Hemolymph cell types of the mussel Mytilus galloprovincialis

Two types of circulating hemocytes were identified in Mytilus galloprovincialis hemolymph: hyalinocytes and granulocytes. The hyalinocytes are agranular cells and show a central nucleus surrounded by relatively small cytoplasm. The granulocytes are larger than hyalinocytes and have smaller nuclei. Three classes of granulocytes (acidophilic granulocytes, basophilic granulocytes and granulocytes containing both types of granules) were distinguished based on staining properties of their cytoplasmic granules. At the ultrastructural level, the hyalinocytes show characteristics of undifferentiated cells. The granulocytes are more differentiated and have membrane-limited cytoplasmic granules. Some granulocytes contain only small granules; others have numerous large granules and some of them contain a mix of granule sizes. Density gradient centrifugation allowed the separation of acidophilic granulocytes from the other types. Immunocharacterization demonstrated that the granulocytes are an antigenically heterogeneous population.