The innervation of salivary glands

Ultrastructure of the parotid and submandibular glands of the Old World Marten (Carnivora; Mustelidae)

The morphology of the parotid and submandibular glands in the marten, a carnivore, were studied and analyzed under a transmission electron microscope. The nature of the granules in both glands, as well as in the acini and in the secretory tubules, is rather mucous. The structure of the secretory tubules is very characteristic, especially the striated ones. The myoepithelial cells are close to the acini and tubules and covered by the basement membrane separating them from the connective tissue, which enhances its epithelial origin. The cytoplasm of the basal parts of the acinar and tubular cells is abundant and separates the nucleus from the secretion granules. Although the morphology of the salivary glands of many carnivores is known, those of the parotid gland of the marten present peculiar characteristics, since they produce a rather mucous saliva and the granules, when forming, are far from the base as well as from the apex of the secretory cells. The submandibular gland contains granules of different densities, an aspect that in general resembles that of other animals.

Protein gene product 9.5-immunoreactive nerves and cells in human oral mucosa

BACKGROUND

Current conflicting information on the innervation of the human oral cavity indicates technical problems such as different detectability of the neural structures according to the various staining methods used and difficulties in reproducibility. The possibility of intraoral regional differences has not been properly considered.

METHODS

Human biopsies of mucosa from different intraoral regions were prepared for immunohistochemistry using protein gene product 9.5 (PGP 9.5; a marker for neuronal structures).

RESULTS

Nerves were found consistently in all the biopsies. The neural pattern showed clear regional differences. Intraepithelial nerve fibers were found in the gingiva, labia, palate, within certain fungiform papillae, and in some salivary excretory ducts. Organized nerve endings were found in varying frequencies in all but one (sublingual) region, appearing as lamellar (Meissner-like), coiled or glomerular neural structures. Merkel cell-neurite complexes were observed in the buccal, gingival, and palatal epithelia. Immunoreactive cells with many similarities to Merkel cells but without a neural connection were also encountered.

CONCLUSIONS

Conflicting results from earlier innervation studies of the oral cavity could be attributed to regional innervation differences. The distribution of the nerves also casts doubt on some of the present theories concerning the function(s) of intraoral nerves, such as the free nerve endings and the Merkel cell-neurite complexes.

The effects of obstruction and secretory stimulation on microlithiasis in salivary glands of cat: light and electron microscopy

Obstruction and increased secretory activity are considered to be important aetiological factors of salivary microlithiasis, which may itself be an aetiological factor of sialadenitis. However, there is a lack of substantial evidence for the importance of obstruction, and investigations on increased secretory activity used pathological doses of pharmacological agents. Therefore further investigation of these factors is essential. Feline parotid, submandibular and sublingual salivary glands, in which microliths occur normally as in man, were examined after ductal ligation to produce obstruction, electrical stimulation of the glandular nerves to produce increased secretory activity, or both. Microliths were detected in: 0 out of 38 untreated, 1 out of 55 ligated, 2 out of 17 stimulated and 2 out of 17 stimulated ligated submandibular glands; 6 out of 29 untreated, 7 out of 46 ligated, 3 out of 12 stimulated and 4 out of 14 stimulated ligated sublingual glands; and no parotid glands. The chi 2 test confirmed that the experimental procedures did not produce an increased occurrence of microliths. Microliths were detected in parenchymal cells, intercellularly in atrophic parenchyma, intraluminally, interstitially and in macrophages. The present investigation indicates that obstruction and increased secretory activity are not important aetiological factors of salivary microlithiasis.

The innervation of salivary glands as revealed by morphological methods

Salivary secretion is nerve mediated. The salivary glands are supplied by parasympathetic and sympathetic efferent nerves which travel to the glands by separate routes. Once in the glands the axons from each type of nerve intermingle and travel together in association with Schwann cells, forming Schwann-axon bundles. Two types of neuro-effector relationships exist with salivary parenchymal and myoepithelial cells: epilemmal (outside the parenchymal basement membrane) and hypolemmal (within the parenchymal basement membrane). Their relative frequencies with either type of nerve differ greatly between glands and species. Salivary blood vessels receive epilemmal innervations by both sympathetic and parasympathetic axons. The classical transmitters--acetylcholine in parasympathetic and noradrenaline in sympathetic axons--are stored in small vesicles. A variety of non-conventional neuropeptide transmitters have also been found in salivary nerves by immunohistochemistry, and they occur in large dense-cored vesicles. Prolonged high frequency stimulation has been found to cause depletion of large dense-cored vesicles from glandular nerves. In recent years afferent nerves have started to be identified and are found in greatest numbers around the main salivary ducts, where they may form a hypolemmal association with the epithelial cells. Functional studies demonstrate complex interactions between parasympathetic and sympathetic nerves. Morphological assessments of changes in the parenchymal cells after nerve stimulations or denervations add greatly to our understanding of the nerve functions. At least four types of influence can be exerted on salivary parenchymal cells by the nerves: hydrokinetic (water mobilizing), proteokinetic (protein secreting), synthetic (inducing synthesis), and trophic (maintaining normal functional size and state). In respect to each role, wide glandular and species differences exist between the relative contributions made by each type of nerve.

