Morpho-functional characterization of haemocytes of the compound ascidian Botrylloides leachi (Tunicata, Ascidiacea)

A morpho-functional study of the colonial ascidian Botrylloides leachi haem ocytes was carried out to propose their classification, relationships and s pecializations. This characterization was obtained by (i) investigations of both living and aldehyde-fixed cells by light and electron microscopy; (ii ) cytochemical and cytoenzymatic assays; (iii) lectin-affinity assays; (iv) phagocytosis and haemagglutination assays; and (v) anti-CD34 immunocytoche mical assay for vertebrate haematopoietic stem cells. Results indicate that the haemoblast is a circulating stem cell and there are at least five haem ocyte differentiation pathways, the last two of which have never been descr ibed in botryllids: (i) phagocytic line (hyaline amoebocytes and macrophage -like cells) share ultrastructural features, the same hydrolytic enzymes an d WGA lectin binding, and are involved in yeast phagocytosis and erythrocyt e rosette formation; (ii) cytotoxic line (granular amoebocytes and morula c ells) with vacuoles containing oxidative enzymes and polyphenolic substrate s; (iii) vacuolated cell line (pigment cells and nephrocytes) involved in c atabolite storage; (iv) compartment cell line (compartment amoebocytes and compartment cells) able to agglutinate erythrocytes and characterized by va cuoles with a moderately electron-dense content, positive to arylsulphatase activity and binding DBA, UEA-I, HPA lectins; and (v) granular cell line i ncludes trophic cells, able to infiltrate the gut epithelium, showing a cyt oplasm filled of PAS-positive vacuoles with arylsulphatase, chloroacetylest erase and beta -glucuronidase activities.