Ontogeny of murine bony semicircular canal form.

The labyrinthine geometry and functional anatomy of the semicircular canals have intrigued scientists for decades, and there has been considerable interest in understanding how these complex structures grow and develop with evidence emerging from human studies that size maturation occurs exceptionally early by comparison with other systems. Here we seek to extend the empirical evidence base and our understanding by investigating canal ontogeny in a non-human species. The postnatal development of the murine bony semicircular canals was investigated to determine the timing of size and shape maturation in relation to ossification of the surrounding otic capsule. Using micro-CT imaging and geometric morphometric analysis of 43 male C57BL/6J mice across six developmental stages (postnatal days 1-48 and 26 months), this study reveals that the semicircular canals undergo rapid growth and shape transformation within the first postnatal week. Ossification begins between P1 and P7, coinciding with the most significant observed increases in canal length and configuration shape. By P14, the canals are fully encased in a thin layer of bone, and subsequent changes are minimal, mostly in size, indicating morphological stabilization. This is further supported by a decrease in size variability as ossification progresses. These findings support the hypothesis that ossification imposes a progressively tighter constraint on canal form, as reflected by the gradual reduction in shape variation past the onset of ossification. Cross-section area shows negligible variation, suggesting early maturation of this feature. These findings highlight a brief but critical window of morphological plasticity in early postnatal life, with implications for interpreting vestibular development and fossil ontogeny in mammals.