(Associated Professor in Animal Nutrition & Physiology)

(Associated Professor in Animal Nutrition & Physiology)
By: A. Riasi (Associated Professor in Animal Nutrition & Physiology) Advanced Digestive Physiology (part 2) Isfahan University of Technology

Salivary gland and salivation
Salivary glands and salivation Secretions contain: Enzymes (amylase and lipase) Water Glycoproteins Salivary glands: Secretions from the salivary glands contain enzymes, water and glycoproteins. These working together and help swallowing. Secretory IgA, lactoferrin and lysozyme in the saliva perform its protective functions. Lactoferrin (LF), also known as lactotransferrin (LTF), is a multifunctional protein of the transferrin family. Lactoferrin is a globular glycoprotein with a molecular mass of about 80 kDa that is widely represented in various secretory fluids, such as saliva, milk, tears and nasal secretions. Lactoferrin can be purified from milk or produced recombinantly. Lactoferrin is one of the components of the immune system of the body; it has antimicrobial activity (bacteriocide, fungicide) and is part of the innate defense, mainly at mucoses. Lysozymes, also known as muramidase or N-acetylmuramide glycanhydrolase, are glycoside hydrolases. These are enzymes that damage bacterial cell walls by catalyzing hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan. Lysozyme is abundant in a number of secretions such as tears, saliva, human milk, and mucus. Large amounts of lysozyme can be found in egg white. Amylase is found in the saliva of omnivores such as rats and pigs, but is absent in carnivores like dogs and cats. Lipases may be found in some young animals that are nursing or on a high-milk diet (e.g. calves). Saliva can serve a neutralizing function if it contains a high concentration of sodium bicarbonate and phosphate (e.g. cattle). The salivary glands have different function: Preparing enzymes Moistens and lubricates feed Water balance Bloat prevention Recycling of N and minerals including Na, P, and S (A 700 kg dairy cow fed a hay-grain diet will secrete about 190 liter saliva/day containing, g total N, 1100 g NaHCO3, 350 g Na2HPO4, and 100 g NaCl) Buffer secretion (normal rumen, pH 5.5 – 7.0, without salivary buffers, pH 2.8 – 3.0) Surrounding each major salivary gland is a connective tissue capsule. Septa of connective tissue from the capsule extend into the gland and divide the organ into lobes and then lobules. A rich vascular and nerves plexus surrounds the secretory units and the ducts. The small tubes go into ducts. Those ducts go into larger ducts that have little stripes on them, called striations. Those go into ducts between the lobes of the gland called interlobar or excretory ducts. The main duct of the salivary glands then goes into the mouth. Actually each salivary gland contains the different components: Secretory units made up of serous, mucous or a combination of these cells types. The cells are arranged as acini or tubulo-acini. Serous cells are shaped like a pyramid. They are joined together in a group that is shaped like a ball. The ball is called acinus, with a small lumen in the center. These cells have centrally located nuclei. Mucous cells are usually shaped like a cube and have flat or oval nucleus. They are joined together to make a tubules, which are very small tubes. These cells make glycoproteins that make saliva wet and slippery. The mucous material (mucinogen granules) in the cytoplasm stains palely in the H&E preparation. Ducts can be classified (from small to large) into intercalated (small ducts leading away from the secretory units), striated intralobular, interlobular and finally the main excretory duct. Goblet cells may be present in the epithelium. Myoepithelial cells wrap around the secretory units and the ducts. Contraction of these cells, which have numerous microfilaments in their cytoplasm. They can squeeze the saliva gland so the saliva comes out faster. Lymphocytes and plasma cells are found in the connective tissue surrounding the acini. Immunoglobulin A (IgA) is synthesized by the plasma cells. The secretory IgA complex, resistant to proteolysis, is then released into the saliva. There is different kind of salivary glands. Parotid gland, mandibular gland, sublingual gland and some minor salivary glands. Purely serous acini are found in the parotid glands of most domestic animals. The parotid glands which are a serous gland are specific in structure as the end piece cells are arranged in spherical form (Edgar & Dawes, 2004). In mucous glands however, the arrangement is of a tubular nature resulting in large central lumen. The peripheral branches of the facial nerve (VII) are related closely with the parotid gland. The walls of the parotid duct are thick due to the unification of the ductules which is also responsible for the drainage of lubules of the gland (Edgar & Dawes, 2004). The duct is situated anterior to the border of the gland and on the surface of the masseter muscle. The duct then curves over the anterior border of the masseter muscle and opens within the oral cavity in papilla adjacent to second upper molars. Mandibular gland are mixed seromucous gland contains a combination of tubules and terminal acini with mucous cells. In cows, sheep and pigs, sublingual gland contains mainly mucous cells. In dogs and cat, this is a mixed seromucous gland. Innervation to the sublingual gland derives from two important sources: 1) Sympathetic innervation from the cervical chain ganglia and 2) Parasympathetic innervation, like the submandibular gland, is