Synaptogenesis in the basilar papilla of the chick.

Synaptogenesis was studied in the basilar papilla of chicken embryos from days 7--21 of incubation. On the 9th day of incubation differentiating hair cells first appeared and a few growing nerve tips made contact with them, although no membrane specializations were apparent at this stage. Synaptic bodies associated with presynaptic membrane specializations were first observed on the 10th day. They lay opposite either supporting cells or afferent nerve processes; in the latter site slight membrane thickenings were occasionally found. During subsequent stages synaptic bodies and the surrounding vesicles increased in number. Synaptic bodies associated with presynaptic membrane specializations, but devoid of contact with afferent nerve endings, were often observed on the 14th day, whereas almost all the synaptic bodies associated with presynaptic specializations were in contact with afferent nerve processes by the 21st day. The efferent synapses were first recognized on the 14th day. These results suggest that in the hair cells of chicken basilar papilla the synaptic bodies and presynaptic membrane specializations appear first and after the synaptic sites are determined by the position of the synaptic bodies, the growing nerve tips seek out and establish synaptic contact at the pre-existing synaptic sites.