MORPHOLOGICAL FATE OF RHOMBOMERES IN QUAIL-CHICK CHIMERAS - A SEGMENTAL ANALYSIS OF HINDBRAIN NUCLEI

Quail rhombomeres two to six (r2-r6) were individually grafted homotop ically into the hindbrain of chick embryos at 2 days of incubation. Ni ne to 10 days after the operation the chimeric embryos were fixed and processed for parallel cytoarchitectural and immunocytochemical study (with an anti-quail antibody) in order to map the anatomical fate of t he grafted tissue. Emphasis was placed on conventionally identified an d distinct neuronal populations composing the sensory and motor longit udinal columns. Grafted rhombomeres consistently developed as complete transverse slices of the chimeric hindbrain. Interrhombomeric cell mi gration was either sparse or restricted to specific nuclei. The crania l nerve motor nuclei showed rhombomeric origins consistent with the pa tterns described in early embryos. Unexpectedly, alar r2 was found to form the auricular part of the cerebellum. As regards the cochlear nuc lei, we found that nucleus angularis derives from r3 to r6, nucleus la minaris from r5 to r6, nucleus magnocellularis from r6 to r7 and nucle us olivaris superior from r5. The nuclei of the lateral lemniscus orig inated between r1 and r3. We also delimited the respective rhombomeric subdivisions of the sensory vestibular and trigeminal columns, both o f which extend from r1 caudalwards throughout the hindbrain. There wer e consistently some interrhombomeric neuronal migrations inside the ve stibular column, some motor nuclei and the reticular formation, involv ing only one rhombomere length. The pontine nuclei, which extended fro m r1 to r7, showed neuronal migrations that crossed several rhombomere s. On the whole, these results represent the first anatomical analysis of the mature avian hindbrain in terms of rhombomere-derived domains.