derived from the submandibular ganglion. A number of minor salivary glands are also present. These seromucous glands include labial, buccal, molar, palatine and (only in carnivores) zygomatic glands. Minor salivary glands do not have connective tissue capsules. Some salivary glands are embedded in the tongue. They are called lingual salivary glands. Don't confuse them with the "sublingual salivary gland," a discrete organ in and of itself. Lingual glands may be of the serous or mucous or mixed type. Some of them open out via ducts onto the surface, some of them have ductwork leading to the "moat" that surrounds the large tongue papillae. Unlike the major salivary glands, the minor salivary glands lack a branching network of draining ducts. Instead, each salivary unit has its own simple duct. Most of the minor glands receive parasympathetic innervation from the lingual nerve, except for the minor glands of the palate, which receive their parasympathetic fibers from the palatine nerves. The production of saliva is an active process that occurring in two phases: 1- Primary secretion that occurs in the acinar cells. This results in a product similar in composition and osmolality to plasma and 2- ductal secretion that results in a hypotonic salivary fluid. It also results in decreased sodium and increased potassium in the end product. The salivary ducts rely heavily on the Na/K/2Cl cotransporter. The duct cells maintain a negative resting membrane potential, and these cells hyperpolarize secondary to the efflux of potassium and influx of chloride with autonomic nervous stimulation. This is unusual, and is referred to as the “secretory potential”, because most excitable cells depolarize (rather than hyperpolarize) with stimulation. The degree of modification of saliva in the ducts turns heavily on salivary flow rate. Fast rates result in a salivary product more like the primary secretion. Slow rates result in an increasingly hypotonic and potassium rich saliva. The parasympathetic nervous system is the primary instigator of salivary secretion. Interruption of parasympathetic innervation to the salivary glands results in atrophy, while interruption of sympathetic innervation results in no significant change in the glands. It was once thought that the sympathetic nervous system antagonizes the parasympathetic nervous system with respect to salivary output, but this is now known not to be true. Stimulation by the parasympathetic nervous system results in an abundant, watery saliva. Acetylcholine is the active neurotransmitter, binding at muscarinic receptors in the salivary glands. Stimulation by the sympathetic nervous system results in a scant, viscous saliva rich in organic and inorganic solutes. For all of the salivary glands, these fibers originate in the superior cervical ganglion then travel with arteries to reach the glands: External carotid artery in the case of the parotid Lingual artery in the case of the submandibular Facial artery in the case of the sublingual In about 50% of total saliva is secreted by the paired parotid glands. Parotid weight of CS, again irrespective of body size, is more than three times that of GR. This means, salivary glands have regressed as ruminants increased fiber digestion. The question arises; do CS and IM then need so much more saliva for buffering purposes? Because as will be seen, all these selective species also have a much denser, evenly distributed rumen papillation than GR. This results in a greater internal surface enlargement facilitating faster absorption of SCFA; hence: little danger of pH depression. First of all, these bigger glands supply more diluting liquid, which reduces retention time. Secondly, CS produce a much higher proportion of thin, proteinaceous serous saliva to carry away much of the soluble plant cell contents set free by puncture crushing of dicots (GR grind fibrous food sideways). There is reason to believe that some of these nutrients (e.g. sugars) are absorbed already in loco, while more solutes are washed, together with excessive serous saliva, down the ventricular groove into the abomasum. This would lead to a certain loss of salivary bicarbonate and to CO2 formation in reaction to the acidic gastric juice. It would, however, initially explain the considerable surplus of HCl-producing parietal cells. There is another reason for much more (and more serous) saliva production in CS and IM: it is a counter-adaptation to overcome the plants' chemical defenses. The phenolic compounds produced by plants form insoluble complexes with protein (tanning effect). Moreover, as protein feed protection experiments have shown, the undigestible tannin-protein complex will be dissolved in the acidic abomasal environment this would be a vital second reason for so much more HCl-production in that thicker abomasal mucosa of selective ruminants. Ruminants produce a high daily output of saliva (6 to 16 L/d in sheep; 60 to 160 L/d in cattle). The secretions from parotid glands are isotonic with blood plasma, have no significant amylase content, change their composition in response to salt depletion, contain urea and alkali. Their secretion responds strongly to mechanical stimulation of the mouth, esophagus, and ruminoreticulum. In contrast, the submaxillary, sublingual, and labial glands produce small quantities of hypotinic, mucous, weakly buffered saliva. The submaxillary and labial glands are strongly stimulated only by feeding and give no response to esophageal or ruminoreticular stimulation. 3 3 Adapted from: Advanced ruminant nutrition Available at:

