Presumptive neurosecretion in Leucochloridiomorpha constantiae (Trematoda) and its possible role in governing maturation

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.

Internal chemical communication within flatworms

An understanding of reproductive function is important for control of parasitic helminths. In cestodes and trematodes virtually nothing is known about regulatory and coordinating mechanisms that control maturation, gamete formation, egg production, and related processes. Neurosecretory neurons have been reported in various species but specific modes of action of neurohormones have yet to be demonstrated. The role of ecdysone is being investigated.

Cytochemical characteristics of the neurosecretory cells ofCeylonocotyle scoliocoelium(Trematoda: Digenea)

Two types of neurosecretory cells (NSC), large a cells and small β cells were detected on the basis of their size and characteristics as shown by aldehyde fuchsin and chrome haematoxylin-phloxin staining. A histochemical survey of the neurosecretory material indicates that it is a glycoprotein. Phosphatases and esterases present in NSC probably help in the transfer of metabolites and their hydrolysis and in the control of secretory material produced in a NSC, while SDH provides the necessary energy for the synthetic activity of the latter. The role of neurosecretion in governing the reproduction of this parasite is discussed.

The structure and possible function of the ventral papillae of Notocotylus triserialis Diesing, 1839

Notocotylus triserialis bears three rows of eversible papillae on its ventral surface. These papillae, which in living worms are firmly applied to the host mucosa, contain numerous pyriform cells. Histochemical tests indicate the presence, within the papillar cells, of protein, lipid, haemoglobin and esterase, and the absence of carbohydrate, acid mucopolysaccharide, RNA, haemosiderin and acid and alkaline phosphatase. Transmission electron microscopy shows the tegument of the papillae to be similar to the non-papillar ventral tegument. The pyriform cells contain many mitochondria with numerous cristae. A mechanism is proposed whereby the musculature of the worm effects the retraction and eversion of the ventral papillae.

Neuropeptides in platyhelminths

The neuropeptide story began in 1928 with the description by Ernst Scharrer of gland-like nerve cells in the hypothalamus of the minnow,Phoxinus laevis.Because these nerve cells were overwhelmingly specialized for secretory activity, overshadowing other neuronal properties, Scharrer termed them ‘neurosecretory neurons’. What was even more remarkable about the cells was that their products were released into the bloodstream to act as hormones, specifically neurohormones. Neurosecretory cells were identified largely on morphological grounds. That is, they could be stained with special techniques, such as chrome-haematoxylin and paraldehyde-fuchsin, although the techniques are far from specific, staining non-neurosecretory cells as well. However, the basis for the ‘special’ neurosecretory techniques is the demonstration of sulphur-containing proteins – so they are indicative of peptide-producing neurones. An alternative characteristic of neurosecretory cells is the presence of large (> 100 nm), dense-cored vesicles at the electron microscope level; these are the so-called elementary granules of neurosecretion, or ENGs. However, implicit in the concept of neurosecretion is that the prime function of the neurosecretory cell is in endocrine regulation, exerting a hormone-like control over some aspect of the organism's metabolism, by controlling endocrine glands and other effector organs. To satisfy this criterion, evidence had to be obtained of cycles of secretory activity within the cell that could be correlated with a change in the physiological condition of the organism.

Parthenogenesis and asexual multiplication among parasitic platyhelminths

Among flatworms with parasitic and commensal modes of existence, parthenogenesis and asexual multiplication appear to be largely confined to the Digenea and Cestoda, the only parasitic platyhelminths that routinely utilize indirect life-cycles. Parthenogenesis is apparently restricted to a minority of adult digeneans and cestodes inhabiting their final hosts, and a survey is made of the particular modes of parthenogenesis (i.e. apomictic, automictic and generative) which are employed by such adults. Asexual (amictic) multiplication, in the form of fissioning, is demonstrated by young adults of the cyclophyllidean cestode, Mesocestoides corti, but is otherwise not exhibited by adult cestodes or digeneans, other than in the perplexing phenomenon of proglottid formation in polyzoic tapeworms. Secondary multiplication is of ubiquitous occurrence in digenean life-cycles in the form of the proliferation which takes place within sporocysts and rediae (germinal sacs) located in the first intermediate host. The controversy concerning the nature of this multiplication is reconsidered in the context of recent findings which have centred on cellular aspects. On the basis of present evidence germinal sac multiplication should be regarded as an asexual rather than a parthenogenetic process. The cestode asexual multiplication which occurs in intermediate hosts is a function of the metacestode stage of development. Metacestode proliferation is only known from about 20 species and 6 families of polyzoic cestodes with approximately half the described instances occurring in the family Taeniidae. The organization of these proliferative metacestodes, findings concerning their totipotent stem cells and the ontogeny of buds and new scolices are all reviewed. Finally, the capacity for population expansion of multiplicative larval digeneans and metacestodes are compared, while the ecological roles and the genetical consequences of both parthenogenesis and amictic multiplication in the two taxa are also examined.