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

The tissue distribution of somatostatin (SST) immunoreactivity was studied in the nasal and forebrain region in the chick embryo. On embryonic day (ED) 3, SST-immunoreactive (ir) cells were first detected in the cells migrating from the olfactory placode. Then, at ED3.5, SST-ir cells and -ir fibers appeared in the olfactory epithelium and olfactory nerve bundles. At ED6-8, one component of the SST-ir fibers was found to separate from the olfactory nerve and it entered the parenchyma of the medial forebrain surface. These SST-ir fibers extended dorsocaudally toward the preseptal area. During this same period, a few SST-ir cells were observed in the medial forebrain adjacent to the SST-ir fibers. SST immunoreactivity in the nasal and forebrain areas was most striking at ED5-8 but a reduction of SST immunoreactivity in the nasal and forebrain areas occurred at ED11 and it virtually disappeared by the day of hatching. These results indicate that the expression of SST in the nasal and forebrain regions is transient in the chick embryo. Since the SST-ir cells did not co-express luteinizing hormone-releasing hormone (LHRH), it, thus, appears that these SST-r cells belong to a different cell population from LHRH neurons that are also found in the olfactory-forebrain axis during embryonic development [23]. However, a close relationship exists between SST-ir cells and -ir neuronal fibers and LHRH neurons. This may play a role in development of LHRH neurons.