Uric acid accumulation in the solitary ascidian Corella inflata

Like many other ascidians, Corella inflata sequesters uric acid. We have identi- fied 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 vacu- oles of nephrocyte cells. Each spherulite is composed of hundreds of thin crystals radially ar- ranged. 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 spheru- lites 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 through- out life which is typically less than a year. J. Exp. Zool. 282:323n331, 1998. © 1998 Wiley-Liss, Inc. It has long been known that a number of ascid- ians sequester urates or other purines in their bod- ies over long periods of time (Goodbody, '65). This is in addition to excreting nitrogenous waste in the form of ammonia and urea (Goodbody, '57; Markus and Lambert, '83). Purine storage has been con- sidered an evolutionary accident involving the loss of enzymes responsible for purine degradation (Goodbody, '65). Others have suggested that the pu- rines are instead stored for use in nucleic acid syn- thesis or other uses (Nolfi, '70) or even as structural elements (Lambert et al., '90). In the Molgulidae urates are stored in the single large multicellular renal sac which also contains calcium oxalate in the form of weddellite (Saffo and Lowenstam, '78). The renal sac has no openings but the amount of stored crystalline material changes with time, suggesting utilization along with precipitation (Nolfi, '70; Saffo, '88). Symbiotic, fungus-like organisms play a role in these changes (Saffo, '82, '90). Most members of the family Ascidiidae have numerous renal vesicles covering the entire left side of the body in association with the gut. These multicellular vesicles have spacious cavities containing concretions which are often, but not always, of uric acid (Goodbody, '65). Several members of the genus Corella have scattered white inclusions in the body wall which are larger than the renal vesicles and have been considered to be urates (Markus and Lambert, '83), but no infor- mation on their chemical composition has been available. In this report we demonstrate that these white inclusions in Corella indeed contain uric acid in the form of crystals organized into spheroids (spherulites) contained in netlike structures of fi- brous connective tissue. The spheroids begin to form intracellularly within nephrocytes shortly af- ter metamorphosis. The number of nephrocytes with crystalline inclusions increases rapidly. Once begun, the quantity of uric acid increases through- out the lifespan of the animal, which is less than a year (Lambert, '68).