Uric acid accumulation in the solitary ascidianCorella inflata

Abstract

Like many other ascidians, Corella inflata sequesters uric acid. We have identified microcrystals of uric acid by X-ray powder diffraction, by the characteristic UV absorption for urates at 292 nm which is abolished by uricase and by methenamine-silver staining in situ. The uric acid is precipitated in birefringent spherulites which are formed intracellularly within vacuoles of nephrocyte cells. Each spherulite is composed of hundreds of thin crystals radially arranged. These 12-μm spherulites accumulate in white structures clearly visible with the naked eye for which we suggest the designation tophus, a medical term used to describe any amorphous concretion. In adults, these tophi are found in several tissues including the body wall, gonads, digestive tract and branchial sac trabeculae. The tophi are often over 200 μm in diameter and include spherulites within nephrocytes as well as free spherulites. Each tophus is surrounded by an epithelium enclosing a thick layer of fibrous extracellular matrix. The first birefringent spherulites are detected 3 days after fertilization, a day before the branchial siphon opens and feeding begins. They form in vacuoles of nephrocytes which do not circulate with the blood but appear to be in fixed locations in the organism. Once begun, the quantity of stored urate increases throughout life which is typically less than a year. J. Exp. Zool. 282:323–331, 1998. © 1998 Wiley-Liss, Inc.