Variations in hemocyte counts in the mussel, Mytilus edulis: Similar reaction patterns occur in disappearance and return of molluscan hemocytes and vertebrate leukocytes

It was asked whether variations in hemocyte counts in a mussel can be explained by mechanisms known to govern the leukocyte number in vertebrates. Hemolymph of 25 freshly collected Mytilus edulis contained (4.2 ± 1.75) × 106 cells/mL including basophilic and eosinophilic granulocytes and 6.6 ± 5.5% hyalinocytes (15 animals). After 12 or 30 days under optimal laboratory conditions, hemocytes in circulation decreased to less than 1 × 106/mL, the lowest number observed being 5 × 105 cells/mL. Within 2 min of a stressful stimulus, cell numbers doubled or increased by a factor of 3 or 4. After stressing mussels by keeping them out of water for 1 h, cell counts were as high as 1.2 × 107 cells/mL. The quick rate of increase in cell counts is not due to hemocyte proliferation. In mussels, returned to optimal water conditions, cell numbers dropped following an exponential decay curve (y = 5.6865 · (0.9936X). Not all hemocyte types decreased in number to the same extent. After a strong decrease in the total cell count induced by injection of LPS, the remaining hemocyte population contained a larger percentage of basophils. This indicated the disappearance of eosinophilic cells from the circulation. Stress situations caused their return. Hemocytopenia or stress-induced hemocytosis in M. edulis, both in conjunction with changes in the percentage of granulocytes present, resembles margination/demargination processes in mammals where the concentration of circulating leukocyte subsets depends on the expression of adhesive receptor–ligand molecules on the surface of specific leukocyte types and vascular endothelial cells. In Mytilus edulis, variations in the concentration of distinct cell groups excluded heart activity to explain cell fluctuations. Furthermore, in this mussel, where hemocyte proliferation is not the reason for rapid hemocytosis, cell divisions were nevertheless demonstrated; they seem to be important in maintaining hemocyte homeostasis as 10–20% of cells in circulation possess the capacity to proliferate. They belong to the group of basophilic granulocytes.