The microfibrillate component of the laminated layer of Echinococcus granulosus equinus contains, except for the zone adjacent to the germinal layer, aggregates of electron-dense bodies displaying a sub-structure of electron-lucent spheres. The tegumentary syncytial cytoplasm contains randomly distributed electron-dense granules, many occurring near the apical plasmalemma, although exocytosis was rarely seen. Granules, similar in size and sub-structure to the bodies of the aggregates, also occur in the internuncial connexions and tegumentary cytons, suggesting that they may be produced in the cytons and released into the laminated layer via the internuncial connexions and tegumentary cytoplasm. Cysts incubated for 0.5-2.5 h in serum- and non-serum-containing media showed differences from non-incubated cysts. The distal half of the syncytium contained a progressive increase in the number of granules (distal: proximal 1.9:1 at 2.5 h; cf. 1:1 in non-incubated cysts), and exocytosis of granules into the laminated layer adjacent to the cyst had occurred. This cannot be attributed wholly to serum proteins. Cysts incubated for 21 h appeared 'normal', suggesting re-establishment of an equilibrium. Since the matrix of the laminated layer is considered homologous to the glycocalyx of other cestodes, the possible protective role played by the granules/bodies, characteristic of Echinococcus spp., is discussed.

SUMMARYChanges in the tegumentary projections of protoscoleces ofEchinococcus granulosus, developing within brood capsules, were examined at the ultrastructural level. The original scant covering of spineless, truncated microtriches was supplemented by a population of microvilli covering the entire surface of protoscoleces in the early stages of formation. As differentiation continued, rostellar hooks were formed by enlargement of single large (T1) microtriches, and normal spined microtriches were produced on the sucker region. These were formed in two ways: by addition of spine material to the truncated microtriches, andde novofollowed by up-lifting from the tegumentary surface. A second type of large microthrix (T2) was present in a band immediately below the developing hooks. These microtriches appeared to be formed from a fusion of existing microtriches, but were subsequently lost prior to scolex invagination. The microvilli were also shed from both the sucker and soma regions, to be replaced by microtriches and small blunt elevations respectively.

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A series of development stages (I-XI) have been devised to describe the development of the cyst wall of the metacestode of Hymenolepis diminuta. The cyst wall possesses tegumentary, muscular, fibrous and inner cyst tissues, the developmental rates and differentiation patterns of which are not identical. The tegumentary tissue differentiates posteriorly. Its microvillus-bearing distal cytoplasm remains simple until scolex retraction, after which rapid increase in depth followed by vacuolation occurs and basal membrane infoldings surround Phase 3 fibrogenesis fibrils. Senescence, which also affects the tegumentary cytons, then ensues. The muscle system development is posteriad and maturation, completed before scolex retraction, is followed by myocyton senescence. Posteriorly differentiated fibroblasts commence Phase 1 fibrogenesis after scolex retraction and the primary fibrous zone is fully established within approximately 6 days. Phase 2 and 3 fibrogenesis develop centrifugally, the fibrils of Phase 2 surrounding the tegumentary cytons and myocytons prior to their senescence, and those of Phase 3 lying more peripherally. The inner cyst tissue, established posteriorly, differentiates anteriorly, centripetally and early, the penultimate stage commencing just before scolex retraction, about 6 days after which the final maturation junctional complexes start development. Neither in vitro excystment nor infectivity of the definitive host can be satisfactorily achieved before the initial development of the primary fibrous zone. This may play a skeletal role during excystment, and is shown to be unaffected by the digestive enzymes which cause loss of cytoplasmic integrity in the outer regions of the cyst.

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