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The fluorescent histochemical technique of Falck for demonstrating catecholamines in sympathetic nerve fibers was modified for use in brown fat. In normal interscapular and mediastinal brown fat, the nerve fibers surrounded the arterial Mood vessels and all parenchymal cells. Both immunosympathectomy and surgical denervation gave an almost identical histological picture of massive loss of nerves from the blood vessels only. The parenchymal innervation remained intact. The blood vessels retained single isolated nerve fibers, which were relatively more numerous on the small vessels and which were connected with the parenchymal innervation. After sympathetic nerve fibers were depleted of catecholamines by treating the rat with reserpine, the parenchymal nerves reestablished catecholamine fluorescence at a faster rate than the blood vessel nerves. Intrinsic (tissue) ganglia in the brown fat were resistant to immunosympathectomy. These ganglia contained cells which were very brightly fluorescent. These results indicate that there are two anatomically, immunologically, and pharmacologically different nerve supplies to brown fat, one to the blood vessels and the other predominantly to the parenchyma. It was concluded that the parenchymal nerve supply is derived from the intrinsic ganglia via 'short' adrenergic neurons, and the blood vessel innervation is derived from the sympathetic chain via 'long' adrenergic neurons.

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