B-50/GAP-43 expression by the olfactory receptor cells and the neurons migrating from the olfactory placode in embryonic rats.

B-50/GAP-43 is a growth-associated phosphoprotein that is commonly expressed in all developing neuronal systems. Using an immunocytochemistry approach, we have investigated the expression of this protein in the rat olfactory system during embryogenesis and neonatal development with a particular emphasis on the early developmental stages of the olfactory placode. Data show that already at embryonic day 12 (E12), a strong B-50/GAP-43 immunoreactivity was detected in few olfactory receptor cells well-recognizable by their positive short neuritic processes. The B-50/GAP-43 expression in the placodal epithelium thus appeared to coincide with the onset of neurite outgrowth. From E13 onwards, there was a rapid increase in the number of B-50/GAP-43-positive olfactory neurons and from E18, the protein was strongly expressed by nearly all neurons. In addition, results clearly demonstrate that as early as E13, B-50/GAP-43 was strongly expressed by many migrating cells which were seen leaving the pit epithelium in association with the first olfactory axons that penetrated the nasal mesenchyme. Many immunoreactive cells were also observed in the presumptive olfactory nerve layer. Experiments of double-labeling showed that B-50/GAP-43-immunostained migrating cells were also stained with anti-neuron-specific enolase (NSE). This confirms the neuronal nature of these early labeled migrating cells. The progressive disappearance of migrating neurons noted during the late stages of embryonic development is discussed in relation with their possible function in the early stages of development of the peripheral olfactory system.