An Electron Microscope Study of Endocytosis Mechanisms and Subsequent Events in Mercenaria mercenaria Granulocytes

Abstract

Foreign elements, molecules, or organisms naturally or experimentally introduced into naive molluscs are, although with exceptions, phagocytosed (see reviews by Cheng, 1975, 1981). Of the two categories of hemocytes common to all bivalves, hyalinocytes and granulocytes (Cheng, 1981), the latter in Crassostrea virginica and Mercenaria mercenaria are found to be the most active from the standpoint of phagocytosis (Foley and Cheng, 1975). There are three ways, involving semi-permanent filopodia, classical endocytosis and afunnel-like pseudopodia, by which bacteria are engulfed by hemocytes. The uptake mechanism has been studied at the electron microscope level in Crassostrea gigas (Ruddell, 1971; Feng et al., 1977), C. virginica (Cheng and Cali, 1974; Cheng, 1975), and Mytilus coruscus (Feng et al., 1977). Furthermore, as a rule, such phagocytosed materials, if digestible, are degraded by intracellularly, and some are removed when cells containing such materials migrate to the exterior through epithelial borders (Cheng, 1977a). Since granulocytes are more phagocytic than hyalinocytes, and one of the major differences between these two types of cells is the occurrence of large numbers of cytoplasmic granules in granulocytes, further studies to resolve the nature of these granules revealed that those in C. virginica are electron-lucid vesicles each of which possesses a complex wall (Feng et al., 1971; Cheng and Cali, 1974; Cheng et al., 1974; Cheng, 1975). However, the granules of M. mercenaria granulocytes, which are membrane-bound vesicles containing a homogeneously electron-dense substance (Cheng, 1975; Cheng and Foley, 1975), have subsequently been demonstrated to be true lysosomes (Yoshino and Cheng, 1976). Furthermore, it has been established that when challenged with foreign materials, there is hypersynthesis of certain lysosomal hydrolases within the hemocytes and their subsequent release into serum (Cheng et al., 1975; Cheng and Butler, 1979; Cheng and Mohandas, 1985; Mohandas and Cheng, 1985). These lysosomal hydrolases, which are associated with intracellular degradation (Cheng and Cali, 1974; Cheng et al., 1974), have also been demonstrated to have antimicrobial properties (McDade and Tripp, 1967; Cheng, 1978).