The noradrenergic innervation of the excretory ducts of the parotid, mandibular and sublingual glands in the rat

Telocytes in parotid glands

The parotid histological structure includes acinar, ductal, and myoepithelial cells, surrounded by a connective stromal component. The parotid stroma is mostly regarded as an inert shell, consisting of septa, which divide the parenchyma. Telocytes were recently identified as a new stromal cell type in various organs, including exocrine pancreas. We aimed to evaluate telocytes presence in parotid stroma and whether their topographical features might support an involvement in parotid function modulation. Serial ultrathin sections of human and rat parotid glands were studied and compared by transmission electron microscopy. Two-dimensional concatenation of sequenced micrographs allowed the ultrastructural identification of parotid telocytes, with their specific long, thin, and moniliform prolongations (telopodes). Telocyte location appeared frequently as a strategic one, in close contact or vicinity of both secretory (acini and ducts) and regulatory (nerves and blood vessels) apparatuses. They were also found in the interacinar and the subductal stroma. Two previously reported telocyte markers (c-kit/CD117 and vimentin) were assayed by immunohistochemistry. Actin expression was also evaluated. Telocytes are making a network, especially by branching of their long telopodes. Elements of this telocyte network are interacting with each other (homocellular connections) as well as with other cell types (heterocellular connections). These interactions are achieved either by direct contact (stromal synapse), or mediated via shed microvesicles/exosomes. Since telocyte connections include both neurovascular and exocrine elements (e.g., acini and ducts), it is attractive to think that telocytes might mediate and integrate neural and/or vascular input with parotid function.

Branchings and course of the larger ducts and accompanying structures within the rat submandibular salivary gland

In the rat submandibular gland, the larger ducts, vessels, and autonomic elements are surrounded by appreciable collagen to form an extensive, ramifying core for the bulk of the parenchyma. This entity is analogous to the armature within a clay sculpture because it provides a framework supporting softer surroundings. Its largest portion, the trunk, begins dorsorostrally at the hilus. Along its caudal course, the trunk emits limbs that, in turn, branch. Rostrally, the trunk and major limbs have their greatest diameters and contain lymphatic vessels and autonomic ganglia that are fewer or absent in the smaller divisions. Four to six orders of branching commonly occur. This architectural device constitutes a physical gradient which peaks at the hilus and is minimal where its components undergo the transition to the lobular parenchyma. The presence of an appreciable amount of connective tissue, nerves, and larger vessels would make this entity firmer than the highly hydrated parenchyma. The major divisions of this grouping are contacted by septa that extend from the organ capsule and demarcate lobules into which smaller divisions penetrate. The parenchyma is consequently ensconced between several strong structures containing abundant connective tissue. The dense collagen encompassing individual components of the biological armature would stabilize duct epithelium and its intercellular channels as well as brace vessels' walls. As a result of this architectural arrangement, the patency of all acinar, duct, and vascular structures is likely to persist despite neck and jaw movements that would otherwise distort them and thus hinder the formation and flow of saliva during feeding. This mechanism may be widespread among organs.

Immunohistochemical localization of [Met5]enkephalin and [Met5]enkephalin-Arg6-Gly7-Leu8 in sympathetic and parasympathetic neurons and nerve fibers projecting to the rat submandibular gland

