Neurons are lost throughout embryonic development in Bdnf-/- and Ntf5-/- mice. At E12.5, the geniculate ganglion is still fused with the vestibular-cochlear ganglion, which explains the greater number of neurons in wild type mice at this age. Neurons are lost by E12.5 in Ntf5-/- mice, indicating that NT4/5-dependency begins earlier in embryonic development than does BDNF-dependency. In Bdnf-/- mice, neurons are first lost between E12.5 and E14.5. Losses continue to be greater in these animals, compared to wild type, throughout the remainder of embryonic development.