Ultrastructural Evidence of Detoxification in the Alimentary Canal of Urechis caupo

Abstract

The marine worm Urechis caupo exhibits specialized secondary lysosomes termed sulfide-oxidizing bodies throughout the epithelial tissues of the alimentary canal which appear to concentrate environmental toxins such as aluminum, silica and sulfur, as revealed by X-ray microanalysis. Ultrastructural examination of the alimentary tract of U. caupo reveals a digestive tube consisting of an inner layer of epithelium lining the central lumen, a layer of connective tissue with embedded muscle fibers, and a peritoneal covering. The thickness, ciliation and morphology of the innermost epithelium show regional variation that correlate with functional specialization, and the main cell types are ciliated, microvillate, and mucusand enzymesecreting cells. In the anterior part of alimentary canal (esophagus, gizzard, and stomach), the epithelium is almost entirely composed of ciliated and mucus cells, serving to move particulate matter and fluids, and the gizzard epithelial morphology reflects comminution of food particles. In the midgut, the epithelium is composed of microvillate and enzyme-secreting cells, suggesting involvement in digestive and absorptive functions. The hindgut, which acts as a water lung and is devoid of any digestive function, is lined by microvillate cells involved in gaseous exchange. The epithelium of the body wall and hindgut have been previously reported to contain numerous sulfide-oxidizing bodies, similar in ultrastructure and elemental composition to those of the alimentary canal described here (Menon & Arp 1992; 1993). These unusual organelles, which form a peripheral defense on all exposed surfaces of the worm, are part of an extensive suite of sulfide detoxification and tolerance mechanisms which enable the worm to flourish in a sulfide-rich environment. Additional key words: worm, alimentary canal, sulfide, sulfide-oxidizing bodies, Urechis