Age-related morphological changes in the C57BL/6J mouse calvaria: implications for stereotaxic neurosurgery

The mouse skull serves as the coordinate framework for stereotaxic neurosurgery, yet whether aging alters surgically critical calvarial geometry has not been examined. We measured three parameters in large cohorts of young (2-3 months) and aged (23-30 months) male and female C57BL/6J mice: bregma-lambda distance and the dorsoventral depth at rostral-lateral (X = {+/-} 2.5 mm, Y = 0) and caudal-lateral (X = {+/-} 2.5 mm, Y = -0.65 mm) calvarial sites. Bregma-lambda distance was significantly reduced with aging in both sexes. Similarly, the rostral-lateral calvaria showed age-related flattening in both male and female mice. The more caudal-lateral site showed smaller but significant age-related flattening, with males exhibiting significantly greater reductions than females. Aging increased caudal-lateral shape variability while reducing bregma-lambda distance variability in males. On the contrary, aging reduced rostral-lateral shape variability in females. These findings demonstrate that the mouse skull undergoes significant, spatially non-uniform, and sexually dimorphic remodeling into old age, where skull shrinks along the midline and flattens more significantly at the rostral than caudal plane. Stereotaxic protocols designed for young adult mice may systematically misestimate skull geometry in aged animals. Age-stratified reference data should be incorporated into stereotaxic practice for aged rodent models.