Force-induced rapid changes in cell fate at midpalatal suture cartilage of growing rats

The application of expansional force induces replacement of the cartilaginous tissue with bone at the midpalatal suture of growing rats. We examined the early cellular events evoked by force by analyzing the expression of proliferating cell nuclear antigen (PCNA), an operational marker of cell proliferation, and of several bone matrix proteins. A rectangular orthodontic appliance was set between the right and left upper molars of four-week-old rats, with 50 g of initial expansional force. Two days after application of the force, the pre-existing cartilage was separated laterally. Mesenchymal cells with stretched shapes were arranged parallel to the expansional force and filled the center of the suture. Only a few of these stretched cells exhibited nuclear accumulation of PCNA. In contrast, many polygonal mesenchymal cells distributed along the inner lateral side of the cartilaginous tissue exhibited strong immunoreactivity for PCNA. Localization of alkaline phosphatase activity overlapped into this proliferating cell zone. Nascent extracellular matrix under the proliferating cells was positive for osteocalcin, indicating commencement of active bone formation. These findings indicated that, among mesenchymal cells subjected to expansional forces, only cells located on the inner side of the cartilaginous tissue proliferate and differentiate into osteoblasts. In agreement with rapid bone growth progression, apoptosis was also observed in the zone of proliferating cells, as measured by TdT-mediated dUTP-biotin nick end labeling (TUNEL) assays.