Otoconial formation in the fetal rat.

The development of otoconia in the fetal rat was investigated by scanning and transmission electron microscopy and by x-ray elemental analysis. The transmission electron microscopical results indicate that primitive otoconia are highly organic appearing but are trigonal in cross section, indicating that they already possess a three-fold axis of symmetry and a complement of calcite. These otoconia develop into spindle-shaped units which accrue fibrous, organic material at an angle to their surfaces. Dumbbell-shaped otoconia, with distinct central cores and peripheral zones, result. These otoconia then mature to the adult crystal configuration having a more cylindrical body and pointed ends. The existence of trigonal, spindle- and dumbbell-shaped otoconia was verified by scanning electron microscopy of fresh-frozen material. Tissues prepared for transmission electron microscopy proved (by elemental analysis) to have been decalcified inadvertently, fortuitously revealing the arrangement of the organic material. Subsequent transmission electron microscopy of dumbbell-shaped otoconia not exposed to fluids during embedment showed that calcite deposits mimicked the arrangement of the organic material. X-ray elemental analysis demonstrated that calcium was present in lower quantities in the central core than peripherally. Findings are interpreted to indicate that organic material is essential to otoconial seeding and directs otoconial growth.