An electron microscope study of crystal calcium carbonate formation in the mouse otolith.

The ultrathin sections of the developing otolith of mouse fetuses 15.5 days to 20 days after birth were observed with the aid of the electron microscope. The first step of otolith formation is an aggregation of organic clusters observed in the sacculus of the 15.5-day fetus. These organic structures are modified and assume a hexagonal shape in the 17.5-day fetus. The unmineralized stages of the otolith referred to as the preotolith, serve as the template for future development. One day after birth, at either end of the preotolith, minute tube-like structures develop in which needle-shaped crystallites are initiated. Crystallites continue to develop throughout the hexagonal template which gave rise to a structure containing many regularly arranged needle-shaped crystallites. Based on the present observations, the mouse otolith is considered a multi-iso-oriented crystal.