A critical period for gravitational effects on otolith formation.

The otoliths of adult animals do not change significantly during space flight. However, during the period when otoliths are first developing, rearing in space produces significantly larger otoliths. Conversely, animals reared on a centrifuge have smaller than normal otoliths. To identify a critical period during development for gravitational effects on otolith growth, fertilized zebrafish (Danio rerio) eggs were reared on a centrifuge for 1 week. The fine structure of their inner ear during development was studied by both light- and transmission electron microscopy. By 16 hours after fertilization (1-g, at 28.5 degrees C), precursors of the otoliths are seen but no sign of a sensory epithelium is present. Mature hair cells, appearing capable of mechanotransduction, are not seen until between 48 and 72 hours after fertilization. Zebrafish reared at 3-g from 1 to 7 days after fertilization exhibit significantly slower otolith growth than did 1-g controls. Fish exposed to 3-g only from 12-36 h after fertilization had slightly smaller otoliths than 1-g controls, but this difference was not significant. Animals exposed to 3-g from 36 h to 7d after fertilization did have significantly smaller otoliths. If the fish use their hair cells to assess otolith weight in a regulatory role, the hair cells would have to be functional. Thus the earliest stage zebrafish, which were not significantly affected by centrifugation, probably did not have an adequate means of sensing otolith weight to correct for the excess weight.