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 thes e cells into granular and agranular haemocytes, as suggested by Cheng. Wher eas granular haemocytes (granulocytes) form a distinct group, agranular hae mocytes are heterogeneous in appearance and ultrastructure. Three types of agranular haemocytes are identified; blast-like cells, basophilic macrophag e-like cells, and hyalinocytes. Also the early stages of granulocyte develo pment, 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 l eucocytes, and their composition may vary greatly between individuals. Not all types occur in each bivalve species; scallops lack granulocytes, and th e hyalinocyte is a poorly defined cell type in several groups. There is evi dence of functional heterogeneity in granulocytes and macrophage-like cells , and the functions of haemocyte types cannot be reliably extrapolated betw een species. Brown cells (rhogocytes) are regarded as part of the urinary s ystem overlapping in tissue distribution and some functions. (C) 1999 Acade mic Press.