Abstract
<jats:p>Otolith organs are specialized structures of the vertebrate inner ear that provide the inertial mass required for maintaining equilibrium. Mammals possess two otolithic organs, the utricle and the saccule, whereas birds and other non-mammalian vertebrates also retain a third one, the lagena, whose development remains poorly understood and whose function is still debated. In birds, the lagena contains thousands of calcium carbonate biomineral particles, termed otoconia. Here, we reconstruct the developmental crystallization of lagena otoconia in the Japanese quail (Coturnix japonica) throughout embryogenesis. We combine multiscale imaging with structural, compositional, and crystallographic analyses across length scales. We show that lagena mineralization precedes cranial bone formation and proceeds predominantly through the growth of existing otoconia rather than continued nucleation. Otoconia develop through progressive particle growth, alignment, and fusion within a pre-existing organic compartment while maintaining a persistent central core, ultimately forming magnesium calcite biominerals. This maturation is accompanied by progressive nanoscale densification, transforming early mineral deposits into mature hierarchical crystals. This work establishes a developmental model of avian otoconia formation and provides new insights into how hierarchical calcium carbonate crystals are assembled during vertebrate development.</jats:p>