Ultrastructure of the Tegumentary Pore-Canal System of the Acanthocephalan Moniliformis dubius

The tegument contains elongate, anastomosing canals which extend about 4 ,u into the hypodermnnis and end blindly. The continuity of these tubules with the free surface is readily demonstrated in tissues fixed with solutions containing colloidal lanthanum nitrate. The canals are lined with tegumentary plasmalemma continuous with that external to the canals but having a trilaminate component about 90 A thick as compared to 120 A for that component subjacent to the epicuticle. The pore-canal complex is interpreted as a stable, structural specialization for amplifying the free surface of the parasite available for chemical interchange with its environment. The tegument of the acanthocephalan Moniliformis dubius includes several structures which have been implicated functionally in processes related to physiological interaction between this parasite and its host (Nicholas and Mercer, 1965; Wright and Lumsden, 1968). The free surface is defined by a plasma membrane and its intimately associated glycocalyx, the so-called "epicuticle. " Elongate, membranelimited caveolae adjoining the surface plasmalemma and vesicles in the peripheral region of the tegument of M. dubius have been described by Nicholas and Mercer (1965) and interpreted as morphological manifestations of pinocytosis (Nicholas and Mercer, 1965; Edmonds and Dixon, 1966). Canalicular structures opening onto the body surface of other acanthocephalan species have been described by Crompton and Lee (1965), Stranack et al. (1966), and Hammond (1967), and surface pores are visible in nonsectioned material studied by scanning electron microscopy (Hammond, 1968). Lee (1966) suggested that these structures represent a system of canals and pores specialized for absorptive function. Further details of the ultrastructure of the acanthocephalan tegument membrane system are reported herein and their possible biological significance discussed. MATERIALS AND METHODS Adult female Moniliformis dubius were removed from the excised gut of albino rats into 25 mM Received for publication 2 May 1969. * Supported by grants from the NIH (AI 08673, 5 TI GM 669) and NSF (GB 7276). t Recipient of a Career Development Award (KO4 AI 23449) from the U. S. Public Health Service (NIAID). tris maleate-buffered saline of Read et al. (1963). The worms were subsequently transferred for primary fixation to ice-cold 6% glutaraldehyde in Millonig's 0.12 M phosphate buffer (pH 7.4) containing 2 mM calcium chloride and 3% sucrose. The worms were immediately cut into small (1 to 2 mm) pieces, fixed 2 hr, and washed overnight in 0.12 M phosphate buffer containing 2 mM calcium chloride and 5% sucrose. The tissue was then postfixed 90 min in ice-cold, phosphate-buffered 1% osmium tetroxide. For some tissue samples, alkaline (pH 7.8) lanthanum nitrate was added to the glutaraldehyde, wash, and osmium postfixative solutions, buffered with 0.1 M cacodylate-HCl (Revel and Karnovsky, 1967; Schatzki, 1969). Material was then rapidly dehydrated in ascending concentrations of cold ethanol and embedded in epon. Sections displaying silver-gray interference colors were cut with diamond knives on a Sorvall MT-2 ultramicrotome and collected on bare copper grids. These preparations were treated with aqueous uranyl acetate (Watson, 1958) and lead citrate (Reynolds, 1963) or left unstained. Electron microscopy was carried out with a Siemens Elmiskop 1A operated at 80 kv. Comparative dimensions of membrane structure were based on microdensitometric tracings of electron micrographs printed on contrast lantern slide plates using a Durst S-45 enlarger equipped with with point source illumination. The slides were cut into strips of appropriate size and scanned with a Canalco microdensitometer. Measurements were taken of the calibrated linear distances between peaks corresponding to regions of significant electron scattering resolved in the primary image of the sectioned membrane. RESULTS The tegument is bordered peripherally by a trilaminate membrane overlaid by the hirsute epicuticle (Figs. 1, 2). The intimate structural association of the epicuticular filaments with this membrane has been described previously (Wright and Lumsden, 1968) and is further indicated in images obtained of ma-