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Cheng, Thomas C. 197 5. 



Functional morphology and biochemistry of molluscan phagocytes. In 

 Pathobiology of Invertebrate Vectors of Disease. Lee A. Bulla and Thomas 

 C. Cheng (eds.) . Ann. N.Y. Acad. Sci. 266 (Pt.V) : 343-379. 



The chapter presents results of research designed to answer such questions 

 as: 1) are there different types of phagocytic cell in mollusks? 2) is one 

 type of cell more important than others? 3) is phagocytic activity in 

 mollusks influenced by ambient factors? 4) how are microorganisms taken 

 into phagocytes? 5) what hydrolytic enzymes occur in phagocytes, and what 

 are the optimum conditions for them to function? 6) what is the fate of 

 bacteria that become degraded intracellularly? 7) do the intracellular 

 enzymes function only within cells? and 8) what are the energy requirements 

 of molluscan phagocytes? Conclusions relative only to Mercenaria mercenaria 

 are reported in this abstract. Much of the work is described in other 

 papers abstracted elsewhere in this bibliography. Mercenaria mercenaria 

 once was believed to have 3 cell types : granulocytes , f ibrocytes , and 

 hyalinocytes. Granulocytes are similar to those of Crassostrea virginica , 

 with large numbers of cytoplasmic granules, restricted primarily to the 

 endoplasm. When permitted to spread against a solid substrate these cells 

 produce thin filopodia, each with a supporting riblike structure which 

 originates in the endoplasm. Cytoplasmic granules of M. mercenaria 

 granulocytes are denser in appearance, and elongate vermiform granules 

 also are present in most cells. Fibrocytes of M. mercenaria are similar 

 to what had been designated as secondary fibrocytes of C. virginica , and 

 are now believed to be degranulated granulocytes. Hyalinocytes of M. 

 mercenaria include less cytoplasm than granulocytes, are less mobile, and 

 are essentially without cytoplasmic granules, but contain vacuoles, 

 vermiform bodies (possibly mitochondria) , and a few retractile bodies in 

 the cytoplasm. Studies with the transmission electron microscope suggest 

 that cells of M. mercenaria originally defined as fibrocytes may be 

 considered as cells at the end of the physiological cycle relative to 

 phagocytosis and intracellular degradation. Cytoplasmic granules of 

 M. mercenaria are membrane delimited and include a homogeneously electron- 

 dense material. Some granules are elongate and vermiform. The functions 

 of these organelles are not known. In addition to dense granules, M. 

 mercenaria granulocytes include mitochondria, membrane-bound electron-lucid 

 vesicles, lysdsome-like bodies, glycogen granules, Golgi apparati, smooth 

 and rough endoplasmic reticulum, and lipid droplets in their cytoplasm. 

 Some cells have a centrosome. Hyalinocytes of M. mercenaria lack large 

 electron-dense cytoplasmic granules, but a few smaller electron-opaque 

 membrane-bound vesicles occur in the cytoplasm. Also contained are 

 electron-lucid vesicles of varying sizes, liquid droplets, glycogen granules, 

 and rough endoplasmic reticulum. Centrosomes and Golgi apparati have not 

 been observed in hyalinocytes. The numbers of bacteria per unit area of 

 each cell type exposed to certain bacterial species was greatest for 

 granulocytes, which was interpreted to mean that molluscan granulocytes are 

 the most important cells in phagocytosis. The only ambient factor that has 

 been studied with respect to phagocytic activity is temp. Phagocytic 

 activity is enhanced at higher temperatures. Bacteria are engulfed by a 

 granulocyte in 2 ways, by filopodia and by endocytosis. In addition to 

 phagocytosis of particulate nonself material, pinocytosis of soluble 

 molecules has been reported. Lysozyme, acid and alkaline phosphatases, 

 B-glucuronidase, and lipase occur in serum and cells of M. mercenaria. 

 Bacteria are not totally composed of carbohydrates. Noncarbohydrate 

 constituents are not passed from primary to secondary phagosomes , but are 

 expelled from primary phagosomes to the exterior of the phagocyte. Large 

 liquid droplets have been found within a spent phagocyte of M. mercenaria. 

 Carbohydrate constituents of phagocytized bacteria are eventually converted 

 into glycogen, which is discharged from cells into serum. It is likely 

 that this carbohydrate, after hydrolysis to glucose, can be distributed by 

 hemolymph to body tissues and utilized for energy production. Phagocytosis 

 appears to serve 2 functions, eliminating certain nonself materials and 

 providing a nutrient source. It is concluded that phagocytosis was 

 originally a nutrient-acquiring process, and the function of defense was 

 acquired later in evolution. The overall process of phagocytosis in 



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