Ultrastructural features of the isolated suprasylvian gyrus in the cat
Authors: Gruner Jean E. , Hirsch J. C. , Sotelo Constantino 
Published: May 2005

An ultrastructural study has been made of the suprasylvian gyrus of the cat after its complete surgical isolation. In acute experiments (36 to 48 hours after isolation) the distribution and type of degenerative changes in axons and axon terminals belonging to extra‐gyral fibers have been studied. The number of dark degenerating boutons nerver reaches 15% of the synaptic population. Such boutons are scattered throughout all cortical layers, but they are more numerous in the superficial than in the deeper layers. The majority of the dark degenerating boutons establish synaptic contact on dendritic spines, less frequently they synapse on dendritic shafts, and only rarely on neuronal perikarya. In some instances the degeneratin presynaptic boutons and their postsynaptic spines seem to follow a simultaneous degenerative process, as they are apparently entrapped in the same astrocytic cytoplasm. Nevertheless, free postsynaptic spines, bearing normal‐looking postsynaptic densities are occasionally observed in the neuropil of the acutely isolated cortex, as well as in subacute (13 days after isolation) and chronic (30 to 46 days after isolation) animals.

In the chronically isolated gyrus, except for the intense, diffuse gliosis, the presence of numerous dark, degenerating, small myelinated fibers and the persistence of some degenerating boutons, the nervous parenchyma seems, from a qualitative assessment, to have a normal appearance.

A quantitative coparison of the various kinds of synapses in the unoperated suprasylvian gyrus and in the chronically isolated gyrus, has been carried out, since this is the only way to study the final result of deafferentation on the synaptic arrangement in the isolated cortex. The number of axon terminals is decreased by 20 to 30% in the chronic preparation. This diminution of synapses is more marked in the superficial than in the deeper layers: thus, the index of synapses in superficial layers is one synapse/10/μm2 in control animals, and decreases to one synapse/17/μm2 in chronic animals. However, in the deeper layers it is one synapse/13/μm2 in control animals and only decreases to one synapse/16/μm2 in chronic animals. The large majority of the lost boutons formerly synapsed on dendritic spines.

In addition to the loss of synapses and to the intense gliosis, other morphological alterations were observed: reduction in the size of all cortical neuronal perikarya, modification of some vascular beds with dilatation of the Virchow's spaces, and spread of the basal membranes for long distances from the capillaries.

A number of pathological features are encountered in the chronically isolated suprasylvian gyrus: (1) loss of afferent fibers, (2) loss of dendritic spines, (3) neuronal atrophy, (4) intense marginal and interstitial gliosis, and (5) alterations of some perivascular spaces; the last feature may give a morphological explanation for the particular electrophysiology observed in this type of cortex; since it may give rise to alterations in the permeability of the vascular barrier, resulting in chemical modifications of the interstitial milieu which may be the cause of the altered neuronal activity.