DEVELOPMENT OF MICROGLIA 391 



and when they take up their positions in the tissue they have small dark 

 nuclei, surrounded by scanty cytoplasm prolonged into two or more 

 thin, wavy, branched processes beset with spines which end freely, that 

 is, they are not anastomosed among themselves nor are they connected 

 with the neuroglia elements." He also holds the view that in a broad 

 sense the microglia of the central nervous system represents from the 

 functional standpoint the reticulo-endothelium of mesodermal tissues. 

 It has been known to fix certain colloids to phagocytose erythrocytes and 

 cellular debris, and it is believed to be concerned in the elimination of 

 substances resulting from metabolism and degeneration of nerve cells. 

 Thus, it is found to participate actively in inflammatory and destructive 

 processes involving the central nervous system, and as a result of the 

 motility of the cells and their increase in size under pathological con- 

 ditions they assume various forms, becoming rod-like, lamellar, or rounded 

 in shape, and frequently contain fat granules. 



Development of Microglia 



The microglia, according to the description of Del Rio-Hortega, 

 does not appear until the last period of embryonic life. In foetuses at 

 term and in new-born animals it is abundant both beneath the pia mater 

 and spreading inward along the course of the vessels of the brain and 

 cerebellum. It is developed later than the neuroglia, at the time when 

 the vessels of the pia mater have reached their full development. The 

 cells which give rise to the microglia are believed to be mesodermal in 

 origin and are first visible immediately beneath the pia mater on the sur- 

 face of the brain and spinal cord ; they are found also in relation with 

 the tela choroidea of the third ventricle below the corpus callosum and 

 fornix, and also beneath the pia mater covering the white matter of the 

 cerebral peduncles. It is also abundantly formed in connection with 

 the vascular folds of the tela choroidea inferior and on the surface of the 

 cerebellum. 



Originating as a layer of rounded or flattened cells beneath the pia 

 on the surface of the brain, the microglial cells afterwards develop pseudo- 

 podia and migrate deeply into the substance of the white and grey matter, 

 and eventually they reach the ependyma lining the ventricles or central 

 canal of the spinal cord. 



The cells beneath the pia are at first rounded, cuboidal, or flattened ; 

 they increase in size and develop irregular bulbous processes ; later, 

 when they reach their ultimate destination, they become fixed and den- 

 drites are formed, on which later the characteristic spines are developed. 

 The cells are said by Hortega to lie in the neuroglia, and their processes 

 do not communicate with each other. 



