Neurogenesis in the brain stem of the rabbit: an autoradiographic study

With the aid of (H3)-thymidine autoradiography, neurogenesis was documented in the nuclear groups of the medulla oblongata, pons, and mid-brain, as well as in the brain stem reticular formation of the rabbit. Following single injections of (H3)-thymidine, counts were taken of intensely labeled neurons within the nuclei of the functional columns related to the cranial nerves, nuclei of several other functional classifications, and nuclei that did not fit into a functional category. In the brain stem as a whole, neurogenesis was found to occur between days 10.0 and 18.5 of gestation: however, the majority of nuclei studied contained intensely neurons only between days 12.0 and 15.0. Only in the pontine nucleus and the tectum were intensely labeled cells observed as late as day 18.5. Directional gradients of histogenesis were often observed within, as well as between, various nuclei. Within the nuclear columns related to the cranial nerves, a clear mediolateral spread of neurogenesis was observable such that nuclei of the motor columns reached a peak in neurogenesis before those in the sensory columns. Likewise, a mediolateral proliferation pattern was seen in the brain stem reticular formation. Other individual directional gradients were discernible; however, in the brain stem as a whole, distinct overall gradients were not observable. In many individual nuclei, gradients in neuron size were observed such that large neurons preferentially arose prior to smaller neurons. Information pertaining to gradients in neurogenesis, as well as to relationships among functionally related nuclei, are discussed.