Effects of mechanical loading on cranial suture mesenchymal stem cell proliferation

Sutures are the primary sites of cranial bone growth, allowing the skull to accommodate the growing brain. External mechanical stimulus has been shown to delay suture fusion and induce tissue remodelling. Recent studies suggest that in vivo cyclic loading increased proliferation of mesenchymal stem cells (MSC) in the coronal suture. The overall aim of this study was to understand how many loading sessions (exposure-response) and how long after loading (time-course) did MSC proliferation increase in the coronal suture. In the exposure-response analysis, mice underwent 1, 3, or 5 loading sessions between Postnatal day 7 (P7) and P11, and in the time-course analysis, treated mice underwent 10 loading sessions between P7 and P21. Loading sessions were 10 min at a frequency of 1 Hz and a force of 10 g (0.1 N). The loading tip was positioned on the posterior aspect of the left frontal bone, dorsal to the coronal suture. EdU labels newly synthesised DNA during the S-phase of cell division, whereas PCNA is abundant in cells during the G1, S, G2, and M phases. The exposure-response analysis showed that when both markers are combined, proliferation levels can be interpreted after a minimum of five loading sessions, at which point a clear increase in proliferation is observed. In the time-course analysis, proliferation was highest immediately after the final treatment session. 24 h after the final loading session, the effects of mechanical loading gradually returned to baseline. The effect of mechanical loading on MSC proliferation is highly dependent on the number of loading sessions and the time point after treatment.