Salivary glands and salivation Saliva has secretory IgA, lactoferrin and lysozyme. Saliva can serve a neutralizing function 4 4

Salivary glands and salivation 5 5

Salivary glands and salivation 6 6

Salivary glands and salivation 7 7 Adapted from: Advanced ruminant nutrition Available at:

The different functions of salivary glands: Preparing enzymes Moistens and lubricates feed Water balance Bloat prevention Recycling of N and minerals including Na, P, and S Buffer secretion 8 8

The secretion cells The ducts Intercalated Striated intralobular Interlobular Main excretory duct Myoepithelial cells Lymphocytes and plasma cells 9 9

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Acinar cells produce proteins and antibacterial peptides. Secretion is stimulated by parasympathetic fibers that enhance metabolic activity of acinar cells and also stimulate contraction of myoepithelial cells to expel fluid into ducts. The ducts add an alkaline secretion to the saliva in response to the hormone secretin synthesized in the duodenum. 12 12

In the left panel you see both serous and mucous secretory areas, in among the strands of skeletal muscle in the tongue. At right, a large serous gland is discharging via a duct (D) into the moat (M) around a vallate papilla. 13 13

Schematic diagram of typical salivary gland. A, Serous endpiece; A’, in cross section. B, Seromucous demilune; B’, in cross section. C, Mucous endpiece; C’, in cross section. D, Intercalated duct; D’, in cross section. E, Striated duct; E’, in cross section. F, Terminal excretory duct. (Modified from Ten Cate AR: Oral histology, ed 2, St Louis, 1985, Mosby.) 14 14

A myoepithelial cell can be seen with its processes surrounding an acinus. (From Nanci A: Ten cate’s oral histology, ed 8, St. Louis, 2013, Mosby) 15 15

Different kinds of salivary glands in ruminants: 16 16

Salivary flow Saliva production occurring in 2 phases: Primary secretion Ductal secretion The salivary ducts rely heavily on the Na/K/2Cl cotransporter. 17 17

Salivary flow The degree of modification of saliva: Fast rate Result in a salivary product more like the primary secretion. Slow rate Result in an increasingly hypotonic and potassium rich saliva. Effect of autonomic nervous system 18 18

