Developmental gradients in the embryonic chick's basilar papilla.

The basilar papilla, the avian counterpart to Corti's organ, was examined cytologically in the embryonic chick (Gallus domesticus) from stages 31 through 46 (hatching). Developmental reconstructions were prepared by using light microscopy to analyse serial sections (1-5 microM) along the entire lagenar length and electron microscopy to examine representative thin sections from the proximal, medial and distal regions. We identified two gradients, the lateral and longitudinal. In the longitudinal gradient, the mid-basal basilar papilla, which lies directly above the statoacoustic ganglion, develops before the proximal and distal regions as evidenced by the maturation of hair cells and afferent synapses. Similarly, in the lateral gradient, tall hair cells and afferent synapses develop before short hair cells and their respective afferent terminals. Efferent synaptogenesis follows the same two gradients several days later than the afferents. Afferent synaptogenesis corresponds with the opening of the perilymphatic spaces and completion of the tectorial membrane.