Dendritic spines are small protrusions that extend from the dendrites of neurons. Most excitatory synapses in mammals are made on dendritic spines. As the dendritic spines hold the key to neuronal circuitry, their structure, composition, function, development, and plasticity are of great interest. In the first part of this dissertation, I used cultured rat hippocampal neurons as a model system to study the distribution of tubulins and microtubules in dendritic spines. Our results indicate the presence of tubulin dimers, but not microtubules in dendritic spines. The presence of tubulin dimers in dendritic spines does not seem to be due to passive diffusion, but requires a dynamic microfilament cytoskeleton. Furthermore, the localization of tubulin dimers in dendritic spines is sensitive to cold treatment, and calcium channels appear to participate in the process that leads to the cold-induced disappearance or retraction of tubulin dimers from dendritic spines. In the second part of this dissertation, I used fluorescence labeling techniques to study the dendritic spines on the processes of cultured rat cortical neurons at different stages during their in vitro development. I report the change in the densities of protrusions on processes, the change in the shapes thereof, and the change in the proportion of protrusions associated with the accumulation of a presynaptic marker, synaptophysin, in cultured cortical neurons during their in vitro development. Furthermore, large numbers of dendritic spine-like protrusions that were not associated with synapses or other processes were observed. The implications of these latter observations for the mechanisms that underlie the formation of dendritic spines in cortical neurons were also discussed.

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