Olfactory marker protein is present in olfactory receptor cells of human fetuses.

Light microscopic observations indicated that cellular differentiation of the primary olfactory pathway in human fetuses was relatively advanced by the end of the first trimester. However, immunohistochemical staining showed that olfactory marker protein was present in the receptor cells only at about 28 weeks post-conception. At about 32 weeks, modest amounts of the protein could be found in the peripheral olfactory nerve layer and a few glomeruli, with increased staining observed in subsequent weeks. The results show conclusively that human fetal olfactory neurons synthesize olfactory marker protein during the later half of gestation.