Morphological evidence for biological control of urate crystal formation in vivo and in vitro

Urate crystal formation, and subsequent gout, occurs in only a minority of hyperuricaemic subjects indicating that factors other than hyperuricaemia are involved. Biological substances (especially proteins) alter crystal growth in vitro independently of uric acid concentration. Physiological crystal formation, known to be under biological control, is characterised morphologically by uniformity of size and constraint of crystal shape. To determine whether pathological urate crystal formation is influenced by the surrounding biological milieu we examined, using scanning electron microscopy, the morphology of urate crystals formed either in vivo or in vitro. We found morphological evidence of biological control of in vivo urate crystal formation and demonstrated that those characteristics could be induced in vitro by the addition of serum, synovial fluid and certain serum proteins to the growth medium during crystal formation. Urate crystal formation is dependent, not only on the degree of hyperuricaemia, but also on the surrounding biological milieu.