The effect of sympathectomy on the occurrence of microliths in salivary glands of cat as studied by light and electron microscopy

Parasympathectomy is followed by a greatly increased occurrence of microliths in the feline submandibular gland, which appears to be because of secretory inactivity. The sympathetic nerves are also important in secretory processes, and so feline submandibular, sublingual and parotid glands subjected to postganglionic sympathectomy for periods from 1 day to 1 yr have now been investigated. Microliths were detected in two out of 28 sympathectomized submandibular glands and four out of 27 untreated glands, and in 10 out of 22 sympathectomized sublingual glands and seven out of 19 untreated glands. There were no significant differences between the occurrence of microliths in sympathectomized and untreated glands. Microliths were not detected in any of 29 sympathectomized and 30 untreated parotid glands. The appearance of the sympathectomized glands was similar to that of the untreated glands. The failure of sympathectomy to affect the occurrence of microliths or the appearance of the glands is possibly because of parasympathetic nerve impulses, which produce continuing secretory activity, and also the spontaneous secretion of the sublingual gland. The results support the concept that secretory inactivity is an aetiological factor of microlithiasis.

Increase of microliths in inactive salivary glands of cat

Secretory inactivity could be a factor in the formation of microliths, and so their occurrence in feline salivary glands after the secretory inactivity produced by parasympathectomy was investigated. Parasympathectomy was followed by a greatly increased occurrence of microliths in the submandibular salivary gland, but not in the parotid and sublingual, which may relate to residual secretory activity in these glands. This discovery suggests that secretory inactivity may indeed be a factor in the production of microliths in human salivary glands, and consequently of chronic sialadenitis and sialothiasis.

The proper role of nerves in salivary secretion: a review

Consideration of the history of this subject has created a perspective which helps one to appreciate how errors in thinking have occurred and why some have persisted, despite subsequent improvements in information. The most enduring misconception has been that the drying of the mouth under stress is due to sympathetic inhibitory fibers, a view that must be eradicated, for such fibers do not exist. The inhibition is due to central influences from higher centers acting on the salivary centers and thereby suppressing reflex activity. Wide variations exist in the neuro-effector arrangements and in the cellular responses in different glands from different species. Myoepithelial cells are usually contracted by both parasympathetic and sympathetic nerves. The blood vessels also receive a dual innervation, but parasympathetic impulses cause vasodilatation as part of secretion, whereas the sympathetic vasoconstrictor fibers are part of a more generalized vascular control system and not a direct part of the reflex secretory sympathetic pathway. Parasympathetic drive usually provides the main stimulus for fluid formation by parenchymal cells, whereas sympathetic nerves tend to increase the output of pre-formed components from certain cells. Absence of nerve impulses causes variable atrophic and other metabolic effects on the parenchymal cells. Evidence is beginning to accrue that certain nerve impulses may influence resynthetic activities. Vacuolation, often found experimentally after strong stimulation, also occurs to a variable extent in certain cells as a normal part of reflex secretion and may therefore have an effect on the components entering the saliva. If rupture of vacuoles occurs, then this may contribute to the salivary amylase that is present in the blood. Recent evidence points to the possibility that, even in a monomorphic gland, not all of the components necessarily enter nerve-induced saliva in "parallel" proportions. Consideration of current information has enabled some provisional generalizations about the roles of the secretory nerves to be suggested in the summary, but they must not be considered immutable.

Histological effects of ductal ligation of salivary glands of the cat

Forty-two submandibular, 32 sublingual and 31 parotid glands have been examined microscopically after ductal ligation avoiding the nerves for periods from 1 day to 1 yr. After an initial increase in size, there was over-all atrophy in all three glands. In the parotid gland the response was uniform and there was progressive atrophy until most of the acini were extremely atrophic. However, in the submandibular and sublingual glands the response was very variable, and although in some glands most of the acini were extremely atrophic, in other glands there was far less atrophy and most of the remaining acini appeared more or less similar to normal despite prolonged ligation. These results contrast with those of earlier experimental studies on ductal ligation, but have similarities with observations on the behaviour of human salivary glands.

Influence of autonomic nerves and drugs on myoepithelial cells in parotid glands of cat

When a single electrical shock is applied to either the parasympathetic or to the sympathetic nerves a pressure rise is seen in the parotid duct of the cat, probably due to myoepithelial cell activation. No secretion occurred on single impulse stimulation. When repetitive stimulation was used both divisions of the autonomic nervous system could evoke secretion. The motor response to sympathetic stimulation was mediated via alpha-receptors while secretion mainly was mediated via beta-receptors.