Histochemical and cytochemical demonstration of zinc cysteinate in the tapetum lucidum of the cat.

Zinc cysteinate is shown histochemically and cytochemically in the tapetum lucidum of the cat. Heavy metal is demonstrated in the paraplasmic rods of the tapetal cells with a sulphide silver method whereas no such reaction can be found in the pigmented epithelium of the retina. These rods are also stained with silver methenamine indicating the presence of reducing groups which probably appeared after hydrolysis of cysteine.