Structure and Functions of Oyster Hemocytes

Abstract

There are a number of characteristics which should be considered in a definitive classification of hemocytes [40], but the initial criterion is most often morphology. Although no general agreement on the number of different cell types in oyster hemolymph has been reached, most investigators divide the hemocytes into at least two major classifications: the granular and the agranular [7,40]. There are apparent differences in the roles that granular and agranular cells play in any organism, and those roles are not necessarily the same for each species. The granular hemocytes are generally larger than the agranular cells and contain walled vesicles (granules) in the cytoplasm. Using light microscopy, investigators have also been able to distinguish acidophilic and basophilic granulocytes [7], stem cells [2], slightly granular cells [40], and differences based on nuclear size [28]. But even with ultrastructural investigation, these studies of oyster hemocytes have not led to a generally accepted scheme of nomenclature and classification. Rather, they have emphasized differences between oyster species and raised conflicting evidence within species. For example, there are major disagreements between invertigations examining the ultrastructure of Crassostrea virginica agranular hemocytes [27,34,48]. Also, the easily distinguishable C. virginia granulocytes was first beleived to be two cell types [30] due to its altered morphology upon degranulation [7].