Encyclopedia of Parasitology

Living Edition
| Editors: Heinz Mehlhorn


  • Heinz MehlhornEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27769-6_23-2


Intermediate Host Retractor Muscle Cytoplasmic Bridge Unequal Division Numerous Vacuole 
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Acanthor is the first larva of Acanthocephala (Acanthocephala/Reproduction, Acanthocephala/Figs. 2 and 3). During the first equal cell divisions after fertilization, two polar bodies usually appear at the end of the embryo that will become the anterior end of the acanthor. Further, equal and unequal divisions show a kind of spiral cleavage resulting in micromeres and macromeres. In a later stage, the central nuclear mass (inside the central syncytium) appears. However, there is no formation of a digestive tract at any phase of development. In addition, the very early embryo attains a syncytial organization. Thus, it is difficult to decide what is ectoderm, endoderm, or mesoderm. During the course of development, the embryo detaches from the floating ovary, and the single eggshell differentiates into the different envelopes.

Mature acanthors consist of three syncytia, the central syncytium (median), the epidermal syncytium (caudal), and the frontal syncytium (Fig. 1). Within the central syncytium, there are ten subepidermal longitudinal muscles and two more centrally located retractor muscles. According to descriptions of Albrecht et al. from acanthors (of three species) that were still inside the mother’s body cavity and enclosed by eggshell envelopes, the subepidermal muscles are connected via cytoplasmic bridges with the central nuclear mass. However, in hatched acanthors of Paratenuisentis ambiguus collected from the gut of its crustacean intermediate host, no connections between the central syncytium and the subepidermal muscles could be found; the muscles were part of the epidermal syncytium (Fig. 1). So probably the bridges described may get lost during the final maturation of the acanthor or the hatching process. The central nuclear mass contains condensed as well as decondensed nuclei. The latter are also found in the epidermal and the frontal syncytium (Fig. 1). The epidermal syncytium forms the outer surface and most of the larva’s body. It contains numerous vacuoles with mucus-like electron-lucent content which are concentrated near the surface of the anterior half (Fig. 1). In acanthors of P. ambiguus , the crypts of the outer membrane are fused underneath the larva’s surface and harbor electron-dense granules which may have a function during the penetration of the larva into the hemocoel of the intermediate host. The electron-dense vesicles as well as the vacuoles with electron-lucent content inside the frontal syncytium could probably also be involved in the task of penetration, chemically supporting the action of the hooks. Stimulated acanthors of Moniliformis moniliformis have been found to discharge chitinase, but this enzymatic activity has not been localized at the acanthor’s body. Hooks are most prominent at the anterior surface and decline in size toward the larva’s posterior end (Fig. 1). Acanthors are rich in glycogen in the cytoplasm between the muscles, nuclei, mitochondria, and inclusions (Fig. 3a).
Fig. 1

Schematic drawing of a hatched acanthor of the eoacanthocephalan Paratenuisentis ambiguus. The frontal syncytium (FS) is rich in electron-dense vesicles (EV) as well as vacuoles containing electron-lucent mucus-like matter (VL). The central syncytium (CS) harbors condensed nuclei (CN) and a few decondensed nuclei (DN). The epidermal syncytium (ES) forms most of the body including fused crypts of the outer membrane which contain round electron-dense granules. The surface of the larva is armed with hooks (H) and body spines (SP). The two retractor muscles (R) enable the larva to perform invaginations of the anterior body. The hind body may contract itself by the action of the ten longitudinal muscle cords (LM, only two are drawn); LI lipid drop (Reitze and Taraschewski unpublished)

