URIC-ACID ACCUMULATION IN THE SOLITARY ASCIDIAN CORELLA-INFLATA

Like many other ascidians, Corella inflata sequesters uric acid. We ha ve identified microcrystals of uric acid by X-ray powder diffraction, by the characteristic UV absorption for urates at 292 nm which is abol ished by uricase and by methenamine-silver staining in situ. The uric acid is precipitated in birefringent spherulites which are formed intr acellularly within vacuoles of nephrocyte cells. Each spherulite is co mposed of hundreds of thin crystals radially arranged. These 12-mu m s pherulites accumulate in white structures clearly visible with the nak ed eye for which we suggest the designation tophus, a medical term use d to describe any amorphous concretion. In adults, these tophi are fou nd in several tissues including the body wall, gonads, digestive tract and branchial sac trabeculae. The tophi are often over 200 mu m in di ameter and include spherulites within nephrocytes as well as free sphe rulites. Each tophus is surrounded by an epithelium enclosing a thick layer of fibrous extracellular matrix. The first birefringent spheruli tes are detected 3 days after fertilization, a day before the branchia l siphon opens and feeding begins. They form in vacuoles of nephrocyte s which do not circulate with the blood but appear to be in fixed loca tions in the organism. Once begun, the quantity of stored urate increa ses throughout life which is typically less than a year. (C) 1998 Wile y-Liss, Inc.