Transmission electron microscopy of intra-tegumental sensory receptors in the forebody of Crepidostomum metoecus (Digenea: Allocreadiidae)

Abstract

Abstract. In the adult fish trematode Crepidostomum metoecus (Braun, 1900), four types of sensory receptors were observed inside the forebody tegument and one type beneath the tegument basal lamina. Two types of sensory receptors extend through the thickness of tegument and have a free cilium inside a pit (types I and II). Two types (III and IV) are nonciliate and entirely intrategumental in location. Type IV receptor with large horizontal and thin vertical rootlets was described earlier in aspidogastreans only. Below the basal lamina, nerve endings in close association with muscle fibres, comparable with those in the Aspidogastrea, were detected.

Microanatomy and ultrastructure of the nervous system of adult Renicola parvicaudatus (Digenea: Renicolidae)

“…In contrast, monociliated sensory papillae in adults show greater diversity at the ultrastructural level, namely through rootlet morphology, shape of nerve bulb, number of dense collars, and especially through the abundance of vesicles in the neuroplasm. The identified components of the free ciliated sensory papillae in adult R. parvicaudatus are also characteristic of simple monociliated sensory endings described for various digeneans at different life cycle stages (Antonelli et al, 2014; Czubaj & Niewiadomska, 1988, 1996; Dixon & Mercer, 1965; Dunn et al, 1987; Pan, 1980; Poddubnaya, Zhokhov, & Gibson, 2020; Podvyaznaya & Galaktionov, 2012; Silk & Spence, 1969; Tihomirov, 2000; Wilson, 1970; Žd’árská & Nebesářová, 2003), as well as for other representatives of the Neodermata group (e.g., Biserova et al, 2016; Lyons, 1973; Rohde, 1989, 1990). The arrangement of cytoskeletal structures is probably of decisive importance for the mechanoreceptive function of sensory bulbs, for example those identified in the S 2 acetabular group in maritae and cercariae of R. parvicaudatus (present study; Denisova & Shchenkov, 2020b).…”

“…In sexual adults, such papillae are evenly distributed over the entire tegumental surface and represent predominantly the terminals of the dendrites of primary sensory neurons. Unciliated papillae have been described in a large number of different members of the Neodermata group (e.g., Biserova et al, 2016; Halton & Lyness, 1971; Hoole & Mitchell, 1981; Rohde, 1989, 1990; Rohde & Watson, 1990; Žd’árská, 1993; Žd’árská & Nebesářová, 2003), and the fine structure of the identified sensory endings in R. parvicaudatus generally corresponds to that in other digeneans, for example, adult flukes of Crepidostomum metoecus Braun, 1900 (Allocreadiidae) (Žd’árská & Nebesářová, 2003). However, we did not observe clearly defined septate desmosomes between these sensory bulbs and the tegument.…”

“…It is worth noting the high orderliness of the cytoskeletal components in these bulbs, namely the presence of a peripheral basket of microtubules and powerful cross‐striated rootlets extending from the kinetosome. Although the presence of these components, as well as a large number of vesicles of varying electron density, do not allow an unambiguous interpretation of the role of the identified unciliated receptors, mechanoreception is considered to be their main function (Czubaj & Niewiadomska, 1996; Žd’árská & Nebesářová, 2003). The presence of invaginations enveloping the surrounding tegumental layer and supported by a dense ring apparently indicates the sensitivity of these nerve endings to stretching and compression of the tegument during locomotion.…”

“…The parasitic lifestyle significantly affects the organization and functional role of the nervous system, which is of great importance for the transmission of these parasites. Many studies provide data on the cytomorphology of the central (CNS) and peripheral (PNS) nervous systems of digeneans of various taxa (e.g., Born‐Torrijos et al, 2017; Czubaj & Niewiadomska, 1990, 1996; Denisova et al, 2023; Dunn et al, 1987; Leksomboon et al, 2012; Podvyaznaya & Galaktionov, 2012; Poddubnaya, Zhokhov & Gibson, 2020; Rees, 1975; Tihomirov, 2000; Žd’árská & Nebesářová, 2003). Most of these studies relate to the individual stage of the life cycle, which prevents the determination of structural changes in the nervous tissue through ontogenesis.…”

Topography and ultrastructure of the tegument of Aphallus tubarium (Rodolphi, 1819) Poche, 1926 (Digenea: Cryptogonimidae), intestinal parasite of the common Dentex dentex (Linnaeus 1758) from Valinco Gulf

“…Surface morphology, especially the numbers, shapes, sizes, and distribution of various tegumental structures such as papillae, sensory receptors, or spines may be helpful in distinguishing closely related trematode species (Choudhury and Nelson, 2000;Moravec, 2002;Zdarska and Nebesarova 2003). Ultrastructural studies of many trematodes give also a clue to the understanding of their morphological structures, and have shown characters that help to determine the correct taxonomic position of those species (Han et al, 2003).…”