Fig. 2

Microscopic images of the tegumental syncytial layer covering Sparicotyle chrysophrii clamps: A Light microscopy, B–J transmission electron microscopy. A Toluidine blue-stained section of gilthead sea bream (Sparus aurata) gill lamellae pinched by the parasite’s clamps. Note the syncytial layer covering the parasite’s clamps in close contact with the host lamellar tissue. B The three most distal clamps in the parasite’s opistohaptor. Note the syncytium covering the luminal and outer surface of the clamps. C Distal end of the clamp sclerites and associated myofilament bundles. Note the heterogeneity of the covering syncytium, which is a thin layer with flat surface at the clamp luminal side and thicker at the outer side and distal end, with an undulating surface. Higher magnification of the tegumental syncytium covering the clamp distal end at the tip (D) and luminal side (E). Note the presence of extracellular vesicles apparently emerging from the syncytial surface in the luminal part of the clamp and the abundance and diversity of vesicle types in the syncytium. F Detail of free extracellular vesicles in the clamp lumen, close to the clamp syncytial surface. G Detail of the undulating syncytial surface in the apparent process of secreting extracellular vesicles to the clamp lumen. H, I Two magnifications of a free extracellular vesicle in the clamp lumen, close to its syncytial surface. Note the bilayer enclosing coarse, moderately electrodense material. J Free extracellular vesicle in the clamp lumen in an area where the syncytium covering the clamp surface contains different vesicle types and multivesicular structures (arrows). Gi, gill filament; Hd, hemidesmosomes; Lu, clamp lumen; LV, large vacuoles; Mi, mitochondria; Mu, clamp myofilament bundles; EV, extracellular vesicles; Sc, sclerite; Sy, syncytial layer; V, syncytial vesicles. Scale bars: (A–C) = 20 µm, (D–E and J) = 2 µm, (F–I) = 200 nm