Parameters of growth in the embryonic and neonatal chick basilar papilla.

The growth of the basilar papilla in the chick cochlear duct was studied utilizing light, scanning, and transmission electron microscopy. The ages of the cochleae investigated ranged from embryonic day 6 to post-hatching day 7. The changes in the length and width of the basilar papilla as well as the establishment of its spatula-like shape were correlated with the maturation of the hair cells' apical surfaces and the changes in the cellular organization of the sensory epithelium. The histological reorganization of the distal hair cell nuclei was concomitant with the broadening of the distal region of the basilar papilla and occurred at a later stage than the reorganization of the proximal hair cell nuclei. Since the stereociliary bundles on all the hair cells are differentiated quite early, it appears that the delayed reorganization of the distal nuclei is associated with anatomical constraints on the cochlear duct, rather than a later differentiation of the distal sensory epithelium. A clear understanding of how growth of the cochlear duct influences both the distribution of hair cells on the basilar papilla's surface and the cellular organization in the sensory epithelium is critical to future studies correlating ultrastructural development with functional maturation of the auditory system.