Exocrine glands of the GIT include the salivary glands and pancreas. Ken Ashwell will talk about the gross anatomy, PDP will talk about microscopic anatomy.
- 1 Gross anatomy
- 2 Histology
- 2.1 Salivary glands
- 2.2 Pancreas
- Lubrication of food to assist deglutition (swallowing)
- Moistening the buccal cavity
- Aiding speech
- Secrete antibacterial agents
- Provide an aqueous solvent for molecules stimulating taste receptors
- Act as a fluid seal for sucking and suckling
- Secrete digestive enzymes
There are two types of salivary gland. Major salivary glands are located some distance from the oral mucosa and their secretions are transferred to the oral cavity by ducts of between 2 mm and 4 cm in length depending on the gland. They include the parotid, sublingual and submandibular glands. Minor salivary glands lie in the mucosa and submucosa of the tongue and walls of the oral cavity
Two main groups of salivary glands:
- Major salivary glands (macro)
- Minor salivary glands (micro)
Major salivary glands
- The parotid gland weighs about 25g and lies below and anterior to the external acoustic meatus between the mandible and the sternocleidomastoid muscle. An accessory parotid gland may be found between the zygomatic arch and the parotid duct.
On a dissection of the side of the head, you can see all three major salivary glands:
- Parotid gland (near ear)
- Submandibular gland (underneath the mandible)
- Sublingual gland (much deeper, more accessible from the floor of the mouth than from the mandible)
- The parotid gland is penetrated by some structures with clinical significance. These include the external carotid artery and its division into the maxillary and superficial temporal arteries, the facial nerve and its final branches (temporal, zygomatic, buccal, marginal, cervical), superficial branches of the mandibular division of trigeminal nerve to the skin anterior to the auricle, deep and superficial lymph nodes. Involvement of the facial nerve fibres by malignant parotid gland tumours may cause paralysis of the muscles of facial expression. The parotid duct carries saliva from the gland to the oral cavity. It ends in a papilla opposite the crown of the 2nd upper molar. This papilla is often palpable in the living.
- In front of auditory meatus
- Has a duct that runs forward that runs forward into the oral cavity, through piercing the buccinator muscle
- Where the parotid gland opens, there is a little flap in line with the crown of your second upper molar.
- There may be accessory parts of the parotid gland (that lie in association with the parotid duct)
- Inside the parotid gland (gross). It gets enlarged in mumps, can develop tumours
- The parotid gland has several vascular structures and a very important neural structure passing through it
- Viewing the right parotid gland from the medial side
- Lt = parotid duct
- Rt = posterior border
- Distinct groove
- Vein = retromandibular vein
- Artery = external carotid artery, which supplies the bulk of the face
- Branches: maxillary artery (deep inside the head and neck) and superficial temporal artery (running across the temple)
- Retromandibular vein
- External carotid artery --> maxillary and superficial temporal
- The facial nerve passes thrrough the parotid gland. This emerges from the stylomastoid foramen and then runs forward through the parotid gland
- Gives off 5 branches that supply all the muscles of facial expression
- Tumours can damage the facial nerve (patient presents not with salivary problems, but with paralysis on one side)
- Important in surgery
Submandibular and sublingual gland
- The other two glands are the submandibular and sublingual glands
- The submandibular gland has superficial and deep parts separated by the mylohyoid muscle. A submandibular duct carries saliva from the gland to the mouth. It is about 5cm long and opens in the floor of the mouth on the sublingual papilla at the base of the frenulum of the tongue. The superficial part of the submandibular gland is easily palpable in the living, medial and about 2 to 3 cm anterior to the angle of the mandible.
- The sublingual gland lies beneath the oral mucosa, in contact with the sublingual fossa on the lingual aspect of the mandible. The gland has 8 to 20 sublingual ducts opening directly into the floor of mouth and into the submandibular duct.
- Submandibular gland is below the mandible (soft lump just in front of angle of mandible on the medial side). (There are also lymph nodes around here that can be enlarged).
- At least part of the gland is superficial. Can divide it into superficial and deep (based on relation to the mylohyoid muscle).
- Deep is superior to the superficial part.
