Tri-dimensional morphometric analysis of astrocytic processes with high voltage electron microscopy of thick Golgi preparations.

A characteristic feature of the astrocytic processes is to assume the form of shin sheets or lamellate coverings of other brain constituents. We analyzed the extensive and finely divided processes of the protoplasmic astrocyte in the molecular layer of the rat dentate gyrus by means of computer electron tomography and stereo-photogrammetry using tilted high voltage electron microscope images of thick Golgi preparations. The surface area and volume of the astrocytic processes were measured and the surface/volume ratios were estimated. The surface/volume ratios of astrocytic processes in the neuropile ranged from 18.9 to 33.0 per microm, and the mean value was 26.2+/-5.0 per microm. The values were roughly comparable to those previously reported for the microdomain of Bergmann glia cell terminal processes in the rat cerebellum, which were estimated from reconstructions using thin serial section electron microscope images. The large surface to volume ratio of the astrocytic processes in the neuropile resulted from the lamellar nature of the processes interposed between other cellular elements, and may reflect the functional activities of the astrocyte. The results suggest the usefulness of the electron tomography and stereo-photogrammetry for three-dimensional morphometrical analysis of the astrocytic processes, although both techniques can be expected to be refined further in order to provide more precise measurements of these complicated processes.