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 : 18-19

4.3. Esophageal ultrastructural features of A. simplex

Thirdly, our ultrastructural data allowed us to subdivide the long esophagus of A. simplex into two morphologically different parts - a short anterior and a long middle - posterior esophagus. The anterior esophageal portion resembles that of the body tegument and retains morphologically similar cytoplasmic inclusions and sunken perikarya, but differs in the absence of knob-like surface outgrowths and the presence of irregular, broad and angular cytoplasmic protrusions along the esophageal cytoplasmic lining. However, the middle-posterior esophageal portion retains only the basic tegumental cytoarchitecture and possesses considerable morphological modifications of the syncytial cytoplasmic lining by variations in the shape of long, bunched, extensive cytoplasmic protrusions, forming a reticular network through the esophageal lumen, and the sunken epithelial bodies are modified into well-developed esophageal glands. In A. simplex , however, based on a light microscopical description by Thulin (1980), the length of the esophagus in adult specimens is about 1/6 of the total body length and it may be divided into two parts, the first of which occupies 2/3 of the total esophageal length. Our TEM data supported another kind of esophageal morphological division, which corresponds to the data observed for schistosomatid blood flukes. As in A. simplex , the anterior portion of the esophagus in S. mansoni has tegumental inclusions identical to those of the external tegument, while in the middle and posterior portions the folded projections contain other cytoplasmic inclusions and the esophageal nucleated cell bodies form so-called esophageal glands ( Morris and Threadgold, 1968; Ernst, 1975). Besides, in the freshwater adult aporocotylid blood fluke, S. inermis , the esophagus is divided into three regions, distinguished by the morphology and arrangement of the apical cytoplasmic lining ( McMichael-Phillips et al., 1994a, b). In the three abovementioned blood flukes, the esophageal epithelial cytoplasm consists of a number of morphological characters, which are unique for each studied species ( Morris and Threadgold, 1968, Ernst, 1975; McMichael-Phillips et al., 1994a; Present study). Generally, in the Trematoda the esophagus varies in the fine morphology from group to group. In the trematode Aspidogaster conchicola , the short esophagus has tegumental traits with a number of uniciliate sensory endings ( Halton, 1972). In a plagiorchiid digenean of the family Paramphistomatidae , Paramphistomum epiclitum , the esophageal lining has irregular corrugations with flattened ridges and secretory esophageal cytons ( Mattison et al., 1992). Another plagiorchiid digenean of the family Gyliauchenidae , Gyliauchen nahaensis , has three morphologically distinguishable esophageal parts ornamented with rugae-like surface projections and secretory esophageal sunken cytons ( Jones et al., 2000). Aporocotylids have a diet of dissolved matter in the blood fluid surrounding them. Interestingly, in the case of plagiorchiid gyliauchenids and paramphistomids with fluid diet (see Mattison et al., 1992; Jones et al., 2000), Jones et al. (2000) suggested that there has been a succession of innovations in the evolution of the feeding of gyliauchenids. Their oral sucker was lost first as a change to a fluid-based diet occurred, and the esophagus lengthened and adopted a major role in the digestion of the food ( Jones et al., 2000). The above suggestion by Jones et al. (2000) may also be true for aporocotylid digeneans. As mentioned above, the absence of an oral sucker is not characteristic of all members of the family Aporocotylidae . Some adult fish blood flukes retain the cercarial anterior sucker, but seemingly lack a pharynx ( Bullard and Overstreet, 2003; 2004; McVay et al., 2011).

