Echinochrome A Release by Red Spherule Cells Is an Iron-Withholding Strategy of Sea Urchin Innate Immunity

Cellular immune defences in sea urchins are shared amongst the coelomocytes - a heterogeneous population of cells residing in the coelomic fluid (blood equivalent) and tissues. The most iconic coelomocyte morphotype is the red spherule cell (or amebocyte), so named due to the abundance of cytoplasmic vesicles containing the naphthoquinone pigment echinochrome A. Despite their identification over a century ago, and evidence of antiseptic properties, little progress has been made in characterising the immunocompetence of these cells. Upon exposure of red spherule cells from sea urchins, i.e., Paracentrotus lividus and Psammechinus miliaris, to microbial ligands, intact microbes, and damage signals, we observed cellular degranulation and increased detection of cell-free echinochrome in the coelomic fluid ex vivo. Treatment of the cells with ionomycin, a calcium-specific ionophore, confirmed that an increase in intracellular levels of Ca2+ is a trigger of echinochrome release. Incubating Gram-positive/negative bacteria as well as yeast with lysates of red spherule cells led to significant reductions in colony-forming units. Such antimicrobial properties were counteracted by the addition of ferric iron (Fe3+), suggesting that echinochrome acts as a primitive iron chelator in echinoid biological defences.

Haemopoiesis in Lepidoptera. III. A note on the multiplication of spherule cells and granular haemocytes

Blood films from early sixth instar larvae of Euxoa declarata (Lepidoptera: Noctuidae) stained in hematoxylin – eosin – alcian blue showed unequivocal examples of mitosis in spherule cells. The improved visibility of mitosis and the estimation of the mitotic index from counts of dividing cells per 1000 cells of each type indicated a far greater potential maximum production of spherule cells and granular haemocytes by mitosis than reported previously. Certain other methods of staining showed similar clear examples of spherule cell mitosis.