In addition to a continuous low-level secretion, salivary secretion may be enhanced by two different types of salivary reflexes: 1. The simple, or unconditioned, salivary reflex occurs when chemoreceptors and pressure receptors within the oral cavity respond to the presence of food. On activation, these receptors initiate impulses in afferent nerve fibers that carry the information to the salivary center located in the medulla oblongata, as are all the brain centers that control digestive activities. The salivary center, in turn, sends impulses via the extrinsic autonomic nerves to the salivary glands to promote increased salivation. 2. The acquired, or conditioned, salivary reflex occurs without oral stimulation. A zoo mammal hearing the sound of a food cart, or a dog watching the preparation of a meal, initiates salivation through this reflex. All of us have experienced such “mouth watering” in anticipation of something to eat. This reflex is a learned response based on previous experience and is an example of feed forward or anticipation regulation. Inputs that arise outside the mouth and are mentally associated with eating act through the cerebral cortex to stimulate the medullary salivary center. Unlike the autonomic nervous system elsewhere in the vertebrate body, sympathetic and parasympathetic controls from the salivary center to the salivary glands are not antagonistic. Instead, both act to increase salivary secretion, but the quantity and consistency of saliva change with the needs of the animal. The rate of saliva secretion depends on the quantity of food in the mouth, whereas its consistency depends on the composition of the food consumed. Both the amount and quality of saliva are commensurate with the need for sensory evaluation of the food as well as the need for lubrication of dry foodstuffs. Parasympathetic stimulation, which exerts the dominant role, increases blood flow through the glands and is accompanied by a prompt and abundant flow of serous saliva. Normally, sympathetic stimulation reduces blood flow but allows a continued saliva output of smaller volume of mucus saliva. Because sympathetic stimulation in humans elicits a smaller volume of saliva, the mouth is drier than usual during circumstances when the sympathetic system is dominant, such as stress situations. Thus, people experience a dry feeling in the mouth when they are nervous about giving a speech. Cats, in contrast, secrete voluminous watery saliva in response to sympathetic stimulation. 19 19 Adapted from Animal Physiology, by Sherwood et al., 2013

Salivary flow Ruminant produce a high daily output of saliva. 6 to 16 L/d in sheep 60 to 160 L/d in cattle 20 20

Salivary glands Total salivary volumes (L d) Characteristics Site of reflexogenic stimuli Parotids Inferior molars Palatine, buccal, pharyngeal Submaxillary Sublingual, labial 3-8 0.7-2 2-6 0.1 Serous, isotonic, strongly buffered Isotonic, strongly buffered Mucous, hypotonic, weakly buffered Very mucous, hypotonic, weakly buffered Mouth, esophagus, ruminoreticulum Mouth during feeding, not cudding Mouth Total volume 6-16 Although parotid saliva is isotonic with blood plasma, there are much higher concentrations of K+ (13mM), HCO3- (112 mM), and HPO4-- (48 mM) and correspondingly lower concentrations of Na+ (170 mM) and Cl- (11 mM). In salt-depleted animals there is a replacement of Na+ by K+ because of the action of aldosterone. The high salivary content of HCO3- and HPO4-- account for its high alkalinity (pH 8.1) and is an important mechanism for neutralization of about one-half of VFAs in the forestomach. The pK value for HCO3- and HPO4-- systems, being 6.1 and 6.8 respectively, help to buffer the ruminal contents in the normal pH range of 5.5 to 7.0. The high content of phosphate also represents a form of recycling, the microbes having a high demand for phosphate to synthesize nucleoproteins, phospholipids, nucleotide coenzymes, etc. Salivary nitrogen, 77% of which comes from urea, provides a useful additional source of NPN for microbial protein synthesis. The high urea content of ruminant saliva may be a critical factor for survival in situations of severe protein deficiency, when even the kidneys can increase renal tubular reabsorption of urea to facilitate its recycling. A basal level of parotid secretion occurs even in the totally denervated or atropinized gland. Reflex-evoked increases in salivation are due to the excitation of the secretory (acinar) cells by acetylcholine liberated by the parasympathetic nerve endings, and this may be blocked by atropine. Electrical stimulation of the sympathetic nerve supply after atropinization produces a transient increase in salivary output followed by a compensatory reduction. This effect is due to the norepinephrine induced contraction of the contractile myoepithelial (basket) cells that surround the acini and small ducts. It leads to an expulsion of stored saliva rather than to an increase in secretion. The increase in parotid blood flow does not exactly parallel the increase in parotid secretion and depends on a noncholinergic parasympathetic mechanism, which is not affected by atropine. Salivary reflexs are integrated in salivary centers located in the hindbrain. The major reflex excitatory input arises from postulated buccal mechanoreceptors located in or near the tooth sockets, and the sensory pathways project mainly to the salivary center on the same side as the receptor stimulation. Thus chewing of ingesta or cud causes a large increase in salivary secretion, one of which in cattle may increase its rate from 2 ml/min to 30 to 50 ml/min. Experimentally, the parotid and other major glands also increase their secretory rates in response to distension of the esophagus, reticulum, reticuloomasal orifice, and ruminoreticular fold as a result of excitation of tension receptors located in these sites. In contrast, little increase is evoked by lightly stroking the ruminoreticular epithelium. Such stimulation primarily excites epithelial receptors and has a lesser effect on tension receptors. Tension receptor induced reflex effects account for the small increases in salivation that occur at the time of each reticular contraction and for the transient large increase in salivation that occurs when the cardia and esophagus are distended. Reflex increases in salivation may be inhibited by concurrent stresses and excitement. The lymphoid tissues of the oral cavity include the ring of tonsils and the diffuse lymphoid tissues in the connective tissues. These are part of the body’s first line of immune response (the lumen of the GI tract, starting from the oral cavity, is physically continuous with the outside world). 21 21

