Ultrastructure of the Tegument of Adult Schistosoma mansoni

Abstract

Electron microscopy of the cuticle of Schistosoma mansoni shows it to be the outer syncytial portion of a unit. It is, like that of all other described trematodes, connected by cytoplasmic tubes to nucleated tegumental cells lying in the parenchyma. The surface of the cuticle is elaborately folded and the matrix is penetrated by many channels. Very few mitochondria are present in the cuticle or in the cells. Electron-dense rodlike inclusions and spherical electron-lucid inclusions are described and occur in both cuticle and cells. Male and female cuticle are compared and the modified cuticle lining the esophagus is described. Some histochemical properties of cuticular components are presented. In recent years a number of papers dealing with the ultrastructure of cestode and trematode teguments have been published. In the light of this research it is now accepted that the cuticle of these worms is but the outer portion of the unit. The anucleate, syncytial cuticle is connected by cytoplasmic tubes to nucleated regions (herein known as cells) lying in the parenchyma. Although several terms are in use to describe these regions we have, for this discussion, retained the old term cuticle when referring to the outer level of the tegument. It must, of course, be realized that in this context the term cuticle is used to describe a metabolically active, cellular unit and not an inert covering. Only a few of the recent papers have, in the authors' opinions, served to advance our knowledge of the structure of trematode teguments beyond this fundamental level. Specifically, the papers of Threadgold (1963) and Burton (1966) have respectively described in some detail the fine structure of the tegument of Fasciola hepatica and Gorgoderina sp. Senft et al. (1961) published a paper on the ultrastructure of the adult Schistosoma mansoni cuticle but their results added little to what was previously known as a result of light microscopy. More recently, Lee (1966) has published a short description of the S. mansoni epidermis ultrastructure. However, his report is of a preliminary nature and his results differ somewhat from those reported herein. This study was undertaken to elucidate the fine structure of the tegument of adult male and female S. mansoni and to provide a startReceived for publication 1 September 1967. ing point for a study of development in the larval stages of this worm. MATERIALS AND METHODS Infected albino mice obtained from the Wellcome Research Laboratories, Beckenham, were killed by cervical dislocation. Worms were removed immediately and fixed both in copula and singly for 1 to 2 hr in chilled 4% glutaraldehyde buffered to pH 7.2 with phosphate. The worms were then postosmicated for 1 to 2 hr in 2% osmium tetroxide buffered to pH 7.4 with veronal-acetate. Whole worms or pairs of worms were embedded in Araldite (Luft, 1961) or Maraglas (Spurlock et al., 1963). Thin sections were cut with glass knives on an L.K.B. ultramicrotome and mounted on bare grids. The sections were then stained for 15 min with a saturated solution of uranyl acetate in 50% alcohol and then for 5 min with lead citrate (Reynolds, 1963). The material was examined on an A.E.I. 6B electron microscope and micrographs were taken at magnifications of 5,000 to 35,000 and subsequently enlarged photographically as required. The histochemical tests were usually carried out on whole worms, although some worms were cut into several segments before fixation. Worms to be tested for phosphatase activity were fixed at 4 C in 2.5% glutaraldehyde in pH 7.2 cacodylate bu fer containing 0.2 Ms sucrose for 1 to 2 hr. In some instances the fixative also contained 0.1 Mi Na 3 glycerophosphate. After fixation worms were washed repeatedly in cold cacodylate buffer containing sucrose and stored for a period of at least 12 hr at 4 C. No new reaction sites appeared when substrate was incorporated into the fixative. There was, however, an enhanced activity at sites which were also active when substrate was not present in the fixative. When tissue fixed in substrate-containing fixative was incubated in a control medium without glycerophosphate very fine deposits of lead salts appeared at the sites which were active in the test incubates. Tissue to be tested for acid phosphatase activity was incubated in a modified, freshly prepared, Gomori's acid phosphatase medium for 40 min at