CONTRALATERAL EFFERENT NEURONS CAN BE DETECTED IN THE HINDBRAIN OUTSIDE OF RHOMBOMERE-4

A group of efferent neurons whose bodies are located contralaterally a nd extend projections across the ventral midline of the hindbrain is c onsidered as a rhombomere 4-specific characteristic. These neurons con tribute to the vestibulo-acoustic nerve. At the level of rhombomere 2, a similar kind of efferents have only been described as a result of s everal experimental manipulations and have been interpreted as being d ue to rhombomere 2 acquiring rhombomere 4 identity. Here is shown that contralateral efferents can also be detected in rhombomere 2 of norma l mouse and chicken hindbrains. These findings indicate that neural pr ocesses crossing the midline should not be considered as a rhombomere 4-specific characteristic. They also imply that the formation of the c ontralateral efferents at different rostro-caudal levels might be unde r different genetic controls, because Hoxb-7, which is not expressed i n rhombomere 2, seems to be essential for their proper formation in rh ombomere 4.