The brains of four adult cats treated with pargyline (a nonhydrazide monoaminoxidase inhibitor) were examined at both the light and electron microscopic levels. Formation of typical mature cilia with the 9 + 2 pattern was observed in neural cells in the following areas: habenula nuclei, interpeduncular nuclei, hippocampus, mammillary bodies, thalamus, and caudate nucleus. The most marked ciliation occurs in the habenula nuclei. In general, glial cells greatly predominate in the formation of cilia. It is not clear whether ciliation in the central nervous system is the direct result of pargyline or if it occurs indirectly as a result of inhibition of monoaminoxidase. These findings are compared with the serotonin effect on ciliation in the embryogenesis of lower forms. It is suggested that pharmacological stimulation of centriolar reproduction without subsequent mitosis may lead to ciliary formation.

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