The growth of cochlear fibers and the formation of their synaptic endings in the avian inner ear: A study with the electron microscope

Abstract The developmental sequence of nerve-epithelial cell contacts, leading up to the formation of the mature receptoneuronal synapse, has been studied in the basilar papilla of chick embryos with electron microscopy. The receptor epithelium before innervation, on embryonic days 3–4, consists of a homogeneous population of primitive cells; hair cells and supporting cells cannot be distinguished. During innervation of the epithelium (embryonic days 5–7), the invading peripheral fibers of cochlear ganglion cells penetrate the basal lamina and form nerve-epithelial attachments with the epithelial cell bases. Once within the epithelium some fibers turn and spread in the transverse dimension across the basilar papilla through channels formed between the basal epithelial processes. Subsequently, nerve-epithelial attachments are observed more superficially within the epithelium. Hair cells and supporting cells differentiate during early synaptogenesis (embryonic days 8–9). Receptoneural synapses, possibly derived from the nerve-epithelial attachments formed during the innervation stage, are first seen during this period. They are characterized by symmetrical or asymmetrical membrane densities, separated by a cleft containing a dense material. At many of these junctions synaptic bodies, as well as dense-cored and coated vesicles, gather in the hair cells. During mid-synaptogenesis (embryonic days 11–13) the hair cells proliferate synaptic bodies, many of which are not located at receptoneural junctions. The preterminal portions of the sensory endings form large swellings, containing flocculent material, endoplasmic reticulum and vesicles. Late in synaptogenesis (embryonic days 15–17) the swellings disappear, while synaptic endings are transformed to foot-shaped terminals. In the hair cells, synaptic bodies not associated with junctions disappear. Efferent synapses are first seen during this period. This sequence of ultrastructural changes, which the developing sensory nerve endings and their target cells undergo in parallel, can be correlated with observations of Golgi preparations from a companion study. These correlations suggest that the innervation of the cochlea involves the following developmental processes. Initially the peripheral fibers of the ganglion cells grow directly toward the otocyst in fascicles. Having reached the base of the primitive receptor epithelium, the axonal endings, including some with growth cones, encounter a barrier in the basal lamina. When they enter some of the fibers attach to the basal end-feet of the primitive epithelial cells. The epithelial cells to which these fibers attach, and eventually innervate, withdraw apically and begin to differentiate into tall hair cells. Fibers destined for the short hair cells, spread across the basilar papilla in transversely oriented tunnels between the basal processes of epithelial cells without forming junctions. When these fibers attach to the basal epithelial processes, the fibers grow radially, as the epithelial cells withdraw and begin to differentiate into short hair cells. Thus, the present study implies that the critical events in the formation of the receptoneural synapses occur much earlier and in a different location than previously supposed. Further differentiation of the hair cells involves the resorption or disappearance of the basal trailing processes, a transitory overproduction of synaptic bodies, and the establishment of the definitive synaptic complexes. In parallel with these changes the differentiation of the afferent endings involves a transitory overproduction of branches and preterminal swellings, and a transformation into foot-shaped terminals with the definitive synaptic junctions. These findings suggest a role for nerve-target cell interactions in the morphogenesis of receptoneural synapses.