Early ear development in the embryo of the zebrafish, Danio rerio.

The zebrafish provides an important model for vertebrate inner ear development. The otic placode becomes visible at approximately 16 hours (at 28.5 degrees C) and forms a vesicle with a lumen by cavitation at approximately 18 hours. Two otoliths appear in the lumen by 19.5 hours, and at about 24 hours the first sensory hair cells are seen, grouped in two small patches, one beneath each otolith, corresponding to future maculae. Staining with fluorescent phalloidin reveals 10-20 hair cells in each macula by 42 hours; between 3 days and 7 days the numbers grow to approximately 80 per macula. Neurons of the statoacoustic ganglion are first visible by staining with HNK-1 antibody at about 24 hours. Serial sections and time-lapse films show that the neuronal precursors originate by delamination from the ventral face of the otocyst; the peak period of delamination is from 22 hours to 30 hours. The system of semicircular canals forms between 42 hours and 72 hours by outgrowth of protrusions from the walls of the otocyst to form pillars of tissue spanning the lumen. Three further clusters of hair cells also become visible in this period, forming the three cristae. Thus, by the end of the first week, all key components of the ear are present. Subsequent growth produces thousands more hair cells; additional neurons probably derive from proliferation of neuronal precursors within the ganglion. Although the timetable is species-specific, the principles of inner ear development in the zebrafish seem to be the same as in other vertebrates.