Changes in gene expression and cell shape characterise stages of epibranchial placode-derived neuron maturation in the chick.

Sensory neurons in the head are largely generated from neurogenic placodes. Previous studies have revealed early events in placode development; however, the process of maturation has not been studied. In this study, it has been shown that placodal neurogenesis follows a sequential progression with distinct stages defined by expression of specific markers. These markers highlight domains of maturation within the stream of migratory neuroblasts that extend between the placode and the neural tube. Commitment to neurogenesis occurs in the apical placode, with the newborn neuroblasts delaminating basally and entering a transition zone. The neuroblasts migrate through the transition zone, differentiating further and becoming post-mitotic as they approach the ganglionic anlage. It has further been demonstrated that this progression from the transition zone to the ganglionic anlage is accompanied by a switch from multipolar to bipolar cell morphology. This sequential progression parallels events observed elsewhere in the nervous system, but here the stages are distinct and anatomically segregated. It is proposed that placodal neurogenesis provides a tractable system to examine the transition between states in neurogenesis.