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 exp ressed in all developing neuronal systems. Using an immunocytochemistr y approach, we have investigated the expression of this protein in the rat olfactory system during embryogenesis and neonatal development wi th a particular emphasis on the early developmental stages of the olfa ctory placode. Data show that already at embryonic day 12 (E12), a str ong B-50/GAP-43 immunoreactivity was detected in few olfactory recepto r 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 E1 3, B-50/GAP-43 was strongly expressed by many migrating cells which we re seen leaving the pit epithelium in association with the first olfac tory axons that penetrated the nasal mesenchyme. Many immunoreactive c ells were also observed in the presumptive olfactory nerve layer. Expe riments of double-labeling showed that B-50/ GAP-43-immunostained migr ating cells were also stained with anti-neuron-specific enolase (NSE). This confirms the neuronal nature of these early labeled migrating ce lls. The progressive disappearance of migrating neurons noted during t he late stages of embryonic development is discussed in relation with their possible function in the early stages of development of the peri pheral olfactory system.