Detection of apoptosis in fusing versus nonfusing mouse cranial sutures

Apoptosis may be involved in maintenance of suture patency. In mice, the posterior frontal suture fuses by postnatal day 45, whereas all remaining cranial sutures remain patent. There are no published reports documenting differences in apoptosis between fusing and nonfusing mouse cranial sutures beyond postnatal day 6 either in vivo or in vitro. In the current study, we hypothesized that apoptosis is required for maintenance of suture patency. We predicted that after normal suture fusion in the mouse, the posterior frontal suture should have fewer apoptotic cells than the sagittal suture. We also hypothesized that all of the sutures should look similar with respect to the number and arrangement of apoptotic cells before suture fusion. The posterior frontal and sagittal sutures were studied on postnatal days 25 and 45. The fragmentation of DNA or terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labeling assay assay, as well as the presence of BCL-10, a specific apoptotic protein, were localized to the leading edge of the sagittal suture calvaria of postnatal day 45 mice. These apoptotic markers were not visualized within the fused posterior frontal suture of postnatal day 45 mice. Posterior frontal or sagittal suture mesenchyme of postnatal day 25 mice showed similar amounts of apoptotic cells. These data indicate that apoptotic cells are present in the patent sagittal suture beyond the period of posterior frontal suture fusion in the mouse. We conclude that apoptosis is an integral component to maintain suture patency in the mouse calvaria.