Genetic patterning of embryonic inner ear development

The commitment of the otocyst to form balance (pars superior) and hearing ( pars inferior) receptors is under the control of patterning genes. Intersec ting patterns of gene expression has been proposed to explain the regionali zation of inner ear sensory receptor development. Insights into the roles o f patterning genes is being acquired from the analysis of inner ear develop ment of null mutation mice. An example of the consequences of the loss of e xpression of a single patterning gene is the effect of a null mutation of p aired-box gene 2 (Pax2) on formation of the cochlea. Pax2 transcripts are e xpressed in the pars inferior of the otocyst and null mutant inner ears sho w agenesis of the cochlea. This is an example of the profound effect that l oss of a single patterning gene can have on inner ear development. However, more typically there is redundancy of gene action during inner ear develop ment as exemplified by the overlapping pattern of expression of two closely related homeobox-containing genes (i.e. Hmx2 and Hmx3) in the pars superio r of the otocyst. Hmx2 and Hmx3 genes are both expressed early in otic deve lopment with Hmx3 transcripts present in the placode. However, null mutatio n of the Hmx3 patterning gene produced only a limited vestibular defect, an d did not result in the agenesis of the vestibule. This result suggests tha t then is redundancy in genes that have similar patterns of regional expres sion in the otic anlagen.