Morphological Adaptations of Intestinal Helminths

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870 THE JOURNAL OF PARASITOLOGY, VOL. 77, NO. 6, DECEMBER 1991Iul#lIIIHHII rlul1u1!111lt!1lllf!( A,t.A^Q1mI>71111101111 11i1I110111110111110111-11111111011 111-100 -,'0 -~IO.5jmEFGH. ..IFIGURE 2. Diagram of the tegumental arrangementin the neck region of Hunterella nodulosa. cm, circularcuticular muscles; er, endoplasmic reticulum; g, golgi;gly, glycogen; lm, longitudinal cuticular muscles; m,microtriches; mit, mitochondrion; n, nucleus; no, nucleolus;r, rod-shaped bodies; v, vesicles (from Hayunga[1977]).reveals the eosinophilic matrix to be a secretoryproduct of the scolex, as large vesicles originatingfrom frontal glands were observed to be transportedtranstegumentally to release their contentsat the parasite-host interface; the failure ofthe frontal glands to stain for proteolytic enzymessuggests that the secretion is not lytic butrather adhesive in nature (Hayunga, 1979a,1979b). Because the frontal glands of H. nodulosaare better developed than those of othercaryophyllideans, they are very likely to play amajor role in attachment and penetration (Mackiewiczet al., 1972). However, the actual functionof both the frontal glands and the Faserzellen or"neck cells" of caryophyllideans remains to bedemonstrated (Mackiewicz, 1972; Hayunga andMackiewicz, 1988).PARASITE EXPLOITATION OF HOSTRESPONSETypically, a parasite is viewed as challenginga host by its presence, then developing some kindof morphological or physiological adaptation toevade the host's response and thus avoid immunerejection. Citing the example of H. nodulosa,Hayunga (1989) inferred instead a more op-JFIGURE 3. Diagram showing variation in tegumentalmicrotriches of Hunterella nodulosa. A. Cross sectionof electron-dense spine. B. Cross section of microthrixshaft. C. Cross section of shaft of a spinelessmicrothrix. D. High magnification view of a rod-shapedbody found in the tegumental cytoplasm. E. Typicalcestode microthrix from the scolex. F. Branching microthrixwith short spine. G. Spineless microthrix fromposterior. H. Branching microthrix. I. Budding microthrix.J. Typical and spineless microtriches from atransitional zone of the tegument. K. Area of degeneratingmicrotriches on posterior part of worm. (FromHayunga and Mackiewicz [1975], courtesy of PergamonPress.)portunistic role for the parasite. Although theactual mechanisms of nodule formation are notknown, he suggested that the adhesive secretionof the frontal glands might also be an irritant andthat this secretion, together with the normallystrong muscular contractions of the worm, woulderode the epithelium and elicit a granulomatousresponse in the host. However, because of theworm's activity, the host is unable to completegranuloma formation. A partial granuloma isformed affording the parasite with a means ofattachment, a sheltered habitat, and ready accessto nutrients found in the intestinal lumen (Hayunga,1977, 1979b).Such a possibility suggests that the parasite isK
HAYUNGA-MORPHOLOGICAL ADAPTATIONS OF INTESTINAL HELMINTHS 871capable of far more than passive evasion of hostdefenses and that, conceivably, parasites mayevolve strategies that actively exploit the host'sresponse for their own benefit (Hayunga, 1989).Careful examination of other parasites may revealthat strategies of exploitation are far morecommon than have previously been thought(Damian, 1987).COST OF SPECIALIZATIONSpecialization in terms of morphological adaptationhas resulted in highly efficient means ofattachment for some intestinal helminths thatpresumably have given these species a competitiveedge. By adapting so well to their preferredsites, cestodes appear to be specialists with regardto space. To a somewhat lesser degree, trematodesand acanthocephalans also may be regardedas specialists. Nematodes seem less limitedin site selection by their anatomy and thus appearto be generalists in this regard, although theirhabitat selection may be restricted by physiologicalfactors. For