Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Neurosciences

Language

English (en)

Date of Award

January 2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Rachel Wong

Abstract

The construction of functional neuronal circuits requires proper coordination of many developmental processes. Neurons must be generated in the proper number and migrate to their mature locations within the nervous system. Upon completion and sometimes during neuronal migration, neurons must elaborate complex dendritic and axonal arbors and then form precise synaptic connections in the background of a plethora of possible synaptic partners. Glial cells must also establish specialized associations with a number of neuronal structures including synapses and axons. In order to create a detailed understanding of how a neuron progresses through all of these stages of development, I have studied a single class of neuron, the retinal horizontal cell: HC), from the stage of cell generation to the establishment of its synaptic circuits and associations with astroglia. HCs are a key component of the circuitry of the outer retina of vertebrates and they function to modulate information transfer from photoreceptors. I examined the genesis of HCs using in vivo multiphoton time-lapse microscopy in the zebrafish retina and discovered a novel precursor cell that is dedicated to the generation of HCs. In contrast to the classical germinal cell, the horizontal precursor cell is unattached to the epithelial surfaces, and instead migrates freely and divides near the final laminar location of mature HCs. I then determined what cellular changes take place in order for postmitotic HCs to establish their specific connectivity with rod and cone photoreceptors. In the zebrafish retina, HCs form well-defined circuits with specific subsets of photoreceptors. I determined how HC dendritic contacts were formed with only a specific subset of photoreceptors by performing in vivo time-lapse microscopy experiments in which a single HC was labeled in the background of all of its proper presynaptic partners. During the course of development, HC dendritic tips formed contacts with both proper and improper photoreceptor presynaptic terminals. However, improper contacts were removed while tips contacting the preferred photoreceptors were preferentially maintained. Thus, the final patterns of connections between HCs and photoreceptors are shaped by a process of refinement. After establishing the events that take place during HC - photoreceptor circuit development, I examined whether M├╝ller glia: MG) influence the development of these circuits. I found that synaptic contacts between HCs and photoreceptors are present before MG processes ensheath the nascent synapses. I also demonstrated that contact with MG did not influence the stability of newly formed HC dendritic tips. Furthermore, preventing the interaction of MG processes with photoreceptor pedicles and HC dendritic tips by targeted MG ablation did not cause the photoreceptor contact with HCs to disassemble. Thus, unlike in other parts of the nervous system, glial contact is not necessary to ensure the stability of newly formed synapses. Together, my findings provide an in vivo view of the cellular mechanisms utilized to generate, localize and establish the circuitry of a single class of interneuron in the nervous system. My experiments also reveal a novel mechanism of cell generation and a contrasting view of the role of glial cells in stabilizing newly formed circuits.

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Permanent URL: http://dx.doi.org/10.7936/K7WH2N1J

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