Digestive system of the marine blood fluke, Aporocotyle simplex (Odhner, 1900) (Digenea: Aporocotylidae) with consideration of the digenean digestive morphology Author Poddubnaya, Larisa G. Author Hemmingsen, Willy Author MacKenzie, Ken text Zoologischer Anzeiger 2023 2023-07-31 305 11 22 http://dx.doi.org/10.1016/j.jcz.2023.05.003 journal article 281827 10.1016/j.jcz.2023.05.003 fef84de4-8e9b-4218-84b9-1c3a3c507240 1873-2674 10376067 3.4. Gastrodermis of A. simplex Along its entire length, the syncytial epithelial lining of the caecum is composed of a morphologically uniform, syncytial layer, possessing considerable variation in the thickness of the epithelial lining from 0.5 to 15.0 μm, without marked differences between its anterior and posterior regions ( Fig. 4A and B ). Most of the luminal volume of the caecum is filled with moderately dense amorphous finely dispersed material ( Fig. 4A, B, I, K ). In addition, there are both dark inclusions of different shape and different kinds of residual bodies within the caecal lumen ( Fig. 4A and K ). The gastrodermal luminal surface is increased by numerous thin, flexible lamellae, which project a short distance into the lumen and range in length from 0.9 to 2.5 μm ( Fig. 4B and K ). The basal plasma membrane of the gastrodermal syncytial lining is thrown into numerous basal invaginations, which may be confined to the lower haft of the gastrodermis, but may extend to the luminal membrane ( Fig. 4C ). The gastrodermal lining is underlain by a thin basal lamina and thicker layer of fibrous extracellular matrix ( Fig. 4B ). Thin fibres of circular muscles are embedded in the surrounding extracellular matrix ( Fig. 4C and D ). The syncytial cytoplasm contains elongated oval or irregularly shaped nuclei with dense patches of heterochromatin, which usually occupy the central or basal positions within the epithelial lining ( Fig. 4A and B ). The dense cytoplasmic matrix of the gastrodermis contains cisternae of granular endoplasmic reticulum, which are usually located in the proximity of the nuclear areas ( Fig. 4B and H ). Golgi bodies give rise to ovoid membrane-bound rounded vesicles ranging from 0.1 to 0.3 μm in diameter, which may occur throughout the syncytial layer ( Fig. 4B, G, H , insert, K). These vesicles contain an agglomeration of finely dispersed, flocculent and moderately dense material ( Fig. 4G and H insert, I). Regularly arranged tubular arrays are scattered throughout the gastrodermal syncytial lining ( Fig. 4E, H, I, J ). Occasionally, multivesiculate bodies (about 0.4 μm in diameter) are dispersed in the upper portion of the syncytial cytoplasm ( Fig. 4G ). Different kinds of residual bodies are scattered throughout the cytoplasm ( Fig. 4C D, F, H, I, J ). Some bodies show a combination of alternate electron-dense glandular and moderately dense finely dispersed material ( Fig. 4F, H, I, J ). The membrane-bound Golgi vesicles appear to fuse with residual bodies ( Fig. 4F and I ). Occasionally, there are large residual bodies (from 2.0 to 2.6 μm in diameter) containing heterogeneous, flocculent and moderately dense material, within which are scattered clumps of electron dense material ( Fig. 4C and D ). In Fig. 4H three stages of development of such bodies may be observed in the gastrodermal cytoplasm. The nascent body ( nb ) is about 0.4 μm in diameter containing moderately dense flocculent, loosely packed material with a few pinpoint clumps of dense material and, a short way off it, the network of tubular structures and Golgi vesicles ( Fig. 4H ). As such bodies develop, their diameter begins to increase from 0.9 μm ( dlb1 ) to 1.5 μm ( dlb2 ) and the amount of both flocculent moderately dense and clumpy dense material increases ( Fig. 4H ). In the worms studied, along the entire length of the caecum there are areas in a secretory-absorptive phase possessing a highly vacuolated luminal surface, where the lamellae may be of various configurations and appear to be recurved to form loops ( Fig. 4B, I, J ). In such gastrodermal areas the percentage of the granular endoplasmic reticulum and the Golgi complexes increases and the amount of Golgi vesicles begins to increase in the gastrodermal cytoplasm ( Fig. 4B and K ). The numerous vesicular-like surface depressions are filled with trapped material from the gastrodermal lumen, demonstrating the subsequent stages of their endocytosis into the gastrodermis ( Fig. 4I–K ). Also, between flattened lamellae an agglomeration of residual material is entrapped ( Fig. 4I and J ).