Identification of early neurons in the brainstem and spinal cord. II. An autoradiographic study in the mouse

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Abstract
Early neurogenesis was studied in two series of mouse embryos with tritiated thymidine (3H‐Tdr) autoradiography to determine (1) the time of origin and (2) the identity of the first postmitotic neurons. Time of origin was studied in embryos cumulatively labeled on the 9th–11th day of gestation (E8, E9, or E10, since the day of finding the vaginal plug is EO) and terminated after one cell cycle (8 hours). Autoradiographs were examined for unlabeled neurons, those which had undergone final DNA synthesis and which were usually also postmitotic. A 16‐somite embryo, cumulatively labeled from the 6‐somite stage, demonstrated 234 such cells in both alar and basal plates from the caudal diencephalon to the cervical spinal cord. Some of these young neurons, therefore, arose prior to neural tube closure, which begins at the 7‐somite stage. Data from other embryos in this series suggests that the first neurons arise at the 1–2 somite stage.Early arising neurons were identified in embryos pulse labeled with 3H‐Tdr during E8–11 and killed after postnatal day (P) 30, when neuronal types could be determined. Autoradiographs were examined for heavily labeled cells, those in final DNA synthesis at the time of treatment. Heavily labeled cells in the youngest mouse in this series (injected at E8½; ca. 8‐somite stage) were primarily small‐to medium‐sized neurons of the brainstem reticular formation, but also some small neurons of the superior olivary nucleus and large primary sensory neurons of the mesencephalic nucleus of the trigeminal nerve. The earliest neurons in the spinal cord appeared early on E9, were medium sized, and were located in the ventral horn and the intermediate zone.Comparisons between the results of this study and those of McConnell and Sechrist ('80) in the chick suggest that the youngest stages of neurogenesis are similar. The first chick nerve cells arise just before formation of the first somites; a slightly later time is indicated in the mouse. Reticular neurons are the earliestarising cells in the chick and make up the majority of the population in each of the youngest chick and mouse stages obtained. Motor and sensory neurons do not begin to originate in large numbers until later, during E3 in the chick and E9 in the mouse.