A cochlear sack innervation develops as dispersed neurons in Pax2 null mice. Details of the innervation of the cochlear sack are shown using anti-tubulin immoncytochemistry in Pax2 null mice (red, A-F) and lipophilic tracing in wild-type mice (insert in C). In contrast to the patterned innervation of the organ of Corti in wild-type mice by radial fibers emanating from spiral ganglia (insert in C) innervation of the cochlear sack of the mutant is provided by an irregular network of nerve fibers (A, C, D). A continuity of this network with the vestibular fibers (B) could not be verified by tracing from the brain, suggesting that even the fibers that extent toward the saccule may not project to the brain for labeling. Higher magnification (E, F) shows a network of neurons that appears to be interconnected by fibers thus generating the network that is so prominent at lower magnification throughout the cochlear sack. Bar indicates 100 um.