Three-dimensionally visualized ossification and mineralization process of the otic capsule in a postnatal...

We aimed to elucidate the ossification process of the otic capsule in postnatal C57BL/6 mice and depict the three-dimensional (3D) process of otoconial mineralization in vivo.

The otic capsules of C57BL/6 mice were stained with alizarin red and imaged/compared using micro-computed tomography on postnatal day (P) between P0 and P8, P10, P15, and P30 and 3-4 months old (P3-4Mo). We reconstructed 3D images of the otic capsule and otoconia and measured the bone mineral density using x-ray absorptiometry on each age.

The 3D reconstructed otic capsule images revealed two ossification centers of the otic capsule at P0. One was observed around the ampulla of the superior semicircular canal and utricle, and the other was observed around the ampulla of the posterior semicircular canal. The cross-sectional views demonstrated that modiolar ossification developed from the base to the apex from P4 to P8. The inter-scalar septum ossified bidirectionally from the modiolus and bony otic capsule from P8 to P15. The mineralized otoconia were first detected in the utricle at P3 and saccular otoconia at P6. The density of the utricle and saccular otoconia showed different growth trends.

To the best of our knowledge, this is the first study to demonstrate the 3D appearance of the otic capsule and otoconia in different developmental stages of mice. We also revealed that modiolar and inter-scalar septal calcification is the final event in the cochlea and that it can be susceptible to pathological conditions (cochlear congenital malformations and hereditary vestibular diseases). The unique features of the ossification process and duration may explain these pathological conditions observed in humans.

3.