Gap junctions and particle aggregates in the tegumentary syncytium of a trematode.

The tegumentary syncytium of a Trematode is studied by transmission EM and freeze-fracture with the following results. (1) Infoldings of the basal plasma membrane suggest that transport of water and solutes occur through the tegument. (2) Heterocellular gap junctions are found between the tegumentary cell bodies and the parenchymal cells. Gap junctional particles, 8 nm in diameter, are visible on the P face of membrane and form an irregular pattern. (3) Orthogonal arrays of small particles (6 nm in diameter) are abundant on the P face of the tegument basal plasma membrane and on the cell necks connecting tegumentary cell bodies to the tegument. (4) Hemidesmosomal particles are found on the E face of the tegument basal plasma membrane. The significance of these structures with respect to tegumentery permeability and exchanges with parenchyma are discussed.

Hemocyte production in trematode-infected Lymnaea truncatula.

In trematode-infected Lymnaea truncatula, as in other lymnaeids, hemocytes are formed in the connective tissue. Mitoses are found singly in blood vessels or connective tissue or occur in hemocyte nodules, developing along the mantle epithelium or associated with blood sinuses. The so-called hemocyte-producing organ in L. truncatula is not equivalent to that in Biomphalaria glabrata, but rather involves the proximal part of the kidney sac. It has a dual structure: the main part, containing podocytes and broad hemal spaces, is adapted for hemolymph filtration; the apical portion, adhering to the mantle and pericardial epithelia, has a thicker connective-tissue frame in which hemocyte nodules may develop. The role of this region of the kidney in hemocyte formation is discussed.

Structural and cytochemical study of the hemocytes in normal and trematode-infected Lymnaea truncatula.

The ultrastructural study of adult Lymnaea truncatula hemocytes reveals a preponderance of spreading cells at different stages of differentiation. A few round cells may represent a distinct population of hemocytes. As in L. stagnalis, spreading cells are remarkable in that they synthesize peroxidase that is stored in secretory granules, but in L. truncatula hemocytes, endogenous peroxidase activity is also localized in multivesicular bodies. The ultrastructure and peroxidase content of hemocytes do not seem to be affected in snails with established parasites. Evidence that parasites interfere with normal hemocyte functions is that in the few capsules that occurred in parasitized snails the hemocytes did not spread normally and their peroxidase granules were resorbed into multivesicular bodies.

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.

Internal defenses of the snail Biomphalaria glabrata.

The three populations of hemocytes and the fixed phagocytes of Biomphalaria glabrata show different affinities for the following injected foreign materials: ferritin, horseradish peroxidase, zymosan particles, latex beads, and live bacteria. Ferritin and horseradish peroxidase, which are internalized by micropinocytosis, induce a moderate activation of hemocytes. Particles trigger a strong activation that consists of mobilization of the cytoskeleton, with formation of pseudopodia and filopodia and modifications of the plasma membrane that enable hemocytes to adhere together, as in encapsulations. Phagocytosis of zymosan and live bacteria induces alterations in mitochondria that may reflect changes in the respiratory activity. Phagocytosis of particles follows different processes: fixed phagocytes ingest latex beads of two different sizes according to the "zipper model", which implies sequential binding between phagocyte receptors and opsonized ligands; zymosan particles are ingested by medium-size hemocytes by a variant of the zipper model in which a limited close contact between cell and particle resembles focal contacts in cell spreading; bacteria or large latex beads are ingested by large hemocytes without apparent close contact following a "trigger model" which resembles macropinocytosis.

Internal defenses of the snail Biomphalaria glabrata.

Injection of foreign particles (zymosan, latex beads, living bacteria) and substances (ferritine, horseradish peroxidase) in Biomphalaria glabrata induces the rapid appearance of tubular double-helical filaments in the hemolymph. This rapid induction may be due to the polymerization of precursor proteins of the hemolymph. Tubular helical filaments are also observed in normal snails, especially in the hemal spaces of the proximal kidney; they seem to belong to the extracellular matrix and form bundles of a few parallel units associated with fine fibrils. Hemocytes adhere to these bundles by means of the filopodia tip or by small areas of the cell surface; consequently, they may become temporarily sedentary in the hemal spaces. The deposition of helical filaments seems increased in ageing snails and in snails parasitized by Echinostoma caproni.