Internal defenses of the snail Biomphalaria glabrata.

We describe three distinct types of cells among Biomphalaria glabrata hemocytes: large cells with a tubulo-vesicular compartment, a component of the endocytic system, and with numerous mitochondria and large aggregates of glycogen particles; medium-size cells poor in organelles and glycogen; and small cells with organelles and few secretory granules. Other small hemocytes can be interpreted as juvenile cells. B. glabrata hemocytes contain few enzymes and do not show specific secretory granules, except for a subpopulation of large cells richer in acid phosphatase vesicles. Hemocytes have different aspects corresponding to different physiological states and their transitions: in quiescent hemocytes, the cell cortex is narrow and organelles are scattered in the cytoplasm, both in circulating cells characterized by thin-folded filopods and large macropinocytic vacuoles and in sedentary cells in which extended filopods connect to the extracellular matrix. In stress-activated hemocytes, the cortical region is thickened by polymerization of actin, and organelles are gathered around the nucleus. Fixed phagocytes are components of the connective tissue; the presence of numerous lysosomes and residual bodies and of acid phosphatase and peroxidase activities suggests a high phagocytic activity. Copyright 1999 Academic Press.