Shirley A Bayer - US grants

Correlated normative and experimental studies are planned to test the hypothesis that the temporal order of neurogenesis is the initial contributing factor in the establishment of neural connections within the olfactory and limbic systems. Project I is to complete normative dating of the time of origin of neurons in the olfactory system using the new, improved progressively delayed comprehensive labeling procedure of 3H-thymidine radiography which allows one to calculate the proportion of neurons arising over single days during development. Morphogenetic studies will be done in conjunction with project I, including a volumetric analysis of morphogenesis in the olfactory bulb, a quantitative analysis of neuroepithelial zones in the basal telencephalon, and a study to follow the migration of olfactory bulb output neurons from their neuroepithelial sources to their final locations. Project II is to examine experimentally the correlations between time of origin of neurons and their patterns of anatomical connections. A) The development of the entorhinal projection to the dentate molecular layer will be traced using iontophoretic injections of conjugated wheat-germ agglutinin and horseradish peroxidase (WGA-HRP) during late embryonic and early postnatal life. B) 3H-thymidine radiography and WGA-HRP iontophoretic injections will be combined to determine the correlation between time of mitral cell origin and their sites of axonal termination in the cortical nucleus of the amygdala. This research will have importance for two reasons. On the one hand, it will allow the testing of a viable hypothesis about one of the mechanisms involved in the regulation of embryonic development of the nervous system. On the other hand, it will fill a gap in our knowledge about the development of the olfactory and limbic systems.

Concurrent normative and experimental studies are planned to test the hypothesis that the temporal order of neurogenesis is the initial contributing factor in the establishment of neocortical neural connections. Project #1 is a series of double-labeling studies (HRP histochemistry and 3H-thymidine autoradiography) examining the correlations between the time of origin of neurons projecting to and from the neocortex and the position of their axon terminals. Experiment 1 will focus on the projection from the magnocellular neurons in the basal telencephalon to the neocortex. Experiment 2 will examine the projections from thalamic sensory relay nuclei (medial geniculate, lateral geniculate, ventrobasal nuclei) to subdivisions of their respective sensory cortices (auditory, visual and somesthetic). Experiment 3 will correlate time of origin of pyramidal cells in the motor cortex with their axon terminal locations in the spinal cord. Preliminary studies in the olfactory system and in the spinal cord indicate that older projection neurons have different targets than younger ones. More generally, these experiments are expected to demonstrate that the time of origin of a neuron specifies where its axon will project in the adult nervous system. Project #2 is a comprehensive quantitative analysis of neuron origin in specific areas of the neocortex using cumulative 3H-thymidine autoradiography, which allows the exact determination of the proportion of neurons originating on single embyronic days. Layers II, III, IV, V, VIa and VIb will be quantified separately in the cingulate, retrosplenial, insular, visual, auditory, somatosensory, and motor areas. This project will complete our effort to provide precise timetables of neurogenesis in the entire rat central nervous system.