Dendritic reorganization in the denervated dentate gyrus of the rat following entorhinal cortical lesions: A Golgi and electron microscopic analysis

Abstract

The organization of the dendritic tree and the morphology of individual dendrites of the dentate granule cell were analyzed qualitatively and quantitatively by means of the rapid Golgi technique 2, 4, 8, 10, 14, 30, 60, and 250 days after unilateral lesions of the entorhinal cortex (EC). Three dendritic field parameters were analyzed from camera lucida drawings of de‐nervated granule cells at each survival time: (1) dendritic field spread, (2) dendritic length, and (3) dendritic branching. Cells in the contralateral dentate gyrus served as controls. Spines were counted at each postlesion interval from material stained with a modification of the Golgi‐Kopsch method. The amount of tissue occupied by dendritic shafts at different postlesion intervals was also evaluated in samples of the ventral leaf of the dentate gyrus prepared for electron microscopy. After unilateral lesions of the EC, dendrites of the granule cells undergo modifications which appear to represent deterioration and recovery. When the dendrites reach the denervated zone, they abruptly change their orientation and tend to follow a course parallel to the granule cell layer. In contrast to normal dendrites, those in the denervated neuropil only occasionally reach the outer boundaries of the molecular layer. At the time of maximal denervation there is often a sudden reduction in dendritic diameter as the dendrite enters the denervated zone. Varicosities are also prominent. The alterations in individual dendrites are not evident 2 days after the lesion, are fully developed 10 days after deafferentation; and disappear for the most part by 30 days postlesion. The quantitative analysis of Golgi‐stained granule cells reveals that there is a 40% reduction in the total length of the granule cell dendritic tree. Electron microscopic analysis confirms the Golgi observations, indicating that the amount of neuropil occupied by dendritic shafts in the denervated zone is initially reduced and later recovers to values close to those observed in control animals. While the apparent dendritic loss is mainly restricted to the denervated zone, significant modifications occur in the inner npnde‐nervated molecular layer; there is an increase in the length of primary dendrites as evidenced by an increase in the distance to the first branch point. There is also a polarized redistribution of 1st, 2nd, 3rd, and 4th branch points toward the middle and outer molecular layer that persists even 30 days after the lesion. The results are discussed in terms of the capabilities of the granule cells of the dentate gyrus to reorganize and remodel their dendritic surface after partial deafferentation.