Migration of branchiomotor neurons to the alar plate in the chicken embryo
It is generally believed that the spinal cord and hindbrain consist of a motor basal plate and a sensory alar plate. This assumption was tested in the chicken embryo by mapping the expression of cadherin-7 and cadherin-6B, in comparison to genetic markers of ventrodorsal patterning (Otp, Pax6, Pax7, Nkx2.2, Shh) and markers for specific motoneuron subpopulations (Phox2b, Islet-1, and Lim3/Lhx4). Results show that cadherin-7 is expressed in a complete radial domain, which occupies the dorsal region of the basal plate. The dorsal limit of this cadherin-7 domain coincides with the ventral border of Pax7 expression, i.e. with the basal/alar plate boundary. The branchiomotor neurons of the chicken hindbrain are born at a medial position close to the floor plate. They extend a cadherin-7-positive axon that grows laterally and exits the hindbrain from the alar plate. Following this axonal trajectory, the cadherin-7-positive neuronal cell bodies translocate laterally to pass through the cadherin-7-positive basal plate domain. Subsequently, the cell bodies traverse the basal/alar plate boundary and assume their final position in the alar plate. After migration is completed, branchiomotor neurons switch expression from cadherin-7 to cadherin-6B. These findings demonstrate that a specific subset of primary motor neurons, the branchiomotor neurons, migrate into the alar plate of the chicken embryo. Consequently, the century-old concept that all primary motor neurons come to reside in the basal plate should be revised.