Autopsy study of a 49-year-old patient demonstrates a rare kidney tumor that had a structure of a mesenchymal renomedullary interstitial tumor (RIO), which had specific characteristics. They include a predominance of the cellular component in the tumor, represented by cells with processes elements (fibroblasts, myofibroblasts), which were surrounded by collagen fibers of interstitium. Apparently, these cells can be involved in the regulation of renin and bicarbonates, the exchange of components of connective tissue (collagen fibers, proteoglycans) and hormones (estrogen, progesterone). Since these tumor cells regulate the synthesis and secretion of the substances, numerous clinical manifestations of RIO can be explained. Most likely, these include increased blood pressure, water-electrolyte disorders, hormonal disorders due to the presence of estrogen receptors, progesterone in the tumor tissue and metabolic disorders (diabetes mellitus). However, these assumptions require further clinical, morphological and immunohistochemical studies.

Keywords: interstitial cells, renomedullary interstitial cell tumor, arterial hypertension