Temporal pattern of innervation in the developing mouse inner ear: An immunocytochemical study of a 66‐kD subunit of mammalian neurofilaments

We have examined the expression of a 66‐kD neurofilament protein (NF‐66) in the developing inner ear. Mouse embryos, fetuses, and neonates were fixed in Methacarn, embedded in paraffin, and sectioned. A polyclonal antiserum raised specifically to NF‐66 and unreactive to NF‐L, ‐M, ‐H, and peripherin was used for immunocytochemical staining. NF‐66 immunostaining was first detectable in the rhombencephalon at embryonic day (E) 9.5. Immunoreactivity was first detected in the statoacoustic ganglion (SAG) early on E10.5. By late E10.5, the first SAG axons were detectable within the intraepithelial spaces of the otocyst. At E12, NF‐66 positivity was detectable in neurites that projected into areas of presumptive vestibular sensory epithelium. Neurites projecting into the presumptive acoustic sensory epithelium were negative. However, at E13, the projections from both the vestibular and the acoustic ganglion (i.e, cochlear duct) were both NF‐66 positive. In the cell bodies, NF‐66 expression appeared earlier in the vestibular than in the auditory neurons. By E16, neuronal somas in both ganglia were NF‐66 positive.