Glia
\
Non-neuronal cells of the CNS include three types of neuroglia that vary in structure and 
function.  Together the neuroglia are as numerous as the neurons of the brain and spinal cord.  
They are astrocytes, oligodendrocytes and microglia.


A.  Astrocytes.  

1)  Astrocyte morphology.  

1) Fibrous astrocytes are found predominantly among the bundles of axons 
that make up white matter; they possess numerous elongtated processes.

2) Protoplasmic astrocytes are present among collections of neuronal cell 
bodies (gray matter).  Their processes are less numerous and considerbly more 
swollen in appearance than those of fibrous astrocytes.

This difference in morphology appears to have no correlate in the functional or 
molecular features of the two astrocyte classes; the two perform similar 
functions (see below) and express the same proteins.  They look different 
simply because they occupy different parts of the central nervous system.


2) Astrocyte chemistry.  

Because of the need to establish and maintain elaborate shapes, astrocytes contain an 
abundance of intermediate filaments that function just like neurofilaments in neurons.  
Astrocyte intermediate filaments are constructed of the protein Glial Fibrillary Acidic 
Protein (GFAP).  


3) Astrocyte location.  The location of astrocytes and their processes reveals much about their 
functions.

a) The entire external surface of the CNS is covered by terminal expansions of 
astrocytic processes.

b) Astrocytic foot processes also cover most of the capillary surface in the 
CNS.  Once thought to be a major constituent of the blood-brain barrier, 
which serves to keep large circulating macromolecules (e.g. immunoglobulins) 
out of the CNS, these foot-processes are now known to serve other functions 
such as the transport of nutrients from the circulation into the CNS.

The blood brain barrier is a function of the endothelial cells that make up the 
capillary walls, and specifically the tight junctions between them that restricts 
the movement of large molecules.


	4) Function of astrocytes:
		
1) Astrocytes serve the general function of maintaining ionic concentrations in the 
extracellular space of the CNS.  To do so, they are permeable to ions, particularly K+.

2) Astrocytes also function as scavengers that remove debris left by injured and dead 
neurons.


B. Oligodendrocytes.  


Oligodendrocytes are small glial cells with few processes that perform the vital function of 
insulating axons of the CNS with myelin.

1) Oligodendrocyte morphology

Slide 151B is of a section through the spinal cord stained so that the nuclei of 
all cells, neuronal and glial, are apparent.  Examine the white matter of the spinal 
cord and note the presence of two prominent types of nuclei; one is larger and 
more lightly stained (astrocytes) whereas the other is small and very darkly 
stained (oligodendrocytes).

A separate and distinct cell type produces myelin along axons in the peripheral 
nervous system.  This is the Schwann cell.

2) The light microscopic structure of myelin.  

a) In the cross section, myelinated axons appear as dark rings 
surrounding central unstained cores.  The rings are the myelin sheaths 
and the cores are the axons themselves. Note the variation in axon size 
as seen by differences in the diameter of the stained rings. 

b) In longitudinal sections, myelin along a single axon appears as a 
series of long stained segments separated from one another by very 
short, unstained gaps.  The long segments are called myelin 
internodes whereas the gaps are nodes of Ranvier.  The latter are 
regions where a short piece of axon is left unmyelinated, so that ions are 
free to flow across the axonal membrane.

For peripheral nervous system myelin, each internode is formed by a single Schwann 
cell; in the central nervous system each internode is formed by one oligodendrocyte 
process (each oligodendrocyte gives off approximately 50 such processes.  Thus 
Schwann cells are dedicated to the partial myelination of a single axon whereas 
oligodendrocytes myelinate portions of as many as 50 different axons.

D. Microglia

The smallest of the neuroglia, the microglia, serve as the resident macrophages of the CNS.  
There is evidence to suggest that microglia are largely of blood monocyte (white blood cell) 
origin, resulting from the embryonic invasion into CNS of cells arising in bone marrow.  

1) As expected for a cell of monocyte origin, microglia express proteins characteristic 
of blood monocytes and macrophages.  This section was immunocytochemically 
stained with an antibody raised against a monocyte surface protein.  

Microglia also express surface glycoproteins selective for macrophages, 
complement receptors and leucocyte common antigen.  In human CNS 
microglia express class II major histocompatability (MHC) factors at levels that 
increase markedly with age.


2) The morphology of resting microglia is simple: they have small somata that give rise 
to a few highly branched processes.

Microglia become activated by injury or inflammation of the CNS.  Activated 
microglia display rounded and enlarged somata; at the same time they display a 
significant upregulation in MHC type II and other surface proteins that 
participate in the immune response.