Neurogenesis in the vomeronasal epithelium of adult garter snakes: 3. Use of H3‐thymidine autoradiography to trace the genesis and migration of bipolar neurons

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Abstract
Use of H3‐thymidine autoradiography and unilateral vomeronasal (VN) axotomy has permitted us to demonstrate directly the existence of VN stem cells in the adult garter snake and to trace continuous bipolar neuron development and migration in the normal VN and deafferentated VN epithelium in the same animal.The vomeronasal epithelium and olfactory epithelium of adult garter snakes are both capable of incorporating H3‐thymidine. In the sensory epithelium of the vomeronasal organ, H3‐thymidine‐labeled cells were initially restricted to the base of the undifferentiated cell layer in animals surviving 1 day following H3‐thymidine injection. With increasing survival time, labeled cells progressively migrated vertically within the receptor cell column toward the apex of the bipolar neuron layer. In both the normal and denervated VN epithelium, labeled cells were observed through the 56 days of postoperative survival. In the normal epithelium, labeled cells were always located within the matrix of the intact receptor cell columns. However, labeled cells of the denervated epithelium were always located at the apical front of the newly formed cell mass following depletion of the original neuronal cell population. In addition, at postoperative days 28 and 56, labeled cells of the denervated VN epithelium achieved neuronal differentiation and maturation by migrating much farther away from the base of the receptor cell column than the labeled cells on the normal, unoperated contralateral side tralateral side.This study directly demonstrates that basal cells initially incorporating H3‐thymidine are indeed stem cells of the VN epithelium in adult garter snakes. These stem cells differentiate and develop into neurons in the normal VN epithelium for normal neuronal turnover. Stem cells, which incorporated H3‐thymidine prior to unilateral VN axotomy, after axotomy differentiate and develop into neurons in the regenerating VN epithelium for replacement of the axotomized neurons. The life span of the labeled VN neurons may exceed 56 days. Postnatal neurogenesis during regenerative repair of the VN organ, however, is accompanied by rapid undifferentiated cell proliferation following axotomy, rapid cell migration, and a tendency for rapid cell turnover.