The inter-relationships of bivalve haemocytes

The morphology, tinctorial properties, ultrastructure and some functions of bivalve haemocytes are reviewed in relation to the simple division of these cells into granular and agranular haemocytes, as suggested by Cheng. Whereas granular haemocytes (granulocytes) form a distinct group, agranular haemocytes are heterogeneous in appearance and ultrastructure. Three types of agranular haemocytes are identified; blast-like cells, basophilic macrophage-like cells, and hyalinocytes. Also the early stages of granulocyte development, and spent granulocytes, may be agranular. The distribution of blast-like cells suggests haematopoiesis may be widespread in connective tissue, with further development of haemocytes in the haemolymph. Consequently, the haemocytes of bivalve haemolymph are less differentiated than vertebrate leucocytes, and their composition may vary greatly between individuals. Not all types occur in each bivalve species; scallops lack granulocytes, and the hyalinocyte is a poorly defined cell type in several groups. There is evidence of functional heterogeneity in granulocytes and macrophage-like cells, and the functions of haemocyte types cannot be reliably extrapolated between species. Brown cells (rhogocytes) are regarded as part of the urinary system overlapping in tissue distribution and some functions.