RESUMO
PIP: The renal medulla appears to exert an endocrine-like antihypertensive action. The main candidate for exertion of this action is the renomedullary interstitial cell. Accordingly, a search for hyperplasia or adenomatous transformation of this cell was conducted. Many fibromas of the renal medulla, previously considered as hamartomas, seemed to be composed of tumorous transformations of renomedullary interstitial cells. These tumors were small (1-7 mm), smoothly outlined, and gray or yellow. By light microscopy they contained lipid-laden cells (oil red O and Sudan black B positive), collagen, and much acid mucopolysaccharide. Semithin sections stained with toluidine blue revelaed many cells having the features of the renomedullary interstitial cell. Noteworthy were toluidine blue positive droplets, clear vacuoles, and elongated processes. By electron microscopy, the similarity of most cells to the renomedullary interstitial cell was evident. The striking features were electron dense osmiophilic droplets, cisternae, cytoplasmic processes, and positive tropism toward blood vessels. It is proposed that the fibroma of the renal medulla be renamed the "renomedullary interstitial cell tumor".^ieng