Fine structure and innervation of the avian adrenal gland

Summary Apart from cholinergic nerve fibers, which make up the main part of efferent fibers to the avian adrenal gland (Unsicker, 1973b), adrenergic, purinergic and afferent nerve fibers occur. Adrenergic nerve fibers are much more rare than cholinergic fibers. With the Falck-Hillarp fluorescence method they can be demonstrated in the capsule of the gland, in the pericapsular tissue and near blood vessels. By their green fluorescent varicosities they may be distinguished characteristically from undulating yellow fluorescent ramifications of small nerve cells which are found in the ganglia of the adrenal gland and below the capsule. The varicosities of adrenergic axons exhibit small (450 to 700 Å in diameter) and large (900 to 1300 Å in diameter) granular vesicles with a dense core which is usually situated excentrically. After the application of 6-hydroxydopamine degenerative changes appear in the varicosities. Adrenergic axons are not confined to blood vessels but can be found as well in close proximity of chromaffin cells. Probably adrenergic fibers are the axons of large ganglion cells which are situated mainly within the ganglia of the adrenal gland and in the periphery of the organ and whose dendritic endings show small granular vesicles after treatment with 6-OHDA.A third type of nerve fiber is characterized by varicosities containing dense-cored vesicles with a thin light halo, the mean diameter (1250 Å) of which exceeds that of the morphologically similar granular vesicles in cholinergic synapses. Those fibers resemble neurosecretory and purinergic axons and are therefore called p-type fibers. They cannot be stained with chromalum-hematoxyline-phloxine. Axon dilations showing aggregates of mitochondria, myelin bodies and dense-cored vesicles of different shape and diameter are considered to be afferent nerve endings. Blood vessels in the capsule of the gland are innervated by both cholinergic and adrenergic fibers.Supported by a grant from the Deutsche Forschungsgemeinschaft (Un 34/1).