Development of the olfactory bulbs in human fetuses (an immunohistochemical study)

An immunohistochemical study of the olfactory bulb (OB) in human fetuses was performed. Immunohistochemical markers for nervous system-specific protein complexes S-100 and SNAP-25 were used. At 20-22 weeks of development, the development of cells expressing protein S-100 was greater in the OB than in the neocortex (the areas of the gyrus rectus and the gyrus orbitalis longitudinalis internus). These quantitative differences indicate heterochronicity in glial differentiation in the OB and cerebral cortex. Immunopositive reactions for SNAP-25 were for the first time detected at the periphery of the OB and in glomeruli in human fetuses at 15-16 and 20-22 weeks of development. Immunohistochemical staining of the OB with antibodies to SNAP-25 indicated that the human olfactory system cannot function prior to 20-22 weeks of development. In the OB of fetuses at 28-29 weeks of development, the intensity of the immunohistochemical reaction showed topological differences. The reaction with antibodies to SNAP-25 in the OB of full-term fetuses was similar to the reaction in the OB of adult humans. The results of these immunohistochemical studies using the reaction for the protein complex SNAP-25 in fetuses of different ages suggest the that the primary olfactory center in humans starts to function no earlier than the 30th week of development.