What is the difference between a MeSH concept and a MeSH descriptor Medical Subject Headings (MeSH) is a very comprehensive vocabulary to index the biomedical literature on Medline or PubMed (which is the major Biomedical literature database). MeSH is arranged into Descriptor > Concept > term hierarchy, but I still do not understand the difference between a MeSH descriptor and a MeSH concept.

For example, for Knee


MeSH descriptor is D007717 
MeSH concept is M0012066 (Under the concepts section in the link)


Even after reading the description here and here, I am still not sure about the purpose of MeSH concept and MeSH descriptor. 

Could anyone please shed some light on it?
MeSH is organized into Descriptor &gt; Concept &gt; Term hierarchy, where all the synonymous &lt;term&gt;'s are grouped together under a &lt;concept&gt;. For example, consider the MeSH terms,


Blood-Brain Barrier
Hemato-Encephalic Barrier
Brain-Blood Barrier


All these terms are group under the umbrella concept of "Blood-brain barrier".