Competence, specification and commitment in otic placode induction

The inner ear is induced from cranial ectoderm adjacent to the hindbrain, D espite almost a century of study, the molecular mechanisms of inner ear ind uction remain obscure. We have identified four genes expressed very early i n the anlage of the inner ear, the otic placode, Pax-2, Sox-3, BMP-7 and No tch are all expressed in placodal ectoderm from the 4-5 somite stage (ss) o nwards, well before the otic placode becomes morphologically visible at the 12-14ss, We have used these four molecular markers to show that cranial ec toderm becomes specified to form the otic placode at the 4-6ss, and that th is ectoderm is committed to a placodal fate by the 10ss, We also demonstrat e that much of the embryonic ectoderm is competent to generate an otic plac ode if taken at a sufficiently early age. We have mapped the location of ot ic placode-inducing activity along the rostrocaudal axis of the embryo, and have determined that this activity persists at least until the 10ss, Use o f the four molecular otic placode markers suggests that induction of the ot ic placode in birds occurs earlier than previously thought, and proceeds in a series of steps that are independently regulated.