Gangliosides occur in much greater amounts in clean isolated neurons and in the neuropil teased from immediately around the neuron cell body and dendrites than in isolated clumps of glial cells. Since the zone of neuropil adjacent to neurons is richest in terminal axons and synaptic endings, these findings indicate a specific concentration of these sialoglycolipids in synaptic membranes.
By using a modified method of Falck, the distribution of nerves containing catecholamines in brown and white fat was studied. The brown fat cells had sympathetic nerve fibers with synaptic varicosities running between all cells. White fat had no sympathetic innervation of the parenchymal cells. Both tissues contained a dense network of fibers around the arterial blood vessels.
With the Falck fluorescent histochemical method for catecholamines, the development of sympathetic nerve fibers in the brown fat of the rat was studied. At birth a low level of catecholamine fluorescence was found in preterminal axon bundles, which have been called here sheathed axons. The fluorescence in these bundles reached a maximum at about 5 days of age. Parenchymal nerve catecholamine fluorescence was discernible at 2 to 3 days of age. These nerves progressively got brighter until adult levels had been reached at about 10–15 days. The sympathetic nerve fiber network seen on arterial blood vessels in the adult was not seen in the young rat until 8–10 days of age. These blood vessel nerves reached maturity at about 15–21 days. The parenchymal nerve innervation was shown to originate in the sympathetic chain. The sympathetic chain gave off sheathed axon bundles which reach the parenchyma by passing along septa or beside blood vessels.
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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