An age-related reduction in the number of dendrites and dendritic spines has been reported in normal aging, and this results in an overall decrease of dendritic complexity. The regressive changes occurring at the dendrite tree affect mainly frontal and temporal cortex and the limbic system in the human brain. The first step in the age-related alterations of dendrites is the loss of spines followed by changes in shape and size of basilar dendrites and then of the branches of the apical shaft (Scheibel et al., 1975). Dendrites are receptor membranes of the neurons, and their spines amplify this function and have been reported to isolate increases of synaptic calcium transport utilized for information storage (Koch et al., 1992). As a consequence, the age-related loss of dendrites and dendritic spines isolates neurons and leads to disturbances in cell-to-cell communication. Because of its dynamic condition, the old CNS is capable of a significant compensating response to the age-related loss of neurons and dendritic retraction by increasing the dendritic growth to fill the neuropil space left by the dendritic trees of dead neurons. In parahippocampal pyramidal cells of cognitively normal individuals the continual growth of dendrites occurs well into the eighth decade of life, whereas in pathological brain aging, for example, AD, this compensating reaction has not been found (Buell and Coleman, 1979).
The dendritic changes due to age have been demonstrated by the Golgi method—an old neurohistological procedure that is still in use. In Golgi-impregnated tissue, staining reveals the complete contours of the cells and of their processes. Nerve cells can be seen against the background because only a small fraction (2 to 5%) of the cells are impregnated, but the reasons of this selective positivity are not known. In the original Golgi method, fresh tissue is hardened in a dichromate solution and impregnated in 1% silver nitrate. In the rapid version of this procedure, the tissue pieces are hardened in 4 parts of potassium dichromate (3.5%) and 1 part of osmic acid (1%) for 2 to 7 days, and then they are transferred to 0.75% silver nitrate for 1 to 2 days. In double and triple impregnations, the tissue samples are returned to osmium-dichromate and then impregnated in the same silver nitrate solution. Although this version of the Golgi method is called rapid, all these steps take long periods of time and several laboratory preparations before obtaining a sample to be analyzed: this increases the possibility to damage the tissue and to see unspecific precipitates.
Several modifications of the original Golgi procedure have been introduced with the aim of reducing the extent and frequency of tissue artifacts. In the Golgi-Cox method, tissue hardening and impregnation take place in a simple bath of potassium dichromate and potassium chromate to which sublimate is added. When the tissue blocks or sections are alkalized, the original yellow compound with divalent mercury is transformed into a black substance. The Golgi-Cox version is more rapid than the original procedure, but the impregnation is rather coarse and never occurs in axons. In the Golgi-Kopsch version the animals are perfused with a freshly prepared mixture of potassium dichromate and formalin (or glutaraldehyde); then the hardened brain is sectioned and impregnated in silver nitrate (0.75%). Perfusion, as the first step of the Golgi-Kopsch procedure, limits its use to studies to be conducted on laboratory animals. Large sampling of tissue, impregnated with different modifications of the Golgi method, have enabled the collection of several data converging on the common conclusion from different laboratories that in the aging brain basal arborizations are lost in selected areas of the CNS.
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