ALTERATIONS IN HYALURONAN SYNTHESIS DURING DEVELOPING JOINT CAVITATION

The mechanisms essential for generating diarthrodial joint cavities be tween skeletal elements in developing limbs remain enigmatic. Histoche mical localization of hyaluronan (HA) at joint interzones concomitant with cavitation led to the postulation that HA may be pivotal in this process, HA synthesis involves the transfer of UDP-glucuronate and UDP -N-acetyl glucosamine to nascent HA by HA synthase. Uridine diphosphog lucose dehydrogenase (UDPGD) activity is responsible for the prior con version of UDP-glucose to UDP-glucuronate. We have assessed the relati onship between the appearance of HA and enzyme activities (quantitativ ely where possible) involved in HA synthesis during metatarsophalangea l joint development in embryonic chicks, Microspectrophotometric asses sment of UDPGD activity using an in situ biochemical assay indicated t hat cells immediately adjacent to forming cavities contained increased UDPGD activity, which was subsequently maintained after cavitation. I mmunocytochemistry showed that high levels of expression of HA synthas e were localized to these same cells. In addition, radiolabeled sulfat e autoradiography showed that cells bordering developing cavities inco rporated relatively little sulfate, suggesting that UDP-glucuronate is utilized in the synthesis of undersulfated or non-sulfated glycosamin oglycans, These results indicate that the differentiation of cells bor dering presumptive spaces may involve alterations associated specifica lly with differential synthesis of HA, which appears to be a primary e vent in joint cavity formation.