MACROPHAGES FROM ARACHNOID CELLS. 387 



The excitation of abnormal activities in cells has been variously interpreted 

 and these experiments are subject to the same limitations. The only difference 

 between the same stimulus applied to connective tissue, endothelial walls, or surface 

 of a serous cavity, lies in the peculiarity of anatomical position. On the trabeculae 

 of the slender arachnoid strands the cells are often given but 4 microns to rest upon. 

 There is little surprise, then, that an increase of protoplasm brings about a pendu- 

 lous appearance. Furthermore, the comparative isolation of these enlarging cells 

 lessens the confusion with neighboring structures as well as permitting a clear view 

 of the cellular inclusions. Material from these experiments, when subjected to the 

 ordinary embedding and section technique, affords very disappointing specimens. 



Although the stimulus of injected particulate matter is an excessive one, it 

 points nevertheless to the physiological activities of this cell-membrane forming 

 the walls of the subarachnoid space. Normally, a certain small quantity of debris 

 finds its way into the subarachnoid cavity; this offers an explanation for the finding 

 of a few macrophages as normal inhabitants of the cerebro-spinal fluid. These 

 appear as the large mononuclear cells withdrawn at lumbar puncture. The capacity 

 for developing macrophages occurs in the earliest months of embryonic life and is 

 never lost by the adult pia arachnoid. These findings are comparable with the 

 reactions in other cells of the body, provided the proper stimulus is given. Differen- 

 tiated mesothelium, such as the arachnoid, peritoneum, and pleura, undifferentiated 

 mesothelium forming the supporting connective tissues, vascular endothelium, and 

 finally neuroglia, may transform into amoeboid wandering cells capable of ingesting 

 particulate matter. As a physiological class they may be embraced by the term 

 macrophage, and as such are concerned with the reparative processes in the body. 



SUMMARY. 



(1) Particulate matter within the subarachnoid cavity causes the lining 

 mesothelial cells to round up and bud off from their attachments. 



(2) As free-moving amoeboid elements these cells fulfill in every way the 

 criteria of the class of macrophages, and as such are concerned with the removal of 

 debris. 



(3) Normally the same type of stimulus, though in very greatly reduced force, 

 results in the formation of the few large mononuclear cells occurring in the cerebro- 

 spinal fluid. 



