The mammalian Leydig cell is a polyhedral epithelioid cell with a single eccentrically located ovoid nucleus. The nucleus contains one to three prominent nucleoli and large amounts of dark-staining peripheral heterochromatin. The acidophilic cytoplasm usually contains numerous membrane-bound lipid droplets and large amounts of smooth endoplasmic reticulum (SER). Besides the obvious abundance of SER with scattered patches of rough endoplasmic reticulum, several mitochondria are also prominent within the cytoplasm. Frequently, lipofuscin pigment and rod-shaped crystal-like structures 3 to 20 micrometres in diameter ( Reinke crystals ) are found. These inclusions have no known function, are found in less than half of all Leydig cell tumors, but serve to clinch the diagnosis of a Leydig cell tumor.   No other interstitial cell within the testes has a nucleus or cytoplasm with these characteristics, making identification relatively easy.