Intra-acinar nerve terminals in four rodent parotid glands

The distribution of autonomic nerves was studied at the ultrastructural level in guinea pig, hamster, mouse and rat parotid glands employing potassium permanganate fixation/staining technique. In the periacinar connective tissue numerous unmyelinated nerves were observed in all the glands examined. Two types of axonal enlargements were encountered, one contained small, dense-cored vesicles (500-600 A in diameter) as well as a few large (approximately 1,000 A in diameter), the other exhibited numerous agranular vesicles (500-600 A in diameter). In all glands, axons devoid of Schwann cell investment were visualized at the base of the acinar cells, inside the basement membrane and in intimate contact with gland cells. In addition, axonal enlargements of both types were encountered in the intervening space between neighbouring acinar cells. Whereas nerve terminals within the gland epithelium were extremely common in the rat, mouse and hamster, their frequency was much lower in the guinea pig. In addition, only parenchymal varicosities of the adrenergic (dense-cored vesicles) type were encountered in the latter species. No intraepithelial nerves were observed in the striated ducts. However, axonal enlargements were present in the subepithelial connective tissue and were most numerous in the hamster parotid gland. The above findings confirm the dual innervation of the serous acinar cells of the guinea pig, hamster, mouse and rat parotid glands by both parasympathetic and sympathetic nerves. Furthermore, there exists a close membrane-to-membrane contact between autonomic nerves and salivary gland cells.