Dear Alba,

As far as I know the number of astrocytes in the CNS is still not well defined. As you say in some stuides it looks like they only account for a 20% of the total glial cells. Depending on the counting technique and the region studied, the proportions can vary from 20 - 40%.

Best wishes

As you stated, the number is still up for debate. Although this isn't something I'm well-knowledgeable about, I have read a paper that states that NG2-glia (or polydendrocytes) are estimated to be nearly as prevalent as astrocytes (see 1st link), which may help explain why the prevalence of astrocytes might be closer to 20% after also considering the prevalence of ependymal cells, neural stem cells, microglia, radial glial cells, and oligodendrocytes.

While this is pure conjecture, I think it's worth keeping in mind that the role of astrocytes in the CNS--one major one being to maintain the BBB--may explain why you might see them in such an abundance. In the brain alone, there are approximately 400 miles of capillaries (capillaries!) (see 2nd link), the distance of which is largely in contact with astrocyte end-feet. This probably goes without saying, but you should also keep in mind that the abundance of certain glial cells may be age-dependent.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1571571/

http://www.ncbi.nlm.nih.gov/books/NBK53086/

I study the brain cells for several years and I can assure you that astrocytes represent the largest population of glial subtype in CNS

Thinking this article might help. Gave it a brief skim. http://tpx.sagepub.com/content/39/1/22.full

You might also want to consider the conditions under which you ask this question. For example, if  the CNS experiences changes such via infection or injury....changes in cell number/distribution may occur. Hope this helps!

I could tell you that the total number of glia cells is circa 20% of the total cells in the brain and microglia cover the 10%

sonia