Rhombomere development in a reptilian embryo.

Rhombomere development was investigated in a reptile, Alligator mississippiensis, using a variety of methodologies: cytoarchitecture (cresyl violet), histochemistry (peanut agglutinin), immunocytochemistry (antibodies to acetylated tubulin, vimentin, calretinin, and acetylcholinesterase), and external and internal morphology of wholemount embryos. Rhombomere boundaries form sequentially until 8 rhombomeres are present at stage 8. From stage 11 onwards, rhombomere borders fade. When present, boundaries of rhombomeres 2 through 5 were distinct. In all embryos, except the earliest stages, neural tissue was divided between the caudal end of the mesencephalon and the rostral end of the rhombencephalon. This area of transection was designated as the isthmus. For these technical reasons, a distinct border between the midbrain and the first rhombomere was not seen and the isthmic rhombomere could not be identified. The interrhombomeric boundary between rhombomere 7 and rhombomere 8 and between the most caudal rhombomere and the spinal cord was not nearly as clear as were the boundaries of rhombomeres 2 through 5. Development of rhombomeres 2 through 5 was investigated in wholemount preparations between stages 5/6 and 11. Qualitative and quantitative observations were made. In these rhombomeres, r2 through r5, rostrocaudal caudal expansion occurs at a slower rate than mediolateral development. This differential growth sculpts the morphology of rhombomeres 2 through 5. Rhombomere development in Alligator shares several features in common with hindbrain segmentation in chick. The identification of rhombomeres in a multitude of vertebrates from a variety of classes suggests that segmentation is a feature common to hindbrain development in all vertebrates.