Abstract
Recent studies have indicated that embryonic stem cells (ESCs) can be a source for the replacement of spiral ganglion neurons (SGNs), auditory primary neurons, and neurite projections from ESC‐derived neurons to auditory sensory epithelia. However, the potential of ESC‐derived neurons for synapse formation with auditory hair cells (HCs) has not been elucidated. The present study therefore aimed to examine the ability of ESC‐derived neurons to form synaptic connections with HCs in vitro. Mouse ESC‐derived neural progenitors expressing enhanced green fluorescence protein (EGFP) were cocultured with explants of cochlea sensory epithelia obtained from postnatal day 3 mice. After a 7‐day culture, neurites of ESC‐derived neurons predominantly elongated toward inner hair cells (IHCs), which play a crucial role in sound transmission to SGNs. Immunohistochemical analyses revealed the expression of synapsin 1 and synaptophysin in the nerve endings of ESC‐derived neurons adjacent to IHCs, indicating the formation of synaptic connections. Transmission electron microscopy demonstrated synaptic contacts between nerve endings of ESC‐derived neurons and IHCs. The present findings show that ESC‐derived neurons can make synaptic connections with IHCs. © 2008 Wiley‐Liss, Inc.