nematodes it would appear thatniche stratification is more a question of whatthey eat rather than where they eat.Although the specialist may be highly efficient,the cost of specialization is dependence on onetype of host, as clearly demonstrated by studiesof host specificity. One need only ask the question,"Whatever happened to the parasites of thedinosaurs?" to envision the risks of a strategy ofspecialization. But, if anything, helminths haveshown themselves to be highly resilient and innovative,as shown by the ability of polystomatidmonogeneans to inhabit both the gills of tadpolesand the urinary bladders of adult frogs, or by thestrategy of neotenic development when a desiredhost is no longer available. The morphologicaladaptations described here represent only oneexample of many highly effective strategies forsurvival.ACKNOWLEDGMENTSI am grateful to Robin M. Overstreet and JohnC. Holmes, both for inviting me to speak at thissymposium and for keeping me on the programdespite the turmoil of world events this year. Iparticularly acknowledge the influence of JohnMackiewicz, who introduced me to the field ofparasitology and to the fascinating Caryophyllidea.Many of the ideas that I discussed werelearned in his classes, and the original researchthat I described was done in his laboratory. Thespecimen shown in Figure 1 was prepared by Dr.Mackiewicz. Observations on competition andsite selection by caryophyllideans were made incollaboration with Anthony J. Grey, a fellowgraduate student, now at the New York StateDepartment of Health in Albany. Finally, I givecredit to the pioneering efforts of prominent parasitologistswhom I have quoted extensively inthis paper.LITERATURE CITEDACKERT, J. E., S. A. EDGAR, AND L. P. FRICK. 1939.Goblet cells and age resistance of animals to parasitism.Transactions of the American MicroscopicalSociety 58: 81-89.BEFUS, D., AND J. BIENENSTOCK. 1982. Factors involvedin symbiosis and host resistance at the mucosa-parasiteinterface. Progress in Allergy 31: 76-177.BEGUIN, F. 1966. Etude au microscope electroniquede la cuticule et de ses structures associ6es chezquelques cestodes. Essai d'histologie comparee.Zeitschrift fuir Zellforschung 72: 30-46.BERGER, J., AND D. F. METTRICK. 1971. Microtrichialpolymorphism among hymenolepid tapeworms asseen by scanning electron microscopy. Transactionsof the American Microscopical Society 90:393-403.BRATEN, T. 1968. The fine structure of the tegumentof Diphyllobothrium latum (L.)-A comparison ofthe plerocercoid and adult stages. Zeitschrift furParasitenkunde 30: 104-112.COLWELL, R. K., AND E. R. FUENTES. 1975. Experimentalstudies of the niche. Annual Review ofEcology and Systematics 6: 281-310.CROMPTON, D. W. T. 1973. The sites occupied bysome parasitic helminths in the alimentary tractof vertebrates. Biological Reviews 48: 27-83.1975. Relationships between Acanthocephalaand their hosts. Symposia of the Society forExperimental Biology 29: 467-504.DAMIAN, R. T. 1987. The exploitation of host immuneresponses by parasites. Journal of Parasitology73: 3-13.DOGIEL, V. A. 1958. Ecology of the parasites of freshwaterfishes. In Parasitology of fishes, V. A. Dogiel,G. K. Petrushka, and Y. I. Polyanski (eds.). LeningradUniversity Press. [English translation by Z.Kabata, Oliver & Boyd, Edinburgh, xi + 384 p.,1961.]ERASMUS, D. A. 1969. Studies on the host-parasiteinterface of strigeoid trematodes. V. Regional differentiationof the adhesive organ of Apatemongracilis minor Yamaguti, 1933. Parasitology 59:245-256.1977. The host-parasite interface of trematodes.Advances in Parasitology 15: 202-242., AND C. OHMAN. 1963. The structure andfunction of the adhesive organ in strigeid trematodes.Annals of the New York Academy of Sciences113: 7-35.GREY, A. J., AND E. G. HAYUNGA. 1980. Evidencefor alternative site selection by Glaridacris laruei(Cestoidea: Caryophyllidea) as a result of inter-
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Morphological Adaptations of Intestinal Helminths

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