Fine Structure of the Cuticle of Female Onchocerca volvulus

Abstract

The fine structure of the cuticle of adult female Onchocerca volvulus, a tissue-dwelling nematode parasite of man, was studied. Although the cuticle is composed of the 3 layers characteristic of nematodes (cortex, matrix, and fiber), there were also unusual features. The most notable modification is in the trilaminate membrane, which covers the external surface of the cuticle. Scanning (SEM) and transmission (TEM) electron microscopy disclose that this membrane is projected into numerous folds, substantially increasing the surface area. Many authors have described the fine structural morphology of the cuticle of various species of nematodes (Watson, 1965a, b; Lee, 1966a, b, 1970; Bonner and Weinstein, 1972). Most nematode surfaces are smooth except for annulations (Weise, 1973), but microvilli have been observed on the surface of Bradynema sp. (Riding, 1970), which inhabits the hemocoel of insects. A light microscopy study of Onchocerca volvulus (Neafie, 1972) shows little more than a deeply striated cuticle. Since this nematode resides within a subcutaneous fibrotic nodule of host origin for long periods of time, the possibility of cuticular structural specialization exists. Therefore a study of the fine structure of the cuticle of adult female 0. volvulus in situ was undertaken. MATERIALS AND METHODS Transmission electron microscopy (TEM) Nodules containing 0. volvulus adults were excised from the subcutaneous tissue of persons living in the endemic area of San Pedro Yepocapa, Guatemala. These nodules were placed immediately into either ice-cold glutaraldehyde buffered with 0.1 M cacodylate (pH 7.2) or into 10% formalin. The nodules were then cut into small pieces and placed into fresh fixative overnight, rinsed in 0.25 M cacodylate buffer, and postfixed Received for publication 4 February 1974. * Supported, in part, by Research Grant AI-02347 from the NIH. in ice-cold 1% OO04 in 0.25 M cacodylate-HCl (pH 7.2) for 3 hr. Adult female worms were teased free from formalin-fixed tissue, rinsed in 0.25 M cacodylate-HCl (pH 7.2), and postfixed in 1% OO,4 in 0.25 M cacodylate-HCl (pH 7.2). Dehydration in all cases was in ethanol and propylene oxide with embedment in Araldite 502. Thin sections were cut with diamond knives on a Reichert OmU2 ultratome, mounted on naked 200/300 mesh copper girds, and stained with uranyl acetate (Watson, 1958) followed by lead citrate (Reynolds, 1963). A Hitachi HU-11A electron microscope, operating at 50 kv, was used for viewing and recording at initial magnifications up to 60,000X. Scanning electron microscopy (SEM) Adult female worms were teased from nodules fixed in 10% formalin, followed by postfixation in 1% Os04 in 0.25 M cacodylate-HCl (pH 7.2). Anterior, middle, and posterior portions were selected. These were then dehydrated through ethanol to amyl acetate, critical point dried and coated with gold-paladium. Viewing was on an AMR Model 1000 scanning electron microscope, operating at 10 kv, with original magnifications of 1,000 and 5,000X.