Mature acanthors of acanthocephalans are enclosed by four eggshells separated by interstices containing granular electron-lucent material (Figs. 2, 3, and 4a). However, eggs of Neoechinorhynchus species become complemented by a fifth envelope (E0) creating a fifth voluminous outer interstice (Fig. 2c). The outermost, first envelope seems to derive from the “fertilization membrane.” Usually, it is thin but can be reinforced by outgrowths of the underlying eggshell (Figs. 3b and 4a). This envelope (E2) was found to contain keratin in all three groups of the Acanthocephala. In palaeacanthocephalans, it forms more or less filiform outgrowths (Figs. 2a and 3a, b) entangling with algae or leaves (the food substrates of the intermediate hosts) once the outermost envelope has disintegrated in the water. In archiacanthocephalans, the second eggshell is interspersed with the respective outermost interstice and seems to function in protecting the egg from desiccation and other negative outer influences (Figs. 2b and 4a). Among archiacanthocephalans, the underlying tripartite third envelope also comprises keratin, while eoacanthocephalans and palaeacanthocephalans do not have keratin in this eggshell. The fourth, innermost eggshell contains chitin among palaeacanthocephalans and archiacanthocephalans (Fig. 3c). In eoacanthocephalans, however, this innermost eggshell lacks chitin. The interstices contain carbohydrates which together with the envelopes seem to have different functions.
Fig. 2

Schematic drawings of the eggshells of Palaeacanthocephala (a), Archiacanthocephala (b), and a neoechinorhynchid eoacanthocephalan of the genus Neoechinorhynchus (c). E eggshells (envelopes), G granular interstices, AC acanthor. (a) Note the filiform outgrowth of the second eggshell. (b) The second eggshell creates a thick mesh of amorphic matter intermingling with the first interstice. (c) Note the existence of five eggshells and five interstices, respectively. The two outermost interstices are loaded with carbohydrates. The eggshell E2 seems to keep the E0 eggshell in position. The E3 eggshell is just a membrane

Fig. 3

TEMs of sections through eggshell envelopes and interstices enclosing acanthors of palaeacanthocephalans treated in different fashions. (a) Egg of Acanthocephalus anguillae incubated according to the electron microscopical PAS method of Thiéry in a mode to visualize glycogen (dark granules in the acanthor). Also note the four eggshells (E1–E4) and the transversally sectioned subepidermal longitudinal muscles (LM); FP filiform protuberance of E2. (b) Egg of Polymorphus minutus incubated with anti-keratin and subsequently with a second antibody labeled with colloidal gold. Note the gold granules on eggshell E2 and its outer filiform protuberances indicating keratin in the second envelope. This section does not show the granular interstice between the third and the fourth envelope. As can be seen under (a), the width of the interstices is different. AC acanthor. (c) Innermost envelope (E4) and acanthor of P. minutus after incubation with lectin wheat germ agglutinin (coupled with colloidal gold granules) in a mode that chitin is visualized. Note the gold label on E4, also the crypts of the acanthor’s outer membrane (CM), and the glycogen granulation between the mitochondria and the vacuole of electron-lucent content (VL) (probably mucus)

Fig. 4

Micrographs showing envelopes, interstices, and an acanthor (B) of the archiacanthocephalan Macracanthorhynchus hirudinaceus. (a) The ultrathin section has been incubated with chitinase and subsequently with lectin wheat germ agglutinin coupled with gold granules. The innermost envelope E4 therefore does not show a chitin-gold-label as it would without the enzyme treatment. According to competition experiments with N-acetylglucosamine and triacetyl chitotriose, it becomes evident that the partly intense label in the granular interstices is due to different carbohydrates but not chitin. The acanthor is not seen. Note that E1 and E3 are tripartite; G1–G4, granular interstices separating the envelopes; E1–E4, envelopes. (b) Light microscopical micrograph of an egg that has been incubated in sodium dodecyl sulfate (SDS) and dithiothreitol (DTE) to extract proteins including keratin. Consequently, the outer two envelopes swell considerably and eventually rupture due to an increase of the osmotic pressure. Therefore, the acanthor seen still enclosed by the keratin-containing E3 envelope and the E4 envelope that has chitin “shoots” out of its enclosure, suggesting that the digestive activity in the gut of the intermediate host largely contributes to the hatching process of the acanthor

As a general rule, the outer envelopes and interstices appear to be ecologically related, accomplishing functions in parasite transmission, etc. In archiacanthocephalan eggs, the outer part of the eggshell swells when exposed to digestive influences so that the inner part containing the acanthor is passively released (Fig. 4b). The interior envelopes (Figs. 2, 3, and 4a) seem to be systematics related and obviously fulfill tasks belonging to the principle requirements of the acanthor.

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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.Institut für Zoomorphologie, Zellbiologie und Parasitologie Universitätsstraße 1DüsseldorfGermany