Neuronal Regeneration in Frog Olfactory System

There is increasing evidence that olfactory neurons can differentiate from the basal cells of the olfactory neuroepithelium in normal adult vertebrates (Nagahara, 1940; Andres, 1965; Thornhill, 1970; Takagi, 1971 ; Moulton, 1971 ; Graziadei, 1971, 1973 ; Graziadei et al ., 1971 ; Graziadei et al ., 1972). Autoradiography at both the light microscope and the transmission electron microscope levels using [H 3]thymidine has been used in our laboratory and we have positively identified the labeled cells as olfactory neurons (Graziadei et al ., 1971, and Graziadei, 1973) . It has been shown further that cutting of the olfactory nerve is followed by complete degeneration of all mature neurons in the olfactory mucosa (Graziadei, 1973) . This degeneration, which is complete in a period of 2 wk in frogs and follows a different time schedule in other animals, is followed by an outburst of mitoses in the basal cells which eventually differentiate into fully mature neurons (Graziadei, 1973) . The maturation of new neurons from basal cells occurs in frogs in a period of 6-8 wk after severing of the olfactory nerve. However, in mammals the process is more rapid and depends on many parameters such as the species of the animal, age, and environmental conditions (unpublished results) . The present experiments are concerned with the growth of axons from new olfactory neurons and the possibility that these axons cross the surgical gap resulting from the cut of the olfactory nerve and reach the olfactory bulb where they reestablish "normal' synaptic contacts within the glomeruli .