Aporocotyle simplex (Odhner, 1900)

Poddubnaya, Larisa G., Hemmingsen, Willy & MacKenzie, Ken, 2023, Digestive system of the marine blood fluke, Aporocotyle simplex (Odhner, 1900) (Digenea: Aporocotylidae) with consideration of the digenean digestive morphology, Zoologischer Anzeiger 305, pp. 11-22 : 12

3.2. The anterior esophagus of A. simplex

The long unbranched esophagus leads directly from the muscular anterior foregut ( Fig. 1D, G and 2A ). The esophageal epithelial lining is a prolongation of the anterior foregut epithelium with sunken cell bodies ( Fig. 2C ). The epithelial syncytial lining of the short anterior esophageal portion projects its cytoplasm into the lumen, possessing luminal irregular, broad and angular cytoplasmic protrusions variable in shape and size ( Fig. 2B, C, E ). Due to these cytoplasmic protrusions, the esophageal cytoplasmic lining varies in thickness from 0.2 to 6.5 μm and contains a number of electron-dense bodies, which are similar to those that fill the body syncytial tegumental cytoplasm and anterior foregut epithelium ( Fig. 2B, C, E ). Besides tegumental dense bodies, numerous rounded vesicles (about 0.2–0.3 μm in diameter), with moderately dense content or empty-looking, fill the cytoplasmic matrix ( Fig. 2B, C, E ). These vesicles may liberate their contents into the esophageal lumen ( Fig. 2B, C, E ). An extensive system of regularly arranged tubular structures is present in the moderately dense cytoplasm of the esophageal lining ( Fig. 2E ). The luminal surface of the epithelial lining is smooth, the basal plasma membrane is surrounded by a thin electron-dense basal lamina and a thick layer of extracellular matrix, below which are thick bands of circular and longitudinal myofibrils ( Fig. 2B and C ). Sunken cell body processes connect to the epithelial lining and open into the epithelial cytoplasm ( Fig. 2C ). The distal portion of these processes contains numerous longitudinally oriented microtubules and electron-dense bodies in their cytoplasmic matrix ( Fig. 2C ). Electron-dense membranous whorls may be present within the anterior foregut as well as within the anterior esophageal lumen ( Fig. 2B ).

Figures

Fig. 1. Anterior foregut of Aporocotyle simplex (A) SEM view of anterior body region, note mouth opening (B, C) LM view of mouth opening, anterior and posterior esophagus, two anterior. and two posterior caeca and caecum bifurcation, note compact cell mass around esophageal portion (D, G) TEM view of anterior foregut portion showing mouth cavity, behind which there is a foregut portion with developed circular and radial muscles and the following anterior esophageal portion, note longitudinal muscle fibres surrounded anterior esophagus (E) Continuation of syncytial tegumental lining of the body into mouth cavity and anterior foregut, note sensory ending around mouth cavity and surface knob-like outgrowths (F) Anterior foregut side showing lateral fold and bunds of circular and radial muscles surrounding foregut syncytial cytoplasm, note well developed nerve plexus (H) Portion of body tegumental cytoplasm, note dense tegumental bodies within cytoplasm and knob-like outgrowths on the surface (I) Anterior foregut syncytial cytoplasm surrounded by circular and radial muscles. Abbreviations: ac, anterior caecum; aes, anterior esophagus; bl, basal lamina; cb, caecum bifurcation; cm, circular muscles; db, dense tegumental bodies; dtc, distal tegumental cytoplasm of the body; elm, esophageal longitudinal muscle fibres; em, extracellular matrix; fsc, foregut syncytial cytoplasm; hd, hemidesmosomes; lf, lateral fold; ko, knob-like outgrowths; maf, muscular portion of anterior foregut; mo, mouth opening; nf, nerve fibers; pc, posterior caecum; pes, posterior esophagus; rm, radial muscles; se, sensory ending; sm, surrounding cell masses; vi, vesicular inclusions. Scale bars: A = 50 μm; B = 200 μm; С = 100 μm; D = 5 μm; E, H, I = 1 μm; F, G = 2 μm.

Fig. 2. Ultrastructure of esophageal epithelial lining of Aporocotyle simplex (A) Transition between muscular area of anterior foregut and anterior esophagus, note different kinds of cytoplasmic protrusions (B) Anterior esophagus showing luminal irregular, broad and angular cytoplasmic protrusions, note tegumental dense bodies in the syncytial cytoplasm (C) Cytoplasmic processes of sunken perikarya filled with dense bodies pass into esophageal syncytial cytoplasm, note vesicles localized close to surface membrane and discharged vesicles (D) Middle esophageal lining showing thinner and branched protrusions filling luminal area, note numerous cytoplasmic vesicles and rare dense secretory granules (insert) (E) Syncytial cytoplasmic layer of anterior esophagus, showing numerous vesicles, some tegumental dense bodies and tubular structures (F, G) Portion of middle esophageal cytoplasmic lining, note vesicles, discharged vesicles and rounded secretory granules. Abbreviatons: cp, cytoplasmic processes of the sunken perikarya; dv, discharged vesicles; esl, esophageal lumen; fl, flat epithelial layer; mt, microtubules; mw, membranous whorls; pr, cytoplasmic protrusions; sc, substance conglomeration; sec, syncytial esophageal cytoplasm; sg, secretory granules; ss, smooth surface; ts, tubular structures; other abbreviations in Fig. 1. Scale bars: A, В = 2 μm; B, E = 1 μm; C, insert, F, G = 0.5 μm.