James W. Hinds

Three-month-old rats were injected intraperitoneally with [3H]thymidine (4.3 microcuries per gram of body weight) and allowed to survive for 30 days. Radioautography of 1-micrometer sections revealed labeled cells in the granular layers of dentate gyrus and olfactory bulb; these were confirmed as neurons by electron microscopy of reembedded 1-micrometer sections.

Abstract Time of origin (final cell division) of neurons and neuroglia of the mouse olfactory and accessory olfactory formations was determined by autoradiography. Animals were injected with thymidine‐H 3 at various developmental stages and killed at or near maturity. In the olfactory formation mitral cells (the largest neurons) arise first, mainly over the three day period from the eleventh day of gestation (E11) to E13, tufted cells chiefly from E13 to E18, and granule cells (the smallest neurons) mainly from E18 to postnatal day 20. Most of the smaller and more superficial peripheral tufted cells arise later than the deeper and larger middle and internal tufted cells. All three types of granule cells have a time of origin extending well into postnatal life, with internal granule cells arising over a longer and later period than periglomerular cells or granule cells of the mitral cell layer. Neuroglial precursors undergo final cell division chiefly between E17 and P10. In the phylogenetically less evolved accessory olfactory formation, mitral cells originate earlier than their homologues in the olfactory formation; mitral cells principally from E10 to E12 and granule cells chiefly from E12 to E18. The results support the concept that some germinal layers of the central nervous system are programmed to produce a succession of cell types, larger cells before smaller ones.

Abstract Cell proliferation and migration in the developing mouse olfactory bulb was studied by autoradiography. Animals were injected with thymidine‐H 3 at various developmental stages and killed one to 96 hours later. All neurons arise in the germinal zone surrounding the ventricle. Until the fourteenth day of gestation (E14) this zone consists only of a matrix (primitive ependymal) layer. From E14 to E17 both matrix and subependymal layers are found, and from E18 to maturity only the subependymal layer is present. The matrix layer produces the mitral cells and some tufted and granule cells; the subependymal layer gives origin to some tufted cells and most of the granule cells. Mitral cell neuroblasts migrate peripherally to reach the primitive mitral cell layer three days after final DNA synthesis. Tufted cell neuroblasts migrate through the developing mitral cell layer to reach their definitive locations; external granule cell neuroblasts migrate past both developing mitral and tufted cells. Neuroglia arise from scattered proliferating glioblasts, originally derived from the periventricular germinal zone. Time of origin and rate of migration of olfactory bulb neurons was checked against the histological development of the bulb. A good agreement corroborated the autoradiographic method used in this study and in previous studies on time of origin of neurons in the central nervous system.

Abstract The techniques of reconstructions of cells from serial thin sections and autoradiography after tritiated thymidine injections have been employed to study the early histogenesis of the cerebral cortex in the embryonic day‐15 (E15) mouse. The autoradiographic studies show that cells below the E15 cortical plate in the intermediate layer are destined to migrate through the preexisting cortical plate cells to take up a more superficial position. Having this information, it has been possible, through reconstructions of large numbers of cells (more than 150) throughout the thickness of the cerebral vesicle, to identify some of the important morphogenetic events of cortical histogenesis. The following scheme is proposed. The first step in neuronal differentiation involves the detachment of the ventricularly directed process of the ventricular cell from the junctional region next to the ventricle. In thin sections, these junctions have the appearance of zonulae adherentes, but freeze cleavage experiments performed in this study show that, in addition, some of them resemble small gap junctions while others appear to be remnants of tight junctions or possibly linear gap junctions. Detachment of the ventricular process accompanies the migration of the nucleus and perikaryon through the ventricular layer. Within the intermediate layer the migrating cells become rounded and sprout numerous processes. Some cells may undergo a mitotic division at this stage. Eventually the differentiating cells sprout a longer lateral process which is oriented tangentially to the pial surface. This process originates from the anterior surface of the soma and at its tip has the characteristics of an axonal growth cone. The cells migrate externally and radially with simultaneous elongation of the primitive axon. In the subcortical plate region of the intermediate layer all cells contain an anteriorly directed axon. Subsequently the cells sprout an apical process which extends into the cortical plate, and the nucleus and perikaryon apparently migrate radially within this process. The result is that the primitive axon first descends into the intermediate layer proper before turning to run tangentially. Dendritic growth and further differentiation begins once the cells reach their definitive position in the cortical plate. One interesting finding is the presence of eight cells in the cortical plate without long anteriorly directed axons. Yet, autoradiographic data show that subcortical plate cells are the immediate precursors of cortical plate cells, and all 28/s28 reconstructed subcortical plate cells have long anteriorly directed axons. Thus, it is possible that the long axon of some cells may be lost as the cells continue to differentiate in the cortical plate. In fact, one cell has been found which appears to be in the process of losing its anteriorly directed axon. A number of molecular layer cells have also been reconstructed. These cells have several processes oriented tangentially to the pial surface. The identity of these processes could not always be determined. Occasional asymmetric synapses have been found between unidentified axons and the horizontal cell soma or its processes. Autoradiographic studies show that horizontal cells have the earliest time of origin of any cortical cell type.