Electron microscopy of thin-sectioned and freeze-fractured preparations of the cerebellum of the weaver mouse indicates that the dendritic spines are morphologically identical to those of their normal littermates. The weaver dendritic spines have been characterized as "unattached" since the synaptic input from the parallel fibers is absent (8-10). The entire region around the dendritic spines is taken up by astrocytic processes in the weaver. The outer fracture face of a normal dendritic spine contains aggregations of 10-nm wide particles in the immediate postsynaptic region. Similar particle aggregations occur in the unattached spines of the weaver. Freeze-fracture preparations reveal rectilinear arrays of particles, having a 7-nm center-to-center distance in the glial membranes. Rectilinear arrays are apparently distributed throughout the astrocyte membrane.

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