Salivary flow The secretions from the parotid glands are: Isotonic with blood plasma Have no significant amylase content Change their composition in response to salt depletion Have a high alkalinity (pH 8.1) Recycling the N and P 22 22

Salivary flow Adapted from: Advanced ruminant nutrition Available at: 23 23

Salivary flow A 700 kg dairy cow fed a hay-grain diet will secrete: 190 l saliva/day 30-80 g total N 1100 g NaHCO3 350 g Na2HPO4 100 g NaCl Cows produce 10 to 32 L of saliva per kg of feed dry matter (DM) with an average of 18.2 L/kg DM. The production of saliva is much higher when roughage is consumed than when grains are consumed. Factors that are important in saliva secretion are the dry matter content of the feed, forage intake, and forage particle size. The saliva produced by cattle, contains 125 milliequivalents (meq)/L bicarbonate and has a pH of 8.4. Cows fed 20 kg of dry matter can produce the equivalent of 3418 to 3617 g/d sodium bicarbonate in their saliva, depending on the level of forage in the diet. Thus, forage intake plays an important role in amount of buffer required by the animal. If a cow is fed a diet with a 70:30 grain to forage ratio, the diet would have to be supplemented with about 0.5% sodium bicarbonate to produce the same natural buffering capacity as a cow fed a diet with a 50:50 grain to forage ratio. Therefore, it is clear that the manner in which we feed dairy cows contributes to the need for supplemental dietary buffer. 24 24

Control of salivary flow A basal level of parotid secretion occurs even in the totally denervated. Excitation of the secretory (acinar) cells by the parasympathetic nerve endings. The increase in parotid blood flow does not exactly parallel the increase in parotid secretion. 25 25

Salivary reflexs are integrated in salivary centers located in the hindbrain. The buccal mechanoreceptors located in or near the tooth sockets have major effect. Chewing of ingesta in cattle may increase salivary secretion from 2 ml/min to 30 to 50 ml/min. 26 26

Results of a study showed that increased chewing time did not result in increased total daily saliva secretion because increased eating and ruminating time decreased resting time, and the accompanying resting saliva secretion. Thus, the net increase in saliva secretion due to increased chewing time was minimal (Maekawa et al., 2002). 27 27

The distension of the esophagus, reticulum, reticuloomasal orifice, and ruminoreticular Little increase is evoked by lightly stroking the ruminoreticular epithelium. Reflex increases in salivation may be inhibited by concurrent stresses and excitement. 28 28

Some feeding factors may affect the saliva flow: Dietary fiber concentration Forage to grain ratio of the diet Maturity of the forage Diet particle size Grinding Grain processing by-products Diet moisture level 29 29

