The secretory part develops later by repeated branching and budding of finer cell cords, to form pregland cells which give rise to acini [ 1 ]. The salivary glands are primarily involved in secretion of saliva. There are other substances also which are secreted by the salivary glands which are found in saliva.
They can secrete proteins in large amounts apically or basolaterally to the saliva [ 1 , 2 , 3 , 4 ]. Saliva is a complex physiological fluid of the oral cavity which coats the teeth and oral mucosa. It contains a myriad of components like enzymes, mucinous substances, antibacterial components etc.
Saliva functions to maintain the oral cavity in the physiological state owing to its lubricating, buffering, antibacterial and immune properties by acting as a physiological barrier to infections. The saliva is a mixed fluid, as it is composed of saliva secreted by both major and minor salivary glands which are both serous and mucous in nature [ 2 , 3 , 4 ]. It contains proline-rich proteins, histatins, cystatin, defensins. Kallikrein, cathelicidin-LL37, lactoferrin and enzymes such as amylase ptyalin , peroxidase, lysozyme, etc.
Saliva in the mouth also consists of desquamated epithelial cells, microorganisms and their products, few inflammatory cells etc. According to Tencate [ 2 ], the formation of saliva occurs in two stages. The first stage involves the formation of saliva by the acinar cells. The acinar cells whether serous or mucous cells produce salivary secretion by ribosomal protein synthesis in the rough endoplasmic reticulum which is followed by the packaging of the proteins by the golgi complex.
The secretions are stored as granules and later released into the lumen by the process of exocytosis or by vesicular mechanism. Exocytosis involves fusion of the secretory granules with the membrane allowing release of the contents into the lumen. Vesicular mechanism involves transport of vesicles filled with secretions from golgi complex to plasma membrane.
Transcytosis involves passage of substances like immunoglobulin A through the acini. Water is taken up by the cells from the bloodstream and the resulting saliva secreted is isotonic. The serous cells produce serous saliva which is thin, watery and is composed of zymogen granules and contains more proteins, while mucous cells produce thick, viscous saliva containing mucopolysaccharides and mucin.
Submandibular gland and other minor salivary gland have both serous and mucous acini, resulting in mixed saliva [ 2 , 3 , 4 , 5 , 6 ].
In the second stage the saliva undergoes changes as it passes through the salivary ductal system into the oral cavity. Saliva secreted from the acini is isotonic or slightly hypertonic when it reaches the intercalated ducts. The intercalated duct cells also release lysozymes and lactoferrin. Striated and excretory ducts are impermeable to water. In the striated duct, reabsorption of sodium and chloride occurs more as compared to the secretion of potassium and bicarbonate ions, which makes saliva hypotonic Figure 3.
Striated duct cells also secrete kallikrein and epidermal growth factor. Thus, saliva secreted into the oral cavity is hypotonic as compared to serum [ 1 , 2 , 3 , 4 , 5 , 6 ]. Modification of saliva. The myoepithelial cells are responsible for the contraction of the acini cells, aiding in the flow and secretion of saliva. In health, the total volume of saliva produced is — ml daily which is contributed by major and minor salivary glands. The resting flow of saliva is 0.
The normal pH of saliva is 6. A number of factors control the quality and quantity of saliva secreted. The control of salivary gland secretion is mediated by the autonomic nervous system ANS. All the salivary gland cells receive ANS supply. Control of secretion is also dependent on the perception of taste and smell. The gustatory stimulus is more important than the masticatory stimulus in controlling the salivary secretion.
The secretion of saliva occurs by the process of stimulus secretion coupling. This refers to the events involving release of neurotransmitter from vesicles in nerve terminals adjacent to parenchymal cells which stimulate them to discharge secretory granules, water and electrolytes as well as contraction of myoepithelial cells.
Norepinephrine activates both alpha and beta adrenergic receptors, while parasympathetic transmitter like acetylcholine activate cholinergic receptors.
The basis for different glands secreting saliva of differing composition can be seen by examining salivary glands histologically. Two basic types of acinar epithelial cells exist:. Acini in the parotid glands are almost exclusively of the serous type, while those in the sublingual glands are predominantly mucus cells.
In the submaxillary glands, it is common to observe acini composed of both serous and mucus epithelial cells. In the histologic sections of canine salivary gland shown above, the cells stained pink are serous cells, while the white, foamy cells are mucus-secreting cells.
Secretion of saliva is under control of the autonomic nervous system, which controls both the volume and type of saliva secreted.
The nerve-mediated salivary reflex is modulated by nerve signals from other centers in the central nervous system, which is most obvious as hyposalivation at times of anxiety.
An example of other neurohormonal influences on the salivary reflex is the circadian rhythm, which affects salivary flow and ionic composition. Cholinergic parasympathetic and adrenergic sympathetic autonomic nerves evoke salivary secretion, signaling through muscarinic M3 and adrenoceptors on salivary acinar cells and leading to secretion of fluid and salivary proteins.
The striated ducts lead into interlobular excretory ducts, lined with a tall columnar epithelium. The glands are divided into lobules by connective tissue septa. Each lobule contains numerous secretory units, or acini.
Images A,B, and C are of three different salivary glands. In image A you can see both secretory acini, and a duct. Can you identify these two features?
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