Increased numbers of mesangial cells per glomerulus, defined as more than 3 nuclei fully surrounded by matrix in one or more mesangial areas, not including perihilar region, on a standard 3-micron-thick tissue section, best evaluated on periodic acid-Schiff (PAS) stain. [HPO_CONTRIBUTOR:Eurenomics_fschaefer, https://orcid.org/0000-0003-2356-950X, PMID:32866505]