Apoptosis in murine calvarial bone and suture development

To study the possible role of apoptosis in calvarial bone and suture develo pment, terminal deoxynucleotidyl transferase-mediated nick-end labeling (TU NEL) was performed on whole mount and sectioned calvariae from mice aged be tween E14 and P6. We also analyzed by in situ hybridization the expression of Msx2, Bmp4 and Bmp7 genes, which are known to act in conserved signaling pathways leading to apoptosis. We found TUNEL-positive cells from E16 onwa rds in the calvarial bones, intervening sutures and fontanelles. TUNEL-posi tive osteoblasts and preosteoblasts were identified at or close to the oste ogenic fronts, areas of intense osteogenic activity, with TUNEL-positive me senchymal cells located in the midsutural mesenchyme. TUNEL-positive osteoc lasts and osteocytes were also observed in a sporadic fashion, as well as T UNEL-positive dural cells. Msx2 was expressed in the sutural mesenchyme and the dura mater. Bmp4 was expressed in the developing bone, underlying dura mater, the osteogenic fronts, and also weakly in the sutural mesenchyme. B mp7 was detected at the same locations as Bmp4 but with noticeably stronger intensity in the meninges and overlying epidermis. We propose that this ap optosis is part of normal suture development, and is integral to the balanc e between bone formation and resorption, so that abnormal apoptosis may lea d to premature (Craniosynostosis) or delayed (Cleidocranial dysplasia) sutu re closure.