The aim of this investigation is threefold: (a) to determine the time of origin of neurons of the rat cranial nerve ganglia; (b) to reexamine the embryonic development of the cranial nerve ganglia in the light ofthese dating results; and (c) to attempt to relate the chronology of these peripheral events to developmental events in those nuclei of the medulla that are intimately associated with the cranial nerve ganglia. Although thymidine-radiography has been used for over 2 decades to investigate the time of origin of neurons, most of these studies dealt with central nervous struc tures. There are relatively few studies available concerning the birth dates of neurons in the peripheral nervous system. In fact, to our knowledge, there is only a single thymidine-radiographic report available dealing with the time of origin of neurons of a cranial nerve ganglion in a mammal; this is the recent study by Forbes and Welt (1981) of neurogenesis in the trigeminal ganglion of the rat. In the present study we determined the birth dates of neurons of the trigeminal, facial, vestibular, "glosso pharyngeal, and vagal ganglia of the rat. We utilized the progressively delayed com prehensive labeling procedure, a method which, in contrast to the single-pulse labeling procedure, allows the exact quantification of the proportion of neurons formed on a particular day.
Table of Contents1 Introduction.- 2 Materials and Methods.- 2.1 Whole-Body Thymidine Radiograms.- 2.2 Paraffin- and Methacrylate-Embedded Embryos.- 3 Development of the Trigeminal Ganglion in Relation to the Trigeminal Nuclei.- 3.1 Time of Origin of Trigeminal Ganglion Cells.- 3.2 Fate of the Two Components of the Trigeminal Anlage.- 3.3 Development of the Trigeminal Nuclei of the Upper Medulla.- 4 Development of the Facial Ganglion in Relation to the Facial Motor Nucleus.- 4.1 Time of Origin of Facial Ganglion Cells.- 4.2 Embryonic Development of the Facial Ganglion.- 4.3 Development of the Facial Motor Nucleus and Nerve.- 5 Development of the Vestibular Ganglion in Relation to the Vestibular Nuclei.- 5.1 Time of Origin of Vestibular Ganglion Cells.- 5.2 Embryonic Development of the Vestibular Ganglion.- 5.3 Embryonic Development of the Vestibular Nuclei.- 6 Development of the Glossopharyngeal and Vagal Ganglia in Relation to the Solitary and Ambiguus Nuclei.- 6.1 Time of Origin of Glossopharyngeal and Vagal Ganglion Cells.- 6.2 Embryonic Development of the Glosso pharyngeal and Vagal Ganglia.- 6.3 Development of some Medullary Nuclei Associated with the Glossopharyngeal and Vagal Nerves.- 7 An Aspect of the Development of the Hypoglossal Nucleus and Nerve.- 8 General Discussion.- 8.1 Sequence of Production of Motor Neurons, Ganglion Cells, and Sensory Neurons of the Cranial Nerves.- 8.2 Placodal Origin of Ganglion Cells.- 8.3 Nerve-Specific Boundary Caps and Their Possible Role in the Guidance of Afferents and Efferents.- 8.4 Sites of Neuron Production, Routes of Migration, and the Relation Between Cytogenetic Zones and Neuroepithelial Zones in the Medulla.- 9 Summary.- Abbreviations.- References.