Oligodendrocytes are the myelin-forming cells of the central nervous system that facilitate transmission of axonal electrical impulses. Using transgenic mice expressing 2',3' cyclic nucleotide 3' phosphodiesterase (CNPase)-enhanced green fluorescent protein, a three-dimensional reconstruction tool and analysis, we illustrate that three morphologically different oligodendrocyte types exist in the hippocampus. Those of the ramified type have the most numerous processes, the largest cell body, occupy the largest area and form beaded-like structures, due to mitochondria aggregates, along the processes. Stellar-shaped oligodendrocytes have smaller cell bodies and their processes cover a significantly smaller area. Those of the smooth subtype have a small cell body with at most two processes. In addition to these types, a large number of oligodendrocytes were found that faintly express CNPase-enhanced green fluorescent protein. More than 50% of the faint type colocalized with NG2 and 91% with oligodendrocyte transcription factor-2, whereas 94% of NG2-immunoreactive and 45% of oligodendrocyte transcription factor-2-immunoreactive cells were faintly CNPase-enhanced green fluorescent protein positive. Based on the complexity of the overall structure, the three types probably represent stages of a maturation process such that one subtype can morph into another. Thus, the least complex 'smooth' cell would represent the youngest oligodendrocyte that matures into the stellar type and eventually progresses to become the most complex ramified oligodendrocyte. Investigation of the distribution pattern revealed that the highest density of oligodendrocytes was found in the stratum lacunosum-moleculare and the hilar region. The distribution analysis of oligodendrocyte subclasses revealed a tendency for different cell types to segregate in large non-overlapping areas. This observation suggests that morphologically, and possible functionally, different oligodendrocytes are topographically segregated.