Neurogenesis in the epithalamus, dorsal thalamus and ventral thalamus of the rat: An autoradiographic and cytological study

Times of final mitotic division for neurons of the epithalamic, dorsasl thalamic and subthalamic nuclei of the rat were determined with the aid of thymidine‐H³ autoradiography. Intensely labelled neurons were observed in the brains of animals injected with radiochemical from days 13 to 19 of gestation. The pattern of distribution of the labelled neurons indicated that neurogenesis in these regions followed caudorostral, lateromedial and ventrodorsal neurogenetic gradients, all of which were found to operate simultaneously. Since neurogenesis in the epithalamus, subthalamus and caudolateral thalamic regions began on days 13 and 14 of gestation, the ventrodorsal and lateromedial proliferative gradients were clearly discerned only within the ventral and dorsal thalamus exclusive of the epithalamus. These directional neurogenetic gradients were apparent throughout the entire thalamus and within individual thalamic nuclei. No neurogenetic pattern based upon neuronal size was observed, i.e., large neurons were not preferentially formed earlier than smaller ones. Detailed information has also been provided on the cytological character of each thalamic nucleus.