Peripolar cells are granulated cells located in the vascular pole of the renal corpuscle. Even though these cells have already been described, there are still many unknown histological and physiological characteristics. We carried out histochemical and immunohistochemical analyses of peripolar cells in sheep and compared their number in both normal and injured kidneys, discriminating according to the age of the animal. We tested HE, Toluidine Blue, PAS, and Masson's Trichrome stains to select the best stain for identification and quantification. Masson Trichrome yielded the best results and was selected for this purpose. We identified the cells by the presence of cytoplasmatic granules and by their position in the vascular pole. We found no statistically significant association between the number of peripolar cells and the age of the animal or the occurrence of lesions. In the immunohistochemical analysis, we found that the cells were positive to α-smooth muscle actin and less consistently positive to NSE and S100 protein. Chromogranin A, cyclooxygenase-2, AE1/AE3, and Wide Spectrum Cytokeratin and desmin yielded negative results. We conclude that although there was evidence of a contractile function, there was no evidence to support that peripolar cells have either a neuroendocrine or an epithelial nature. 1. Introduction The kidneys are the main urinary system organ, being responsible for maintaining the body hydroelectrolytic balance. Besides these vital homeostatic functions, they are also responsible for the excretion of metabolic degradation by-products, arterial tension regulation, and hormone production (mainly erythropoietin and renin) [1]. In 1979, Ryan et al. described a new kind of cells found in the vascular pole of renal corpuscle in sheep [2]. These were designated by peripolar cells (PPCs) due to their ring-like distribution around the vascular pole [3]. They are located near the inflexion point between the parietal and visceral layers [4]. They were initially classified as epithelial cells due to their location above the basal membrane of Bowman’s corpuscle, and by an abundance of junctions between the basal membrane and the PPCs [2, 5]. PPCs’ main identifying characteristic is the presence of cytoplasmatic granules. These granules have been shown before to be stainable for optical microscopy by PAS, Toluidine Blue, Methylene Blue, and Trichrome methods [2, 5–7]. PPCs are also present in many other animals including humans [4–10]. The percentage of PPCs in an animal varies with species and age: in humans, they were found in 6.5% of the
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