DEVELOPMENT AND MIGRATION OF OLFACTORY NEURONS IN THE NERVOUS-SYSTEM OF THE NEONATAL OPOSSUM

The neonatal opossum (Monodelphis domestica) was used to assess how di fferent populations of cells are generated in the olfactory region, an d how they migrate along pathways to the central nervous system. Devel oping nerve cells were immunocytochemically labelled using antisera di rected against two specific markers of olfactory receptor neurones: ol factory marker protein (OMP) and the dipeptide carnosine. In new-born opossums both carnosine and OMP are already co-expressed in primary ol factory neurones and in those axons that extend towards the olfactory bulb. Expression of these markers in olfactory receptor neurones durin g the first postnatal days reflects the advanced developmental state o f this system compared to other regions of the central nervous system (such as the cortex and cerebellum), which are highly immature and les s developed in comparison with those of new-born rats or mice. A secon d, distinct population of carnosine/OMP expressing cells was also iden tified during the first postnatal week. These neurones were present as clusters along the olfactory nerve bundles, on the ventral-medial asp ect of the olfactory bulb and in the basal prosencephalon. The distrib ution of this cell population was compared to another group of well ch aracterized migratory neurones derived from the olfactory placode, whi ch express the decapeptide GnRH (Gonadotropin-releasing hormone, also known as LHRH). GnRH was never co-localized with carnosine/OMP in the same migratory cells. These observations show that distinct cell popul ations arise from the olfactory placode in the neonatal opossum and th at they migrate to colonize the central nervous system by following co mmon pathways.