For schistosomatid blood flukes, it has been suggested that the digestion of host erythrocytes commences in the esophageal region of the digestive tract, and that the final phases of digestion occur in the lumen of the intestine (Morris, 1968; Ernst, 1975). Schistosomes have a glandular esophagus, and secretory cells are modified sunken epithelial bodies, which are a source of digestive enzymes, and digestion of blood cells begins in the esophagus ( Ernst, 1975; Halton, 1997). Present data on the A. simplex esophagus are in complete agreement with those obtained for schistosomes. As shown in our study, the esophageal morphological features of A. simplex might suggest a digestive-absorptive function for the middle-posterior esophageal region surrounded by compact cellular complex, in which sunken glandular perikarya of different development stages and their cytoplasmic processes mix with muscle cells as well as with extensive nerve fibres. In A. simplex each esophageal secretory granule has heterogeneous content with a dense thin outer halo and less dense inner core. Such granules are surrounded by the lucent or moderately dense matrix of the vacuole, within which each granule is embedded. Such a glandular appearance shows morphological similarity with digestive vacuoles described in the gastrodermal digestive cells of polyopisthocotylean monogeneans ( Poddubnaya et al., 2015; Cable and El-Naggar, 2021). In the cercaria of the freshwater aporocotylid, S. inermis , the esophageal secretory granules are similar to those of A. simplex , but there is no information on their vacuolar appearance ( McMichael-Phillips et al., 1994a, b). However, in schistosomatid S. mansoni , esophageal secretory granules of varied shape display a highly structured morphology, each granule consisting of a dark substance with crystalline structure ( Morris and Threadgold, 1968) or with rays of tubules ( Bogitsh and Carter, 1977). Usually, particular attention has been concentrated on the cytochemical composition of the esophageal secretory product in S. mansoni ( Ernst, 1975; Bogitsh and Carter, 1977). Ernst (1975) indicated that the posterior esophageal portion of S. mansoni contains substantial acid phosphatase activity both in the lining of the esophageal lumen and in the sunken perikarya connected to this lining. The reaction product in this region occurs in some membrane-bound vesicles and in the basal infoldings of the cytoplasmic lining. These acid-phosphatase-positive vesicles are found in the esophageal cell bodies and in the Golgi complexes and granular endoplasmic reticulum, suggesting that these proteins are being synthesized and packaged in the cell bodies and then transported to the esophageal cytoplasmic lining. Also, the cytochemical results of Bogitsh and Carter (1977) have shown that esophageal granules contain carbohydrate-containing endopeptidase, which is capable of digesting hemoglobin. Biochemical and molecular data indicated proteolytic pathways of haemoglobin digestion in schistosomes ( Halton, 1997). Immunocytochemistry has shown that the complex of hydrolytic proteinases (cysteine endopeptodases) is involved in the sequential degradation of haemoglobin to readily absorbable peptides, all of which are expressed in the schistosome gastrodermis ( Dalton et al., 1995).

The abovementioned cytochemical data for S. mansoni is supported by our morphological data on the A. simplex esophagus. In studied specimens of A. simplex , clots of host blood cells in various stages of decay were found in close proximity to epithelial cytoplasmic protrusions within the lumen of the posterior esophagus. As a result of blood cell disintegration, the conglomerations of moderately dense substance, in which small vesicles and dense formless inclusions are embedded, may be seen in the esophageal lumen.

Additional data by Ernst (1975) shows that, like the tegument, the anterior portion of the esophagus contains negligible cytochemically demonstrable acid phosphatase activity. Contrary data by Cesari (1974) show that most of the acid phosphatase activity in S. mansoni males is located in the epidermis, but in the opinion of Ernst (1975) the techniques employed by Cesari (1974) do not allow the determination of the original cytological site of the soluble enzyme activity measured in the supernatant fractions. Also, Pappas (1988) suggested that it is basically correct that the tegument is the primary source of nutrients for adult digeneans during short-term in vitro incubations, but that “trematodes do not readily feed under artificial (in vitro) conditions, so the value of such experiments remains questionable” (see Pappas, 1988, p. S110).

Additionally, besides differentiated esophageal secretory perikarya, there are undifferentiated cells, cells at the beginning stage of their differentiation and disintegrated cells in the compact cellular masses surrounding the middle and posterior esophagus of A. simplex . Taking into account that esophageal glandular cells are considered to be an initial site of blood processing, with blood cells being lysed as they pass through the esophagus, the new esophageal glandular cells should be produced prior to the initiation of blood feeding and be accompanied by apoptosis of utilized esophageal glands. The presence of extensive nerve fibres supporting the foregut of A. simplex suggests that such processes are controlled by the nervous system.