  The Medical Subject Headings (MeSH) thesaurus is a controlled vocabulary
widely used in biomedical knowledge systems, particularly for semantic indexing
of scientific literature. As the MeSH hierarchy evolves through annual version
updates, some new descriptors are introduced that were not previously
available. This paper explores the conceptual provenance of these new
descriptors. In particular, we investigate whether such new descriptors have
been previously covered by older descriptors and what is their current relation
to them. To this end, we propose a framework to categorize new descriptors
based on their current relation to older descriptors. Based on the proposed
classification scheme, we quantify, analyse and present the different types of
new descriptors introduced in MeSH during the last fifteen years. The results
show that only about 25% of new MeSH descriptors correspond to new emerging
concepts, whereas the rest were previously covered by one or more existing
descriptors, either implicitly or explicitly. Most of them were covered by a
single existing descriptor and they usually end up as descendants of it in the
current hierarchy, gradually leading towards a more fine-grained MeSH
vocabulary. These insights about the dynamics of the thesaurus are useful for
the retrospective study of scientific articles annotated with MeSH, but could
also be used to inform the policy of updating the thesaurus in the future.
