Gorgoderina vitelliloba : an ultrastructural study on the development of the tegument from the metacercaria to the adult fluke

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

The digenean complex life cycle includes various morphological forms with different locomotory and behavioral activities, and the functional specialization of their nervous system is of importance for the transmission of these parasites. Adult digeneans acquire many adaptive features associated with the final settlement in a vertebrate host. Our study describes the general morphology and ultrastructure of the nervous system of the adult renicolid digenean Renicola parvicaudatus parasitizing the renal tubules of herring gulls. Using immunocytochemical and electron microscopic methods, we identified the distinctive characteristics of ganglia and synapses in the studied species. A comparative analysis of the organization of the nervous system of adult individuals and their continuously-swimming stylet cercariae revealed a number of stage-related differences in the composition of ganglia, the distribution of serotonin- and FMRFamide-immunoreactive neurons, the cytomorphology of neuron somata and free sensory endings. Thus, in adults, the presence of FMRFamide-positive neuron somata, accessory muscle bundles in the ganglionic cortex, and eight types of neuronal vesicles was detected, but no glia-like elements were identified. Their neurons are characterized by a larger volume of cytoplasm and also show greater ultrastructural diversity. Although the sensory papillae of adults do not vary in their external morphology as much as those of larvae, their sensory bulbs are more diverse in cytomorphology. Following our previous data on the "support" cell processes related to various tissues of the larvae and considered as glia-like structures, we also briefly present the identified features of the parenchyma, attachment organs and excretory system of adult individuals. The excretory system of adult R. parvicaudatus is characterized by the presence of unique terminal cells with several flame tufts, which are not typical either for the larvae of this species or for other digeneans studied so far. We also used molecular phylogenetic analysis to clarify species identification.

The glands of trematode cercariae of the family Microphallidae Travassos, 1920

The ultrastructure of the glands of microphallid cercariae is described. The fully-formed cercariae possess only penetration glands opening onto the surface of the tegument. Cercarial embryos have tegumental glands transferring their secretion in the tegument. In all studied species two anterior pairs of penetration glands produce identical granules while two posterior pairs produce heterogeneous granules of various shapes and sizes. Tegument cells are divided into mucoid and two other types of tegument glands (TG1 and TG2). The mucoid glands of Microphallinae cercariae produce large fibrous granules, while those of Maritrematinae cercariae produce osmiophil ones. TG1 cells of Maritrema subdolum produce large rod-shaped granules and TG1 cells of Levinseniella brachysoma, Microphallus sp. and M. claviformis produce two types of osmiophil granules. TG2 produce small rod-shaped granules. The association of gland ultrastructure with their function and features of the microphallid life-history are discussed.

Ontogenetic changes in helminth membrane function

During their life-cycle many parasites experience a wide range of environments including free living and those provided by a variety of intermediate and final hosts. The nutritional requirements of parasites are met by physiological processes adapted to exploit the physicochemical characteristics provided by different hosts. In helminth parasites these adaptations are frequently expressed on the tegumentary surface. As an example of adaptations within the Trematoda, the control of monosaccharide transport in Proterometra sp. is described. Environmental sodium, although not directly involved in the uptake process, nevertheless regulates the expression of transport capabilities. In the Cestoda, the uptake of monosaccharides and amino acids is described for Hymenolepis diminuta. The metacestode of this tapeworm inhabits the blood system of an arthropod, and the adult the gut of a mammal. There are quantitative and qualitative differences in the amino acids and monosaccharides in these two environments and these are reflected in the transport mechanisms exhibited by the two forms of the life-cycle. In Echinococcus granulosus the transfer of amino acids, sugars and macromolecules across the membranes of hydatid cysts and protoscoleces is described. The major difference between these two stages in the life-cycle relates to the ability of hydatid cysts to absorb macromolecules, whereas protoscoleces are impermeable to these compounds. The potential for future work is emphasized.