Zoning inside the renal fascia: The anatomical relationship between the urinary system and perirenal fat

Objectives To examine the anatomical relationship between the urinary system and perirenal fat, and to clarify the zoning inside the renal fascia. Methods Using computed tomography images from 50 men, we examined perinephric veins to reveal vessel communication in perirenal fat. Nine cadavers were dissected to investigate connective tissue continuity and vessel communication inside the renal fascia. Eight retroperitoneal specimens were macroscopically observed: four from the anterior and four from the posterior aspects. One specimen was used to obtain retroperitoneal transverse sections to study macroscopic anatomy and histology. Results Perinephric veins were classified into four types (superior, middle, inferior and lateral) using computed tomography. Most of the inferior perinephric veins were connected to the ipsilateral gonadal vein. In the cadaveric study, the superior and middle perinephric veins communicated with veins deriving from the ipsilateral adrenal gland. A fibrous connective tissue gap between perirenal fat and renal hilar fat was observed in posterior aspect dissection. From the gap, we could dissect the urinary system from perirenal fat en bloc along with a thin fibrous connective tissue layer. Communicating vessels between perirenal fat and the urinary system were rare. Conclusions Our results suggest that perirenal fat belongs to the connective tissue of the gonad and the adrenal gland. The urinary system is separate from perirenal fat, and is located on the dorsal side inside the renal fascia. This concept of zoning inside the renal fascia is valuable particularly in retroperitoneoscopic donor nephrectomy.