Direct evidence for the migration of LHRH neurons from the nasal region to the forebrain in the chick embryo: a carbocyanine dye analysis.

To examine the possible migration of luteinizing hormone-releasing hormone (LHRH) neurons from the olfactory placode to the brain, the epithelial cells of the olfactory placode of the chick embryos were labeled with a fluorescent carbocyanine dye DiI at embryonic days 3.5-4.0. The embryos treated with DiI were incubated in ovo for 1 to 7 days. The DiI-labeled cells were first detected in the olfactory epithelium and olfactory nerve one day after the application of DiI. Two to four days after the application of the dye, the labeled cells sequentially appeared in the rostral and medial forebrain, and in the septo-preoptic area. The distribution pattern of DiI-labeled cells closely resembled that of LHRH neurons. Double staining for DiI and LHRH demonstrated that DiI-labeled cells co-expressed LHRH. However, there were a number of DiI-labeled cells which did not co-express LHRH. These results provide evidence for the actual migration of LHRH neurons from the olfactory placode to the septo-preoptic area during the development, and also suggest that some non-LHRH neurons in the forebrain are of olfactory placodal origin.