Three-dimensional structure of astrocytes in the rat dentate gyrus.

Structural features of astrocytes in the rat dentate gyrus were studied by means of light and high-voltage electron microscopy of Golgi-impregnated materials, conventional electron microscopy, combined Golgi-electron microscopy, and immunohistochemistry for glial fibrillar acidic protein. Astrocytes in the dentate gyrus were of the protoplasmic type and were classified into six subtypes based on the location of their somata; i.e., astrocytes in the polymorph layer, in the subgranular zone, in the granular cell layer, at the border of the granular cell layer and the molecular layer, in the molecular layer, and subjacent to the pia surface. Stereoscopic observations of 5-micron-thick sections of Golgi-impregnated materials revealed three-dimensional structural details of astroglial processes not apparent in either the light microscope or in conventional thin-section electron microscopy. Most of them were basically thin sheets, varying in shape and size in accordance with their sites. In the granule cell layer, thin veillike sheets or lamellae, originating from three kinds of astrocytes (subtypes 2, 3, and 4), intervened between granule cell somata, whereas in the plexiform and molecular layers small leafletlike appendages originating from astrocytes (subtypes 1, 2, 4, and 5) intermingled with one another, making spongelike conglomerates. Thus astrocytes in the subgranular zone and those at the border of the granule cell layer and the molecular layer showed prominent regional differentiation of their processes among layers. In addition to these sheetlike processes, thin threadlike processes were also common.