Electron microscopic identification of luteinizing hormone-releasing hormone-immunoreactive neurons in the medial olfactory placode and basal forebrain of embryonic mice.

Luteinizing hormone-releasing hormone is a decapeptide found in the brain and nose of all vertebrates that have been examined by immunocytochemical procedures with antiserum to luteinizing hormone-releasing hormone. It regulates the release of both luteinizing hormone and follicle-stimulating hormone from the gonadotropes of the anterior pituitary gland and promotes mating behavior. After about 11 days of embryogenesis in mice, luteinizing hormone-releasing hormone-immunoreactive cells are detected by immunocytochemical procedures in the medial olfactory placode, in the primordium of the vomeronasal organ. As they leave the olfactory placode, they run under the epithelial layer of the nasal septum associated with vomeronasal and terminalis nerves. Clustered, they stream toward the primordium of the olfactory bulb, passing along its ventromedial surface. Eventually, the largest numbers reach the septal and preoptic areas of the brain. Electron microscopic immunocytochemistry showed that luteinizing hormone-releasing hormone-immunoreactive product is accumulated just outside the nuclear envelope and in the lumen of rough endoplasmic reticulum adjacent to the cell nucleus of cells in and adjacent to the olfactory placode. As luteinizing hormone-releasing hormone-immunoreactive neurons migrate, they assume a fusiform shape and the immunoreaction product extends from the area around the nucleus throughout the cytoplasm, notably in processes which extend toward the direction of migration. Before and during migration, luteinizing hormone-releasing hormone was not detected in the Golgi apparatus or neurosecretory granules. It is inferred that as far as ultrastructural evidence is concerned, these neurons do not have a secretory function before they attain their target organs.