niches stem cells.