A fine structural study of the cells that proliferate in the partially denervated dentate gyrus of the rat.

Tritiated thymidine autoradiography has established that after interrupting the commissural afferents to the dentate gyrus a number of non-neuronal cells proliferate in the molecular layer. In the present study the fine structure of the proliferating cells was analyzed by reembedding the 2-microns thick plastic sections of the dentate gyrus which had been previously coated with a nuclear emulsion and processed for light microscopic autoradiography. The location of the labeled cells was plotted with a camera lucida and a few ultrathin sections were taken from the re-embedded sections. In these the labeled cells were re-identified and photographed in an electron microscope. Most of the identified proliferating cells exhibited the following morphological features: The nuclei were irregularly oval, sometimes with deep indentations and contained dense clumps of chromatin; their diameters ranged between 4.5 and 6.5 microns. The cytoplasm was generally disposed to one side of the nucleus and often extended into a few broad processes. The Golgi apparatus was well developed. Many rosettes of free ribosomes were scattered throughout the cytoplasm, and the rough endoplasmic reticulum usually consisted of a few short cisternae. Small multilamellated bodies were common, but dense inclusion bodies were infrequent. The observations reported in this paper suggest: 1. that the nonneuronal cells which proliferate in a neuropil undergoing a mild denervation are morphologically closely related to microglia; 2. that in young adult animals these cells do not seem to have been previously involved in intense phagocytic activity; and 3. that the proliferating cells are present in the neuropil at the time of the denervation.