| Summary: | Embryonic stem cells are pluripotent cells with the potential to differentiate into any cell type in the presence of appropriate stimulatory factors and environmental cues. Their broad developmental potential has led to the proposal that in the future, the use of human embryonic stem cells or their differentiated progeny may be beneficial in regenerative medicine. In particular, a current goal in the field of clinical neurology is to use stem cells in cell-based therapies for motor neuron disease (MND) or amyotrophic lateral ~clerosis. MND is a progressive neurodegenerative disease that specifically affects upper and lower motor neurons and leads ultimately to death from respiratory failure. Stem cellderived motor neurons could conceivably be used to replace the degenerated cells, to provide authentic substrates for drug development and screening and for furthering our understanding of disease mechanisms. However, to reliably and accurately culture motor neurons, the complex pathways by which differentiation occurs in vivo must be understood and reiterated in vitro to direct embryonic stem cells towards motor neurons. This thesis presents the work I have performed on the directed differentiation of embryonic stem cells towards motor neuron fates. I describe the various experimental approaches I took in attempts to produce motor neurons in vitro. My studies reveal that it is possible to deploy the signals used during normal development to direct the differentiation of both human and mouse embryonic stem cells into neural and neuronal cells, including motor neurons. Two major limitations precluded my analysis of pure motor neuron cultures: first, the high concentrations of the ventralising morphogen, SHH, apparently required to direct embryonic stem cells towards motor neuron fates, and second, the difficulties encountered in culturing purified motor neurons. However, using a mixed culture, I obtained evidence that motor neurons and their progenitors fail to survive in medium conditioned by mutant SOD1-G93A astrocytes.
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