The olfactory placode (OP) is an epithelial thickening derived from the anterior neural ridge. The OP gives rise to the olfactory and vomeronasal sensory cells, supporting cells within these epithelia and specialized glia that ensheathe the olfactory nerve. Of interest to reproductive biologists is the observation that gonadotropin releasing hormone (GnRH) neurons differentiate within the OP and then use the olfactory nerves as a guidance mechanism to enter the CNS where they become widely dispersed in the forebrain. This application focuses on two major issues: how during ontogenesis is the diversity of GnRH neurons and other placodal derivatives generated and what cellular or chemical signals mediate the GnRH neuronal migration into and then through the CNS. The first category of experiments examines lineage or cellular associations. Using 3H-thymidine incorporation combined with immunocytochemistry (ICC) we shall determine if time of withdrawal from the cell cycle (birthdate) can account for when and how far GnRH cells migrate. We shall also analyze the temporal potential of the OP stem cell population both in embryogenesis and in the adult. To develop a fate map of the OP, application of the vital dye, DiI, or a replication deficient retrovirus, again combined with ICC, will be used. These methodologies will detail the normal progeny of the OP and provide critical information on its temporal and/or spatial organization. Commitment to selected lineages will be studied using the retroviral technology or by single cell injection at both the placodal stage and earlier in ontogeny before neural tube closure. The second cluster of the experiments highlight the biology of the GnRH neuronal migration. Do the specialized ensheathing cells, with their mixed Schwann cell and astrocyte phenotype, play a role in the entrance of GnRH neurons into the CNS and/or their final distribution? this study requires light and electron microscopic ICC. Does the CNS produce any diffusible signals that mediate migration or that promote GnRH neuronal survival? For these experiments we have developed a collagen gel matrix system that can be used to test for both chemotaxis and chemotrophism.
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