- The submandibular gland has a duct that runs forward, to the floor of the mouth, under the tongue
- The duct runs alongside the sublingual gland
- Sublingual gland has ducts that run up and open into the cleft underneath the tongue, where it meets the cleft of the floor of the mouth.
- The lingual nerve carries the parasympathetic fibres to the sublingual and submandibular glands
- The submandibular ganglion lies on the stylohyoid muscle (parasympathetic postganglionic nerve cell bodies that control the gland)
- The third important nerve to note is the hypoglossal nerve. It spirals behind the vagus nerve and passes between the internal carotid artery and internal jugular vein lying on the carotid sheath. After passing deep to the posterior belly of the digastric muscle, it passes to the submandibular region to enter the tongue. It supplies motor fibres to all of the muscles of the tongue, except the palatoglossus muscle, which is innervated by the vagus nerve (cranial nerve X) or, according to some classifications, by fibres from the glossopharyngeal nerve (cranial nerve IX) that "hitchhike" within the vagus. It controls tongue movements of speech, food manipulation, and swallowing.
View of the underside of the tongue
- Sublingual papilla at the base of the frenulum of tongue. Has 2 holes that are the openings of the submandibular gland.
- There is a sublingual fold on either side, which has the sublingual gland beneath it, and its ducts open at the anterior aspect into the oral cavity
The pancreas is a retroperitoneal structure about 12 to 15 cm long, which extends transversely across the posterior abdominal wall from the duodenum to the spleen. It lies mainly in the epigastrium and left hypochondrium.
It consists of a head, neck, body and tail. Projecting from the lower left part of the head is the uncinate process, which passes posterior to the superior mesenteric A. and V. The body is described as having anterior, posterior and inferior surfaces, and superior, anterior and inferior borders.
Functions of the pancreas
- Endocrine (or metabolic) functions:
- Hormones produced in islets of Langerhans
- Glucagon - stimulates glucose production via gluconeogenesis and glycogen breakdown
- Insulin - promotes glucose uptake by cells and glycogen production and storage
- Exocrine functions
- Secretion of pancreatic juice into the duodenum
- Released into duodenum via main and accessory pancreatic ducts
- Pancreatic juice contains:
- Enzymes for the digestion of proteins and fats
- Alkaline-rich fluid to neutralise stomach acid
Ducts of the pancreas
- The main pancreatic duct, which drains into the duodenum at the greater duodenal papilla. The main pancreatic duct is joined by the bile duct to form an hepatopancreatic ampulla. Various sphincters (choledochal, sphincter of main pancreatic duct and sphincter of the hepatopancreatic ampulla) control flow of pancreatic juices and bile into the 2nd part of the duodenum
- The accessory pancreatic duct, which drains into the lesser duodenal papilla
- The duodenum wraps around the head of the pancreas, and the tail is associated with the spleen.
- We normally divide the pancreas up into a series of regions:
- Head - embraced by the 2nd part of the duodenum
- Neck region
- Tail, projecting up towards the spleen
- Pancreatic notch in the head of the pancreas, with the uncinate process projecting off it
- Found in the upper abdomen - tail goes up to LV1, head goes down as far as LV2
- Ducts that run through it (either drain the pancreatic juices towards the duodenum, or another duct coming down is the bile duct, from the gallbladder/liver).
- Greater duodenal papilla (with a dilated region of the duct before it - hepatopancreatic ampulla) - where the bile and pancreatic secretions are fed into the duodenum
- An accessory pancreatic duct opens above it, which opens into the duodenum at the lesser duodenal papilla (a little pimple)
- Pancreatic carcinoma - nasty disease, very aggressive, diagnosed in the late stages
- Exocrine enzymes can dissolve body tissues if there is a blockage of release (pancreatitis - the pancreas digests itself, with cysts forming in the pancreas).
