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.
“If testosterone levels drop in men like they did in the mice, I think we’ll start to see men coming forward saying, ‘I don’t feel like myself,’ and we’ll find out about it that way,” Moley says. “You might also ask, ‘Wouldn’t a man notice if his testicles shrank?’ Well, probably. But we don’t really know how the severity in men might compare with the severity in mice. I assume that something is happening to the testes of men, but whether it’s as dramatic as in the mice is hard to say.”