Candidate: Cynthia L. Fuller

Degree of: Doctor of Philosophy

Department: Biological Sciences

Title: An Examination of Neurons in the Olfactory Bulb of Adult Zebrafish: Morphology, Distribution, Cellular Interactions, and Structural Stability Following Deafferenation

Committee: Dr. Christine A. Byrd, Chair
Dr. John Jellies
Dr. John Spitsbergen
Dr. Peter Hitchcock

Abstract: The zebrafish is becoming an increasingly popular model for studies involving olfactory function, yet there is still much to be learned about the anatomy and circuitry of different cell types in the olfactory bulb. This study focuses on identifying the morphology and distribution of output neurons and interneurons in the olfactory bulb of adult zebrafish, Danio rerio. Furthermore, this investigation examines the cellular interactions of the primary output neuron, the mitral cell, and addresses the issue of neuronal plasticity by considering the structural stability of this cell type following loss of afferent innervation.

Using retrograde tract tracing with various dextrans and live tissue culture, we were able to label several types of output neurons in the olfactory bulb including mitral cells, ruffed cells, and ganglion cells of the terminal nerve. Mitral cells were the most numerous output neurons in the olfactory bulb. These cells, located primarily in the glomerular layer and superficial internal cell layer, had variable-shaped somata that ranged in size from 5-20µm in diameter and 22-156µm 2 in surface area. These cells typically had a single dendritic tuft, although some cells possessed multiple primary dendrites. Even mitral cells with multiple dendrites appeared to contact a single glomerulus, a finding that suggests olfactory coding in these teleosts may be more similar to mammals than previously suggested. This information provides further background for olfactory coding and processing studies in this key model system.

Another focus of this study was to examine the structural integrity of mitral cells following denervation of the olfactory nerve. A comparison of dendritic complexity at 3 to 8 weeks post-deafferentation showed that mitral cell morphology was affected by loss of afferent input. This investigation demonstrated that innervation was required in order to maintain dendritic density, but did not appear to alter significantly major dendritic processes even 2 months following ablation. This finding suggests that mitral cells are extremely plastic structures that may be capable of reforming synaptic contacts if afferent targets are re-established following injury.

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