Saliva as a source of systemic biomarker
Saliva plays an important role in various processes that take place in the oral cavity. In addition to that, a close interrelation between saliva and blood exists, making it possible for systemic biomarkers to enter saliva. It has recently been demonstrated that saliva in cattle contains potential markers of bloat and in pigs might indicate ongoing stress, pregnancy, inflammation and viral infections. Saliva is an extraordinary fluid in terms of research and diagnostic possibilities. Its composition in electrolytes, hormones and especially its proteome contains information about feeding status, nutritional requirements and adaptations to diet and environment, and also about health status of animals. It is easy to collect on a non-invasive and routine basis without any need for special training. Therefore, the analysis of salivary proteomes is going to emerge into a field of high interest with the future goal to maintain and improve livestock productivity and welfare. Moreover, the comprehensive analysis and identification of salivary proteins and peptides in whole and glandular saliva is a necessary pre-requisite to identify animal disease biomarkers and a powerful tool to better understand animal physiology. Saliva contains thousands of proteins and peptides of glandular or blood origin and therefore is suggested to hold physiological and pathological information accessible by proteomic research. Saliva helps to study the influence of diet on taste perception, food choice or avoidance as well as to obtain the nutritional status of an animal. It is furthermore a pool of putative diagnostic information encrypted in proteins (e.g. BSP30, immunoglobulins), hormones (e.g. progesterone, cortisol) and nucleic acids (e.g. viral RNA), all possibly helping us to secure health in farm animals. However, especially the salivary proteome is highly variable, dependent on mechanical and chemical stimulants and vulnerable to proteolysis. Therefore, today's saliva sampling and preservation techniques highly rely on freezing the samples to −80 °C for long-time storage to reduce degradation to a minimum. Nevertheless, future strategies also need to cope with saliva sampling in farm animals from tropical or desert areas, where the cooling chain is often unsustainable. Only by developing robust techniques, we will be able to meet the future challenges of the nutritional needs of a growing world population under ongoing climate change. In achieving this goal, we might be able to support health monitoring and livestock production worldwide. 30 30 Adapted from: E. Lamya, and M. Maud, 2012

Mastication Chewing of food is fast and irregular with variable amplitude. The differences of chewing and rumination: Rumination is much more slowly and evenly Rumination is usually on one side Rumination occasionally changed to the opposite side Mastication involves the premolar and molar teeth. The lower jaw is much narrower than the upper jaw, and the flat condyles of the temporomandibular joints allow considerable lateral movement. The teeth of the lower jaw are beveled, so that they are lower on the outer side and higher on the inner side, with the conserve occurring in the upper jaw. Thus each chew begins with the lower jaw below and lateral to the upper jaw on one side. As the chew develops, the lower teeth move upward and inward against the upper teeth, so that food is subjected to a mixture of shearing in grinding actions. 31 31

Mastication Direct and indirect effects of mastication:
Break the stem and leaf fragments of food and to cud solids into small particles. Movements of the teeth excite sensory buccal mechanoreceptors. The direct importance of mastication is to break the stem and leaf fragments of food and to cud solids into small particles to increase the number of portals entry for the ruminoreticular microbes. The indirect importance of mastication is that movements of the teeth excite sensory buccal mechanoreceptors, which provide the most potent natural excitatory inputs to both the salivary and gastric centers. Thus chewing movements, above all other stimuli, lead to increase salivation, particularly on the side on which the chewing is occurring, and to increases in the rate and the amplitude of primary and secondary cycle contractions of the ruminoreticulum. Foodstuffs that require little chewing on ingestion and that evoke little rumination subsequently fail to boost salivation and forestomach motility. They may have adverse digestive consequences. 32 32

Swallowing The swallowing has three phases: Oral Phase
Pharyngeal phase Esophageal pahse The phases of swallowing are controlled by central pattern-generating circuitry of the brain stem and peripheral reflexes. The oral, pharyngeal, and esophageal phases of swallowing are independent of each other. The peripheral manifestation of these phases depends on sensory feedback through reflexes of the pharynx and esophagus. 33 33

(Associated Professor in Animal Nutrition & Physiology)

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