Tissue interactions and cell differentiation: neurone-sensory cell interaction during otic development.

Statoacoustic ganglion neurones (SAG) are produced by the same group of cells (otic placode) that produce all of the receptor cells that populate the sensory areas of the inner ear. The observation that ingrowth of SAG neurites to presumptive sensory areas of the inner ear preceded cytodifferentiation of those receptor cells suggested a causal relationship. Results from in vivo, in ovo and in vitro studies do not support a causal relationship. These studies support the hypothesis that the programme for labyrinthine sensory cell differentiation is intrinsic and does not require the extrinsic stimulus of neuronal interaction to trigger its expression. In contrast, developing statoacoustic ganglion neurones appear to require a trophic influence that is supplied by either their peripheral or central target tissues for their survival and maturation in vitro. A mechanism for the ingrowth of SAG dendrites to their appropriate target sites within the inner ear proposes that attractant fields produced by areas of differentiating sensory cells act to guide the nerve growth cones of ingrowing SAG neurites to the appropriate tissues. Preliminary results from a heterochronic series of SAG implants to common age otocysts suggest that these SAG neurones are capable of responding to the attractant fields which are produced by presumptive labyrinthine sensory epithelium over an extended period of otic development. Both in ovo and in vitro studies suggest that spatiotemporal patterns of extracellular matrix molecules may be important components of the attractant fields which are produced by the sensory areas of the developing inner ear and may ultimately result in the specificity of their neuronal connections.