Cell death during development of the cochlear and vestibular ganglia of the chick

This report documents and quantitates the naturally occurring death of neurons in the cochlear and vestibular ganglia during the normal development of the chick embryo. The data are compared with the amount of cell loss in the cochlear and vestibular nuclei to evaluate the possibility that the death of the neurons in the same sensory pathway may be related to the formation of their connections. Neurons of the cochlear and vestibular ganglia of the chick were counted and their cell bodies measured with light microscopic methods at the beginning of their period of synapse formation, embryonic day 8 (E8), at the end of the period of synapse formation (E14), and again at a more mature stage, post-hatching day 14 (P14). The number of neurons declined between E8 and E14 in the cochlear ganglion by 25% (from 11170 to 8353) and in the vestibular ganglion by 24% (from 12687 to 9613). The neuronal population remained relatively stable between E14 and P14. Cell counts at each age were accompanied by morphological descriptions of the neurons and of their synaptic targets in the sensory epithelia of the inner ear. During the period of cell loss the mean diameters of the cell bodies in the cochlear ganglion increased slightly, from 9.5 to 111 μm and reached 17.6 μm at P14, while those in the vestibular ganglion increased from 10.4 to 13.3 μm and reached 20.9 μm at P14. The period of cell loss in the cochlear and vestibular ganglia coincides with the stages in their development when the sensory neurons have already contacted their target cells in the primary sensory nuclei and in the receptor epithelium. It is during this period that the transformation from the immature axonal endings to the definitive types of synapses begins. Thus the present data are consistent with the view that the formation of central as well as peripheral synapses is involved in determining the extent of neuronal death during the development of sensory neurons.