A HISTOPATHOLOGIC AND IMMUNOHISTOCHEMICAL STUDY OF SMALL NODULES OF RENAL ANGIOMYOLIPOMA - A COMPARISON OF SMALL NODULES WITH ANGIOMYOLIPOMA

Small mesenchymal nodules (SNs) are observed in some cases of renal an giomyolipoma (AML), with or without tuberous sclerosis. They are compo sed of blood vessels and/or nonvascular smooth muscle cells (SMCs) and /or fat cells. We examined 20 cases of AML, performed detailed histopa thologic and immunohistochemical studies of SNs, verified the histolog ic relationship between SNs and AMLs, and compared the SNs of the tube rous/nontuberous sclerosis groups. Seventy-seven SNs were observed in five cases of AML. The SNs were 0.11 mm to 20.0 mm in diameter. The lo cation of small-sized SNs in the kidney was variable; almost all of th e SNs larger than 3.25 mm were in the renal capsule. The small-sized S Ns contained mainly epithelioid-type nonvascular SMCs. Blood vessels a nd fat cells were not observed in the small-sized SNs but appeared gra dually in the large-sized SNs. Almost all of the SNs were rounded lesi ons, and no fusion was observed between the SNs. Nonvascular SMCs of a ll of the SNs and AMLs were positive for vimentin, alpha-SM actin, and S-100 protein. The SNs less than 1.13 mm in diameter were negative fo r HMB-45; the nonvascular SMCs of AMLs and of SNs greater than 1.13 mm in diameter were positive for HMB-45. Nonvascular SMCs of SNs and AML s showed a neurogenic phenotype. The SNs of the nontuberous sclerosis group contained only SMC components, whereas the same-sized SNs of the tuberous sclerosis group contained SMCs, fat cells, and blood vessels . The SNs of the nontuberous sclerosis group may not increase in size or may grow slowly. Some of the SNs of patients with tuberous sclerosi s grow to become AML. Although the SNs in patients with nontuberous sc lerosis do not contain any blood vessels or fat cells, their SMCs show the histologic and immunohistochemical characteristics of AMLs; this indicates that SNs are the ''buds'' of AML.