What is the definition of Glia?

ito at mzdmza.zdv.uni-mainz.de ito at mzdmza.zdv.uni-mainz.de
Wed Oct 27 10:22:09 EST 1993


I am writing a paper about the distribution of glia in the embryonic
Drosophila central nervous system (CNS). I have a problem concerning the
definition of glial cells. I would greatly appreciate it, if people in this
newsgroup could tell me your opinion.

The insect CNS contains several "genuine" glial cells that come from the
same ectodermal region as neurons (so called "neurogenic ectoderm). On the
other hand, the outermost cell layer of the CNS comes from the mesoderm;
the cells are somehow incorporated to the CNS structure during the
development. The tracheol, fine tracheal capillary, also make a network in
the CNS to provide oxygen to the neurons. 

There are several ways of glia definition:

DEVELOPMENTAL DEFINITION
In this case, glia is defined as the non-neuronal cells in the CNS that
derive from the same region of the ectoderm as neurons. In case of
vertebrates, the glia should derive from the envaginating nerve cord. In
insects, they should derive from the neurogenic ectoderm region of the
blastoderm, from where several neural as well as glial precursor cells
segregate. 

FUNCTIONAL DEFINITION
Other people use the wider definition in favor of the brevity. They define
glia as all the non-neuronal cells in the CNS: all the cells in the CNS
that support neurons, regardless of their origin. 

HYSTOCHEMICAL DEFINITION
In this case, glia is defined as a kind of cells which express certain
glia-specific protein. Some vertebrate people say that glia should be GFAP
positive. GFAP is "glia fibrillary acidic protein" that makes the
glia-specific intermediate filaments. You can stain glia with anti-GFAP
antibody.


In case of vertebrate CNS, oligodendrosites and astrocites are termed
"neuroglia". I think microglia come from mesoderm (am I right?). The
microglia is not called "neuroglia" but it is still in the "glia" cell
category. There is yet another kind of glia (ependymal cells) but I have no
idea about its origin.

      Vertebrate CNS Glia
               neuroglia (glia which share the common origin with neurons)
                        astrocite (including radial glia in chick embryos)
                        oligodendrocite
               microglia (from mesoderm)
               ependymal cells (from   ???)

Is the above classification correct?

I was told that some people insist on excluding microglia from glia
category.  Is it true? What about the ependymal cells?  

Is GFAP exist only in the astrocite and oligodentrocite? Or does also
microglia or ependymal cells contain GFAP?


I am trying to make a comprehensive glia catalog of the Drosophila
embryonic CNS. So I have to decide which definition to use. 

Actually it is difficult to know the origin of certain cell type before we
name them. For example, although the outermost sheath cell layer
("perineurial cells" or "sheath cells") of the insect CNS was shown to be
of mesodermal origin, there is no guarantee that all the inner non-neuronal
cells are of ectodermal origin. So it is not easy to consistently perform
the Developmental Definition. As there is no known-antibody against insect
GFAP or other glia-specific glia-ubiquitous molecule, Histochemical
Definition is not available for my purpose. Therefore I am inclined to use
the Functional Definition, which will include the most various cells in the
CNS. 

Is my approach adequate? Any suggestion, correction, and criticism are 
most welcome.



------TU NE CEDE MALIS, SED CONTRA AUDENTIOR ITO.------

  Dr. ITO, Kei
  Institute of Genetics, University of Mainz
  Saarstr. 21, D-55122 Mainz, GERMANY
  tel   :  (49) 6131 - 39 32 93
  fax   :  (49) 6131 - 39 58 45
  email : ito at mzdmza.zdv.uni-mainz.de
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