The gut and protonephridia.

<p><b>A</b>. Transmission electron micrograph of the gut epithelium of an <i>S. mansoni</i> male. The cytoplasm of the syncytial gastrodermal epithelium (ga) is rich in rough endoplasmic reticulum (rer) and Golgi apparatus, typical of a cell synthesizing proteins and glycans for export; unlike the tegument it lacks obvious secretory inclusions. The luminal surface is extended by numerous thin lamellae (l) 3–5 microns long, in place of the conventional microvilli of an absorptive gut. Dense aggregates (“blobs”) of erythrocyte stroma (s) lie adjacent to the lamellae, together with paler lipid droplets (d). <b>Inset</b>, Stromal blob containing a hemozoin (Hz) pigment granule (g), with several more free granules adjacent. P, G and L denote Parenchyma, Gastrodermis and Lumen, respectively. <b>B</b>. Some (but not all) lipid droplets (d) have a dark ring of Hz around the periphery. <b>C</b>. The distribution of lipid droplets in the gut lumen, the epithelial syncytium and the surrounding parenchyma, is suggestive of transcytosis (i.e., the process of metabolite import into vesicles on one side of a cell followed by their release on the other side). <b>D</b>. Video frame from a feeding experiment during which an adult male <i>S. mansoni</i> regurgitated gut contents. Dark Hz demarcates the bifurcated gut (arrowheads) while a thin line of Hz (arrows) can be seen passing up the lumen of the posterior esophagus. (Numerous out-of-focus erythrocytes surround the worm. Dotted outline denotes ventral sucker.) <b>E</b>. Dorsal aspect of a female <i>S. japonicum</i> worm stained with FITC-labelled pea-nut agglutinin showing the bilateral distribution of flame cells (green dots) and protonephridial tubules running towards the main lateral collecting ducts on either side of the body. <b>F</b>. Higher magnification showing that each tubule terminates in a flame cell. The green dots are intense aggregates of O-glycan at the point where the flame cell connects to its tubule. Scale bars: A, 2 µm, inset 1 µm; B, C 0.4 µm; D, E 100 µm, F, 20 µm.</p>