106 



THE NERVE TISSUE 



NEUROGLIA. 



Although the peripheral nerves are supported by connective-tissue 

 frameworks, the nerve tissue of the brain and spinal cord is supported 

 hy the special tissue called neuroglia. Additional, less conspicuous 

 mesodermal elements, called microglia, enter into the nerve tissue 

 of these regions with the blood vessels. Arising from the primitive 

 cells of the neural tube, several t^,iDes of neuroglia cells are differ- 

 entiated. 



Ependyma Cells.— The columnar epithelial-like cells lining the 

 embryonic neural canal de^'elop long processes extending across the 

 wall of the developing tube. In the course of development these pro- 

 cesses are lost, so that in the mature animal there remain columnar 

 cells with tapering ends projecting into the 

 tissue of the cord and brain. Cilia are 

 usually present on the free surface of these 

 cells. In certain regions of the brain there 

 are vascular invaginations, called choroid 

 plexes, where the epend>-mal cells lose their 

 cilia, become cuboidal, and act as a secretory 

 epithelium. 



Astrocytes.— The astrocytes are stellate 

 cells with processes attached to blood ves- 

 sels, and are the largest type of neuroglia 

 element. The nucleus is large and oval, with 

 scant scattered chromatin and no nucleolus. 

 Two types are usually recognized, fibrous 

 astrocytes and protoplasmic astrocytes. 

 Fibrous astrocytes, also called spider cells, are characterized by 

 long, usually unbranched processes containing fibrous elements. 

 They occur abundantly among the myelinated fibers of the white 

 matter of spinal cord and brain. (Fig. (U.) 



The protoplasmic astrocytes, which occur in gray matter, have a 

 stellate cell body with granular cytoplasm and many short, stubby, 

 branching processes. The abundance of the processes, as shown in 

 Golgi ])reparations, has led to calling them mossy cells. 



Microgliocytes.— These elements appear as small cells in their 

 resting state and have fine-branched processes, but are capable of 

 becoming ameboid and phagocytic. The nuclei of these cells are 

 the smallest and stain more deeply than those of other cells found 

 in the spinal cord and brain. 



Fig. 61. — Types of neu- 

 roglia cells. A, protoplas- 

 mic astrocyte; B, fibrous 

 astrocyte. 



