Phagocytic and enzymatic activities of cells and urn cell complexes in the coelomic fluid of the marine worm Sipunculus nudus (Sipuncula)

Particulate components of the coelomic fluid of the marine worm Sipunculus nudus were studied by light microscopy and their phagocytic and enzymatic activities examined. Coelomocytes include haemerythrocytes (about 90%), granulocytes and hyalinocytes (about 9%). In addition, multicellular structures, i.e., urn cell complexes are found with low frequency (about 1%). Haemerythrocytes are nucleated cells, containing the respiratory pigment haemerythrin, which show neither phagocytic nor enzymatic activities. Granulocytes and hyalinocytes (about 7% and 2% of total coelomocytes, respectively) may be distinguished into two different subpopulations, small and large cells, which may appear as round or amoeboid. Small cells appear positive to the anti-CD34 antibody for stem cells. The present study demonstrated that both circulating granulocytes and hyalinocytes represented main phagocytes, since they were able to recognise and engulf yeast cells in vitro and contain inducible hydrolytic and oxidative enzymes. In addition, both phagocytes produced superoxide anion, and the neutral red retention suggested that granulocyte granules were lysosomes. The phagocytic ability of urn cell complexes appears controversial: their ciliated basal cells can trap numerous yeast cells and show positivity for some hydrolases, but evidence of engulfment has never been observed. Lysozyme-like activity was found in coelomocyte lysate and to a lesser extent in cell-free coelomic fluid.