Biochemical Differentiation and Intercellular Interactions of Migratory Gonadotropin-Releasing Hormone (GnRH) Cells in the Mouse

Abstract

GnRH cells are first detected in the olfactory placode of the mouse on Gestational Day 11.5 (E11.5). Between E12.5 and E15.5 they migrate across the nasal septum and by E16.5 attain their adult distribution within the forebrain. In the present study, we used immunocytochemistry at the light and electron microscopic level to study the biochemical and morphological differentiation of gonadotropin-releasing hormone (GnRH) neurons and the cellular associations that they make during this migratory process. On E12.5, when the majority of GnRH cells are still in the nasal septum, only 15% of the population can process the pro-GnRH precursor to the amidated decapeptide, Two days later (E14.5), when most of the cells have advanced into the forebrain, 79% contain mature GnRH. In keeping with these light microscopic observations, ultrastructural analysis reveals that on E12.5 GnRH immunologic reaction product is confined to the outer nuclear envelope and rough endoplasmic reticulum. By E14.5 these migratory cells in the nasal septum have more reaction product in the rough endoplasmic reticulum and some of the Golgi cisternae are also immunopositive. Neurosecretory granules, some of which are immunoreactive, also appear at this stage. We had anticipated that the expression of GAP-43 would coincide with axonal elongation and pathfinding in GnRH neurons. Instead, GAP-43 was expressed at the early migratory stage of GnRH cells and its expression declined rapidly after these neurons had entered the forebrain and commenced axonal outgrowth. Hence, on E12.5, 62.3% of GnRH cells in the nasal septum are immunopositive for GAP-43, while only 12.6% of the forebrain population at the same embryonic stages express the protein. Similarly, on E14.5 GAP-43 is expressed in 50.7% of the nasal septum GnRH cells but only 11.6% of cells in the E14.5 forebrain express this protein. While in the nasal septum, GnRH neurons migrate only within the confines of the olfactory and vomeronasal axonal fascicles. During this part of the migratory route, the cells maintain close membrane apposition with each other and with the axons and ensheathing glia of the nerve fascicles. Once in the forebrain, GnRH neurons no longer maintain these associations, nor do they follow any defined anatomical structure. These findings indicate that although GnRH cells express their unique neuropeptide early in their ontogeny, their differentiation continues and is coordinated with their migration. The migration of these neurons across the nasal septum relies on axonal fascicles of the olfactory and vomeronasal nerves. In the forebrain, however, GnRH cells must utilize alternative guidance mechanisms to complete their migration.