Fine Structure of the Tegument of the Miracidium of Fasciola hepatica L.

Abstract

The fine structure of the tegument of the miracidium of Fasciola hepatica is described. Ciliated epithelial cells overlie a thin discontinuous layer of cytoplasm. This is thickened between adjacent epithelial cells to form prominent intercellular ridges. The thin layer of cytoplasm is associated with diffuse extracellular fibrous material. Beneath the thin layer are circular and longitudinal muscle fibers and beneath these again are large (ca. 10, in diameter) vesiculated cells. The vesiculated cells communicate with the thin cytoplasmic layer by means of narrow cytoplasmic connections running between adjacent muscle fibers. Small electron-opaque vesicles are found in the vesiculated cells, thin cytoplasmic layer, and intercellular ridges. Flattened membranous bodies are also present in the intercellular ridges. The surface of the miracidium is covered with a layer of alcian blue-positive material which appears to be continuously secreted and is presumed to be acid mucopolysaccharide. The apical papilla has an array of circular and longitudinal muscle fibers attached to each other by numerous small desmosomes. The surface of the papilla lacks ciliated epithelial cells and is covered instead by a layer of corrugated cytoplasm. The structure of the miracidium of Fasciola hepatica has been described in detail at the light microscope level by Mattes (1949) and Dawes (1960), and there have been many studies on other miracidia (e.g. Price, 1931; Lynch, 1933; D6nges, 1964). The ultrastructure of the flame cells of the miracidium of Fasciola was described by Kiimmel (1958, 1959), and that of the miracidial eyespots by Kiimmel (1960), Isseroff (1964), and Isseroff and Cable (1968), but other aspects of fine structure do not appear to have been investigated. The above accounts give some indication of variations in structure, at least between miracidia from different families. Lee (1966) reviewed the structure and composition of the helminth body wall and pointed out that little work has been done on the fine structure of the body wall of larval trematodes. Descriptions of rediae have been given by Rees (1966), and Krupa et al. (1967); of a cercaria by Belton and Harris (1967); and of a daughter sporocyst by James et al. (1966). With the exception of the daughter sporocyst, the body surface was an anucleate cytoplasmic syncytium separated from the body wall musculature by an acellular basement membrane. In the case of the daughter sporocyst, the outer layer was nucleated. The miracidium stage in the digene life cycle has a body surface covered with discrete epithelial cells and Dawes (1960) has shown that, in Fasciola at Received for publication 17 June 1968. least, these cells are shed prior to penetration of the miracidium into the snail. This paper presents the results of an investigation into the fine structure of the tegument of the miracidium of Fasciola hepatica. MATERIALS AND METHODS Infected sheep livers were obtained from the local slaughterhouse and mature flukes dissected out. Undeveloped eggs were teased from flukes into distilled water and incubated for 12 days at 25 C in the dark. Mass hatching of miracidia into a minimal volume of water was brought about by exposure to light. Miracidia were fixed at 4 C, 20 min after exposure of eggs to light, in either Zetterquist's buffered osmium tetroxide for 1 to 4 hr (after Kay, 1965), or 3% glutaraldehyde in cacodylate buffer pH 7.2 for 30 min followed by postfixation in osmium for 4 hr. Miracidia were dehydrated in alcohol and embedded in an araldite-epon mixture. Prior to sectioning, individual miracidia were isolated in small pieces of cured resin and oriented transversely or longitudinally on top of a perspex rod, being fixed in position with araldite adhesive. Sections were cut on a Huxley ultramicrotome and collected on grids coated with formvar and carbon. Specimens were stained either with uranyl acetate during dehydration followed by lead citrate on the grid, or by both uranyl acetate and lead citrate on the grid. Sections were viewed in an AEI EMB6 electron microscope operating at 60 KV.