Through the use of serial sectioning of dog cerebral cortex tissue, holes or perforations could be revealed in the larger postsynaptic densities (PSDs), in confirmation of the earlier work of Peters and Kaisermann-Abramof (1969. Z. Zellforsch. Mikrosk. Anat. 100:487-506). These holes appeared in serial sections which happened to be cut both parallel and normal to the plane of the synaptic junction. Cleft material was absent in that part of the synaptic cleft opposite this hole. Somestimes the presynaptic membrane opposite the hole was indented into the presynaptic cell. In addition, most of the synaptic vesicles in the presynaptic cell close to the membrane were clustered at that part of the membrane opposite the edge of the density disk. The meaning of the hole and of the other features mentioned above for the function of the density is not known at present.

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