Localization of calcitonin gene-related peptide mRNA in developing olfactory axons

During development of the olfactory pathway, calcitonin gene-related peptide (CGRP) expression is regulated both temporally and spatially. We had previous evidence that between E13 and E19 CGRP mRNA was present at the level of olfactory axons but the resolution of light-microscope in situ hybridization did not permit the axons to be distinguished from the closely apposed ensheathing cells. In this study, the localization of CGRP mRNA was studied at early developmental stages (E13-15) through in situ hybridization at the transmission electron-microscope (TEM) level. CGRP transcripts were observed exclusively in axons and not in ensheathing cells. The distribution of transcripts in the axons suggests that they are associated with intermediate filaments rather than microtubules. In addition, a careful ultrastructural analysis provided evidence that polysomes and membrane-bound ribosomes are present in such axons, suggesting that the peptide could be synthesized locally.