Indiana University-Purdue University Indianapolis (IUPUI) / Background: Suture expansion is a very important clinical approach to correct maxillary width deficiency, but it has a high potential for treatment relapse. Accelerating bone formation and mineralization in the midpalatal suture during suture expansion is beneficial in preventing relapse of the arch width and reducing the retention period. Pyk2 is tyrosine kinase which has been shown to mediate signaling pathways that are involved in the process of bone remodeling. Pyk2 knock-out (KO) mice have augmented bone formation and increased bone mass, suggesting that therapeutic strategies that inhibit Pyk2 may be useful to enhance bone remodeling and prevent suture relapse during suture expansion. Objectives: To determine if Pyk2-deficiency affects midpalatal suture bone mass and bone remodeling with or without suture expansion in mice. Methods: Thirty-six Pyk2-KO and thirty-six wild type (WT) 6 week-old male mice were randomly assigned into three groups: receiving no expansion force (0 g), 10 g or 20 g force of rapid maxillary expansion for 14 days. Half of the mice in each group were used for histology analysis; the other half was assigned for fluorescence analysis. Suture width, maxilla width and bone volume/tissue volume around suture bone edges were measured using micro-CT. Histological analyses of osteoclasts (tartrate resistant acid phosphatase, TRAP), osteoblasts (alkaline phosphatase, ALP) and chondrocytes (alcian blue) were performed. Results: The BV/TV ratio was significantly higher in Pyk2-KO control mice compared to WT control mice. Suture expansion in WT and Pyk2-KO mice led to an increase in bone marrow spaces around the suture edge and significantly reduced BV/TV. Expansion also led to a significant increase in suture width, suture fibrous area, osteoclast number, cartilage area and hypertrophic chondrocyte number. However, BV/TV in Pyk2-KO mice was significantly higher than in WT mice at both the 10 g and 20 g force levels. In addition, Pyk2-KO exhibited reduced suture width, maxilla width, fibrous area and osteoclast number per bone surface (OC.S/BS) compared to WT mice under expansion forces. Cartilage area and hypertrophic chondrocyte number were increased by force but were independent of mouse genotypes. Conclusion: Pyk2-KO mice have higher BV/TV and narrower suture width compared to WT mice, which may be due to decreased osteoclast activity. The higher BV/TV of the midpalatal sutures of Pyk2-KO mice following suture expansion may suggest the presence of a more stable suture that has a reduced potential for relapse. Therapeutic strategies to inhibit Pyk2 during RME may be beneficial in increasing bone mass and preventing relapse of the suture.