Transient expression of somatostatin immunoreactivity in the developing olfactory-forebrain region of the chick.

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SST-ir cells and fibers appear in the olfactory-forebrain axis in early chick embryos but these SST-ir elements virtually disappear by the day of hatching (Murakami and Arai, 1994). The time of appearance and distribution of the SST-ir neural elements are similar to those of LHRH neurons which have been reported to originate in the olfactory placode and migrate into the forebrain. The results of double-staining for SST and LHRH indicate that these two types of peptidergic neurons belong to two different cell populations. The placodal epithelial cells of the embryonic day 2.5-3.5 (Stages 17-23) chick embryos were labeled with a fluorescent carbocyanine dye, DiI.Four to five days after DiI application, the labeled cells were recognized in the medial forebrain and in the septo-preoptic area. Double staining for DiI and SST in the brain showed that some of SST-ir cells in the medial forebrain were labeled with DiI.These results indicate that SST-ir cells originate in the olfactory placode and migrate into the medial forebrain. Moreover, a number of LHRH neurons in the medial forebrain surface are found to be accompanied by a small subset of transient SST-ir fibers extending from the SST-ir olfactory nerve. It is possibility that these transient SST-ir fibers provide structural support for the migration of LHRH neurons from the nasal region to the septo-preoptic area. Electron microscopic or conforcal laser microscopic study is needed to elucidate whether leading processes of LHRH neurons contact with a transient SST-ir fibers.