Ultrastructural characterization of the cuticle of Onchocerca volvulus microfilaria

Previous ultrastructural studies on various microfilariae have suggested the presence of a trilaminar membrane at the parasite cuticle. In the present report, the formation of the cuticle of Onchocerca volvulus microfilariae was studied at different developmental stages by transmission electron microscopy of samples obtained from subcutaneous nodules of untreated patients. The cuticle is formed by the layering of surface coat fibrillar components of the hypodermal cells. No plasma membrane is found at the cuticle of the microfilaria. The use of freeze-fracture replication confirmed the absence of a classic plasma membrane at the parasite cuticle. Our results suggest that immunogenic determinants of the microfilaria cellular elements are hidden from the exterior by the acellular cuticle. This may explain the lack of cellular reaction usually found around living O. volvulus microfilariae in the dermis of onchocerciasis patients.