Autonomic innervation of salivary glands in the armadillo, anteater, and sloth (Edentata)

Interlobular excretory ducts of mammalian salivary glands: structural and histochemical review

In the major salivary glands of mammals, excretory ducts (EDs) succeed striated ducts. They are for the most part interlobular in position, although their proximal portions sometimes are on the periphery of a lobule, where they occasionally retain some of the structural features of striated ducts. Based on a survey of a broad range of mammalian species and glands, the predominant tissue type that composes EDs is pseudostratified epithelium. In some species, there is a progression of epithelial types: the proximal EDs are composed of simple cuboidal or columnar epithelium that, in the excurrent direction, usually gives way to the pseudostratified variety. Secretory granules are visible in the apical cytoplasm of the principal cells of the EDs of only a few species, but histochemistry has shown the presence of a variety of glycoproteins in these cells in a spectrum of species. Moreover, the latter methodology has revealed the presence of a variety of oxidative, acid hydrolytic, and transport enzymes in the EDs, showing that, rather than simply acting as a conduit for saliva, these ducts play a metabolically active role in gland function. It is difficult to describe a "typical" mammalian ED because it can vary along its length and interspecific variation does not follow a phylogenetic pattern. Moreover, in contrast to intercalated and striated ducts, ED cellular features do not exhibit a relationship to diet.

Selective innervation of different target tissues in guinea-pig cranial exocrine glands by sub-populations of parasympathetic and sympathetic neurons

This study has used multiple-labelling immunohistochemistry and quantitative analysis to examine the projections of subpopulations of parasympathetic and sympathetic neurons to different vascular and secretory structures in five cranial exocrine glands of guinea-pigs. Multiple subpopulations of parasympathetic axons, identified by immunoreactivity (IR) for various combinations of peptides, innervated arteries, arterioles, ducts and acini in sublingual, submandibular, parotid, lacrimal and zygomatic glands, although axons were absent from ducts in the parotid gland. Most parasympathetic axons contained IR for vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), with or without enkephalin (Enk). The proportion of parasympathetic axons that contained Enk-IR varied greatly between target tissues and glands: Enk-IR was more common in axons supplying secretory ducts, acini and arterioles than in axons innervating more proximal arteries; Enk-IR was less common in axons supplying the lacrimal gland than axons supplying the submandibular, lacrimal and zygomatic glands. Sympathetic axons with IR for tyrosine hydroxylase (TH) innervated arterial vessels in all glands, but innervated secretory structures only in the salivary glands. Sympathetic axons supplying proximal arterial segments often contained NPY-IR and sometimes also contained IR for dynorphin. Dynorphin-IR was more common in axons in the parotid, lacrimal and zygomatic glands than in the sublingual and submandibular glands. In contrast, axons supplying arterioles, ducts and acini lacked peptide IR. These results indicate that neuronal pathways regulating proximal arteries in cranial exocrine glands are different from the neuronal pathways regulating arterioles and acini, and may be different from neurons projecting to proximal secretory ducts. Furthermore, the peptides enkephalin, NPY and dynorphin are likely to make variable contributions to autonomic neurotransmission in different arterial segments and in different cranial exocrine glands.

Nerve interactions in salivary glands

In the salivary reflex, not only secretory cells are activated, but also myo-epithelial cells are contracted to support these cells and promote the flow of saliva, and blood vessels dilate to meet the increased demands of the tissues. The various effector cells often receive nerves from both parts of the autonomic system, and interactions may occur when the nerves act on the same type of effector, or on different types of effectors. While in an experiment electrical stimulation of the sympathetic trunk may decrease a parasympathetic salivary flow by causing marked vasoconstriction, this does not occur in the salivary reflex, since the vasoconstrictors do not take part. On the contrary, the normal sympathetic vasoconstrictor tone of the resting gland is easily overcome by activity in parasympathetic vasodilator nerves when secretion starts. Pronounced synergism can be demonstrated between sympathetic and parasympathetic secretory nerves. In dogs, for instance, in which sympathetic secretion is beta-adrenoceptor-mediated, this is marked in the case of fluid secretion. In rats and rabbits, in which beta-receptors elicit secretion of amylase, the potentiating interaction among the nerves is striking when amylase secretion is considered. Even the random release of acetylcholine from the post-ganglionic parasympathetic axons, by itself insufficient to evoke secretion, can increase the sympathetic effects. Motor nerves interact with secretory nerves by causing myo-epithelial contraction, mechanically promoting secretion. Interactions between the nerves in their long-term regulatory function on the sensitivity of the acinar secretory and myo-epithelial cells can also be demonstrated.