**3. Secretions of salivary gland**

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 [1–4].

#### **3.1. Saliva**

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 [2–4].

#### *3.1.1. Composition of saliva*

Saliva is composed of 99% water and 1% of components such as inorganic ions like sodium, potassium, chloride, bicarbonate, calcium, magnesium, fluoride, thiocyanate, hydrogen phosphate etc. It contains proline-rich proteins, histatins, cystatin, defensins. Kallikrein, cathelicidin-LL37, lactoferrin and enzymes such as amylase (ptyalin), peroxidase, lysozyme, etc. Immunoglobulins A, G and M, glucose, amino acids, urea, uric acid, lipid molecules and blood group antigens, epidermal growth factors, factor VII, factor VIII and factor IX are also present. Saliva in the mouth also consists of desquamated epithelial cells, microorganisms and their products, few inflammatory cells etc. [2–4].

#### *3.1.2. Formation of saliva*

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. Parotid gland and von Ebner's gland is purely serous gland, while sublingual, glossopalatine and palatine glands have more of mucous secretions. Submandibular gland and other minor salivary gland have both serous and mucous acini, resulting in mixed saliva [2–6].

**Figure 3.** Modification of saliva.

**S. no Time Event**

cells which give rise to acini [1].

**3.1. Saliva**

*3.1.1. Composition of saliva*

*3.1.2. Formation of saliva*

**3. Secretions of salivary gland**

**Table 3.** Chronology of salivary gland development.

6 Salivary Glands - New Approaches in Diagnostics and Treatment

1. Sixth week Development of the primordia of parotid and submandibular salivary gland

a lumen by the action of microfilaments at apical areas of cell to become ducts. The secretory part develops later by repeated branching and budding of finer cell cords, to form pregland

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

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

Saliva is composed of 99% water and 1% of components such as inorganic ions like sodium, potassium, chloride, bicarbonate, calcium, magnesium, fluoride, thiocyanate, hydrogen phosphate etc. It contains proline-rich proteins, histatins, cystatin, defensins. Kallikrein, cathelicidin-LL37, lactoferrin and enzymes such as amylase (ptyalin), peroxidase, lysozyme, etc. Immunoglobulins A, G and M, glucose, amino acids, urea, uric acid, lipid molecules and blood group antigens, epidermal growth factors, factor VII, factor VIII and factor IX are also present. Saliva in the mouth also consists of desquamated epithelial cells, microorganisms

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

2. Seventh to eighth week Development of sublingual salivary gland 3. Third month Development of minor salivary gland

proteins in large amounts apically or basolaterally to the saliva [1–4].

and minor salivary glands which are both serous and mucous in nature [2–4].

and their products, few inflammatory cells etc. [2–4].

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 [1–6].

#### *3.1.3. Saliva secretion in health and disease*

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 750–1000 ml daily which is contributed by major and minor salivary glands. The resting flow of saliva is 0.2–0.4 ml/min. Salivary flow at rest refers to as unstimulated saliva, whereas salivary flow in response to a stimulus refers to stimulated saliva having a flow rate of 2–5 ml/min. The normal pH of saliva is 6.4–7.4 [1, 2].

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. Alpha adrenergic receptor stimulation results in protein secretion while beta adrenergic or cholinergic stimulation results in low protein secretion and secretion of water and electrolytes. Substance P stimulates alpha adrenergic and cholinergic secretion of saliva. The following flow chart (**Figure 4**) shows the events associated with stimulus secretion coupling which involves the basic process of receptor stimulation which results in increase in the concentration of a secondary messenger, which will further trigger additional events leading to a cellular response [3–6].

Copious watery saliva is secreted in response to parasympathetic stimulation and thicker saliva in response to sympathetic stimulation. Other factors affecting saliva composition are flow rate, circadian rhythm, duration of stimulus, nature of stimulus and diet. During sleep very little saliva is secreted by major salivary glands and majority of the saliva secreted is by the minor salivary glands. Concentration of saliva depends on rate of flow and not on nature of stimulus [2–6].

Historically, it was suggested that parotid salivary gland secretes a hormone called parotin which was considered to have a protein-anabolic function and deficiency resulted in diseases such as chondrodystrophia fetalis, Kaschin-Beck disease, etc. [7].

An increase in the flow of saliva is referred to as sialorrhea (ptyalism), while a decrease in the salivary flow is referred to as xerostomia (dry mouth). Ptyalism is observed after insertion of new orthodontic appliance, in pregnancy, epilepsy, cerebral palsy and Parkinson's disease. Xerostomia is observed in menopause, patients treated by radiation therapy, old age, prolonged use of tranquilizers, amphetamines, antihypertensive and anticonvulsant drugs. A number of systemic conditions affect the functioning of the secretion of salivary glands. Hyperthyroidism, pernicious anemia, vitamin D deficiency, multiple sclerosis and poorly controlled diabetes mellitus affect the salivary glands. Autoimmune diseases like Sjogren's syndrome, Mikulicz's disease affect the salivary gland secretion as the parenchymal elements are affected. Inflammatory, infective and neoplastic diseases also disrupt the activity of salivary gland secretion. Salivary secretion is influenced by hormones. For example antidiuretic hormone facilitates water reabsorption by striated duct, aldosterone causes increased sodium reabsorption by striated duct, testosterone and thyroxine increase salivary secretion [2, 8, 9].

**4. Significance of salivary secretion**

which maintain the integrity of enamel and repair.

The saliva has a number of important functions as mentioned below.

**Figure 4.** Flowchart depicting sequence of events following neural stimulation.

Protection: the saliva contains mucin and glycoproteins which provides it with lubricating properties and moistening the oral cavity, thus preventing friction between the oral structures during physiological functions like mastication. The constant flow of saliva provides clearance of accumulated food debris and microorganisms. Mucins also provide thermal and chemical insulation. Proteins, glycoproteins and mucins form a coating called pellicle formation. Saliva acts as a source of calcium, phosphate, fluoride, statherin and proline rich protein

Secretions of Human Salivary Gland http://dx.doi.org/10.5772/intechopen.75538 9

**Figure 4.** Flowchart depicting sequence of events following neural stimulation.
