Immunohistochemical detection of EP1 and EP2 receptor in rat growth plates and in cultured chondrocytes. Protein expression of EP1 and EP2 receptor was studied using isoform-specific antibodies. The EP1 receptor showed strong expression in the proliferative and hypertrophic zone but marginal expression in the reserve zone, with some negative cells (marked by arrow). In contrast, the EP2 receptor was distributed throughout the whole growth plate. In vitro the EP1 and EP2 receptors were only expressed in subpopulations. EP1 showed strong positivity in chondrocytes organised in a cobblestone pattern and surrounded by matrix, whereas fibroblastic-shaped cells were only occasionally and moderately positive for EP1. The highest expression for EP2 could be demonstrated in dividing cells and polygonal cells embedded in matrix (marked by arrow). In fibroblastic cells, only minimal to slight positivity was found in a small number of cells. Magnification 200 × . r, reserve zone; p, proliferative zone; h, hypertrophic zone.