The localization of [Met5]enkephalin, [Met5]enkephalin-Arg6-Gly7-Leu8, vasoactive intestinal polypeptide and tyrosine hydroxylase immunoreactivities was studied in the submandibular gland of adult Sprague-Dawley and Wistar rats using the indirect immunofluorescence technique. Immunoreactivities for [Met5]enkephalin and [Met5]enkephalin-Arg6-Gly7-Leu8, a proenkephalin A-derived octapeptide, showed identical distributions. A large number of enkephalin-immunoreactive nerve fibers were detected around secretory acini, along intercalated ducts, convoluted granular tubules, intra- and interlobular ducts, as well as in close contact with blood vessels. The submandibular ganglia contained several enkephalin-immunoreactive neurons and nerve fibers. In the superior cervical ganglion numerous enkephalin-immunoreactive neurons and nerve fibers were also detected. Immunohistochemical co-localization studies indicated that [Met5]enkephalin and [Met5]enkephalin-Arg6-Gly7-Leu8 immunoreactivities co-exist with vasoactive intestinal polypeptide in a subpopulation of neurons of the rat submandibular ganglia, in nerve trunks along the salivary ducts of the gland, and in nerve fibers around the acini. Uni- or bilateral superior cervical ganglionectomies for 1-4 weeks resulted in a complete disappearance of tyrosine hydroxylase immunoreactivity in the glandular parenchyma, while moderate tyrosine hydroxylase immunoreactivity was seen in some neurons of the submandibular ganglia. Abundant [Met5]enkephalin-Arg6-Gly7-Leu8-immunoreactive nerve fibers were still seen around the acini and blood vessels, as well as close to salivary ducts. These operations did not affect the [Met5]enkephalin-Arg6-Gly7-Leu8-immunoreactive neurons in the submandibular ganglia. Many principal neurons in the superior cervical ganglion contained both [Met5]enkephalin-Arg6-Gly7-Leu8 and tyrosine hydroxylase immunoreactivity. Nerve ligation experiments indicated that [Met5]enkephalin-Arg6-Gly7-Leu8-immunoreactive sympathetic fibers project along the external carotid nerve. Accordingly, nerve fibers were found around the acini and blood vessels as well as in nerve trunks along the salivary ducts of the submandibular gland, showing co-localization of [Met5]enkephalin-Arg6-Gly7-Leu8 and tyrosine hydroxylase. Taken together, these observations suggest that the nerve fibers of the rat submandibular gland containing proenkephalin A-derived peptides are of both sympathetic and parasympathetic origin.

Effects of acetylcholine, isoprenaline and forskolin on electrolyte and protein composition of rabbit mandibular saliva

1. The major purpose of this study was to investigate cellular regulation of the ductal transport processes in salivary glands which act to modify the electrolyte composition of primary saliva and cause it to become hypotonic. This was achieved using an isolated mandibular gland preparation by observing the effect of different stimuli on the electrolyte composition of saliva secreted at the same flow rate, on the assumption that these stimuli do not influence primary saliva composition. The effects of the same stimuli on the volume of primary fluid secretion and on protein secretion were also observed. Proteins were measured in total and as individual components after their separation by high-performance liquid chromatography. 2. Acetylcholine was used as a 'Ca2+-mobilizing' agonist (i.e. one which both elevates intracellular Ca2+ concentration and activates protein kinase C). Isoprenaline was initially used to elevate intracellular cyclic AMP concentration but was subsequently abandoned in favour of forskolin. 3. Acetylcholine was a very potent stimulus of primary fluid secretion. By contrast, isoprenaline and forskolin were essentially without effect, even when superimposed on acetylcholine stimulation. 4. As judged by saliva electrolyte composition, increasing the concentration of acetylcholine enhanced ductal absorption of Na+ and Cl- and secretion of K+ (and presumably HCO3-). Forskolin had the opposite effect: when superimposed on submaximal acetylcholine stimulation it caused saliva concentrations of Na+ and Cl- to remain high and K+ low (i.e. it inhibited ductal transport processes). The inhibitory effect of forskolin on ductal transport could be overcome by increasing the concentration of acetylcholine, and vice versa. 5. Acetylcholine, isoprenaline and forskolin each increased salivary protein secretion, although the kinetics of secretion differed. The spectrum of proteins secreted in response to the three stimuli was the same. The relative proportions of the individual proteins was influenced by the strength of stimulation (i.e. the proportions at high total protein output differed from those at low total protein output) but not apparently by the nature of the stimulus. 6. Thus, the three major secretory processes in the rabbit mandibular salivary gland respond differently to the two major signal transduction mechanisms. For primary fluid secretion, Ca2+ is stimulatory and cyclic AMP almost without effect; for ductal transport, Ca2+ is stimulatory and cyclic AMP inhibitory; and for protein secretion both Ca2+ and cyclic AMP are stimulatory.