Distribution of parathyroid hormone-related protein (PTHrP) and type I parathyroid hormone (PTH) PTHrP receptor in developing mouse mandibular condylar cartilage

The mandibular condylar cartilage undergoes endochondral bone formation and is an important growth site in the mandible. Parathyroid hormone-related protein (PTHrP) has received attention as a physiological regulator attenuating chondrocytic differentiation and preventing apoptotic cell death. In order to examine the localization of PTHrP and its receptor during fetal development of the condylar cartilage, an immunohistochemical study of PTHrP and the type I PTH/PTHrP receptor was carried out. At day 15 of gestation, the condylar cartilage was evident and some chondrocytes showed positive staining for PTHrP. At day 16, the cartilage was increasing in length and width, and PTHrP was localized in the flattened and hypertrophic cell layers. After day 17, when endochondral bone formation had already started, PTHrP was mainly observed in the flattened cell layer and in a few layers of the hypertrophic chondrocytes. The localization of the type I PTH/PTHrP receptor was similar to that of PTHrP on days 15 and 16, and was broadly distributed at day 18. Apoptotic chondrocytes were scarcely observed on days 15 and 16, and only a few cells were present in the erosion front at day 18. This temporal and spatial localization of PTHrP and the type I PTH/PTHrP receptor suggests that PTHrP is a possible autocrine/paracrine factor regulating condylar chondrocytic differentiation during development.