Cyclooxygenase (COX) expression in rat growth plate chondrocytes in vitro and in situ. Protein expression of COX-1 and COX-2 was studied using isoform-specific antibodies. Both COX isoforms could be detected in all zones of the growth plate. In cultured growth plate chondrocytes, COX-1 was expressed in all cultured chondrocytes with high intensity in paranuclear areas (marked by arrow). COX-2 protein was detected in extranuclear regions as well as in cell processes (marked by arrow) of a sub-population of the cultured cells only. r, reserve zone; p, proliferative zone; h, hypertrophic zone.