392 THE PINEAL ORGAN 



The nuclei in the early stages of development are easily visible in 

 specimens stained with the ordinary nuclear dyes. They form two or 

 three layers beneath the pia and resemble the nuclei of lymphocytes and 

 proliferating endothelium, and in cases of injury or disease may be readily 

 mistaken for these. 



The microglia cells having attained their full development become 

 " fixed " and are described as being in the resting condition. They are 

 found throughout the nervous system in the grey and white matter, being, 

 however, more abundant in the grey matter than in the white. It is 

 probable that the normal number of cells is maintained during adult 

 life by amitotic division of the nuclei, although mitotic figures have 

 occasionally been seen (Del Rio-Hortega). Having reached their full 

 development and entered the resting phase, in a fixed position, they may 

 retrace their stages of development in reverse order ; the processes 

 gradually thickening and becoming shorter, until they assume the form 

 of pseudopodia, and the body becomes rounded. This process of devolu- 

 tion is seen in cases of injury to the brain, when the cells having assumed 

 amoeboid characters, migrate towards the focus of inflammation, hsemor- 

 rhage, or degeneration and there act as phagocytes, engulfing leucocytes, 

 erythrocytes, and broken-down nerve tissue. Their function is, there- 

 fore, similar to that of leucocytes and the cell-elements of the reticulo- 

 endothelium, and before the ordinary methods of staining were supplanted 

 by the special methods of staining with silver carbonate, the appearances 

 were interpreted as those of inflammation as it occurs elsewhere in the 

 body generally and attended by the accumulation of leucocytes and pro- 

 liferation of endothelium. 



Relations of Microglia to Nerve-cells and Vessels 



Microglial cells, like oligodendrocytes, are found as satellite cells of 

 large neurones, such as the Purkinje cells of the cerebellar cortex. They 

 may be associated either with the body of the cells or with its processes. 

 They may be distinguished from oligodendrocytes by their small, deeply 

 stained, and irregularly shaped nuclei and by the spines on their processes. 

 Microglia cells are also found in relation with the adventitia of the vessels 

 lying in the white or grey matter. 



As might be expected, microglial cells are present in the retina and 

 in the optic nerve, and they have been found in the fibrous plaques of the 

 human pineal organ. 



