MeSH Tree Structures
MeSH descriptors are organized in 16 categories: category A for anatomic terms, category B for organisms, C for diseases, D for drugs and chemicals, etc. Each category is further divided into subcategories. Within each subcategory, descriptors are arrayed hierarchically from most general to most specific in up to thirteen hierarchical levels. Because of the branching structure of the hierarchies, these lists are sometimes referred to as "trees". Each MeSH descriptor appears in at least one place in the trees, and may appear in as many additional places as may be appropriate. Those who use MeSH should find the most specific MeSH descriptor that is available to represent each concept of interest.
For example, articles concerning Streptococcus pneumoniae will be found under the descriptor Streptococcus Pneumoniae rather than the broader term Streptococcus, while an article referring to a new streptococcal bacterium which is not yet in the vocabulary will be listed directly under Streptococcus. Accordingly, the user may consult the trees to find additional subject headings which are more specific than a given heading, and broader headings as well. For example, under Abnormalities, there are specific abnormalities:
Congenital Abnormalities C16.131
Abnormalities, Drug Induced C16.131.042
Abnormalities, Multiple C16.131.077
22q11 Deletion Syndrome C16.131.077.019
DiGeorge Syndrome C16.131.077.019.500
Alagille Syndrome C220.127.116.11
Alstrom Syndrome C18.104.22.168
Angelman Syndrome C22.214.171.124
In the MeSH Browser, each descriptor is followed by the number that indicates its tree location. It may also be followed by one or more additional numbers, in smaller type, and truncated at the third level, indicating other tree locations of the same term. The numbers serve only to locate the descriptors in each tree and to alphabetize those at a given tree level. They have no intrinsic significance; e.g., the fact that D12.776.641 and D12.644.641 both have the three digit group 641 does not imply any common characteristic. The numbers are subject to change when new descriptors are added or the hierarchical arrangement is revised to reflect vocabulary changes.
Last Reviewed: November 10, 2021