Pancreas - posterior
- Structures lying behind the pancreas:
- Left crus of diaphragm
- Inferior vena cava
- Left kidney
- Left suprarenal gland
- Duodenum wrapping its head
- Bile duct entering its head from behind (bile from liver)
- Portal vein (from the splenic vein and the inferior mesenteric vein)
- Coeliac trunk --> splenic artery
- Superior mesenteric artery (supplies small intestine and a large amount of the large intestine)
These are glands that empty their secretions (saliva) into the oral cavity. They are compound exocrine glands, and can be classified in several ways:
- According to size (major and minor groups)
- According to location (submandibular, sublingual, buccal (cheek), labial (lips), palatine)
- By the nature of their secretions (serous, mucous, or mixed sero-mucous)
- Minor salivary glands: are intrinsically found in the structures of the oral cavity (e.g. labial, buccal, lingual and palatine). They all secrete saliva fairly continuously keeping the mucous membrane of the mouth moist.
- Major salivary glands: are larger extrinsic parotid, submandibular and sublingual glands located outside the oral caivty with large excretory ducts leading into the oral cavity.
Acini (secretory portion)
The secretory unit of a salivary gland is a saclike dilation at the beginning of each intercalated duct. Its narrow lumen is surrounded by a single layer of cuboidal cells, either serous or mucous in type. Each acinus is partially surrounded by myoepithelial (basket) cells, which copmpress the glandular cells, expelling the secretions into the lumen of the acinus
- They make tylin, salivary amylase, water
- Parotid gland is mainly serous (watery secretion - for washing, diluting)
- Sublingual gland is mainly mucous (sticky substance)
- Submandibular gland is mixed gland (mucous substance)
Each gland has an extensive duct system. Saliva is expelled from the acini, by the contraction of myoepithelial cells. it is forced into the 1st segment of the intralobular duct system (intercalated duct), inserted between the secretory and excretory parts of the gland. This duct is lined with simple squamous or low cuboidal epithelium. These cells become taller as they lead into the striated duct, the next in line of the intralobular ducts. The simple columnar cells, lining the striated ducts, have basal striations due to basal infoldings of the plasmalemma in which are located mitochondria. These ducts are secretory as they modify the ionic composition of saliva by reabsorbing sodium and excreting potassium ions. The distal segment of the striated duct becomes stratified columnar as it enters the interlobular septa to become an interlobular duct. Several interlobular ducts unite to forma lobar duct, which drains an entire lobe and join to form a single excretory duct that empties into the oral cavity.
- Intercalated duct = a little duct inserted between the acini (the smallest ducts)
- We have different names for ducts as they get bigger and bigger as we go higher up in the tree
- Compound tubulo-acinus gland
- Pale staining = mucous glands, nucleus pushing towards the edge
- Dark staining = serous glands, round nucleus
- Making watery solution
- Mixed glands = pale with a dark cap; crescent shaped dark cap, called a demilune
- Arrangement: it's always a mucous gland with a serous demilune (not around the other way)
- Myoepithelial cells
- Contract, squeezing the secretions through the ducts.
- Branching: Acinus --> intercalated ducts --> striated (intralobular) duct --> interlobular
- Ducts have a necklace of cuboidal cells (unlike arteries). Arteries/veins have endothelium (simple squamous) with muscle around
- Collagen septa separate the lobes into lobules
- In an exam, he could ask us to draw a submandibular salivary gland and label it
Parotid salivary glands
These are the largest of the paired salivary glands located near the ear. They are enclosed in a CT capsule with septa that divide the gland, into lobes and lobules. They are composed entirely of serous acini whose product is rich in enzymes (proteins).
Submandibular salivary glands
These are smaller, paired glands located under each side of the mandible in the floor of the mouth. They are serous with a variable mucous component. The gland has a less dense CT capsule than the parotid and septa penetrate the parenchyma of the gland to divide it into lobes and lobules. The duct system contains all the types of ducts found in the parotid, including a large excretory duct (Wharton's) that open near the frenulum of the tongue. Some mucous acini are capped with serous cells, (serous demilunes).
Sublingual salivary glands
These are the smallest of the major salivary glands located under the mucous membrane of the tongue, near the midline of the floor of the mouth. The gland lacks a capsule and is divided into many lobes by CT partitions. Most of the lobes are composed largely of mucous acini with serous demilunes, whereas others are entirely mucous. Serous acini are rare. The duct system is deficient, lacking almost entirely striated and intercalated ducts.
The pancreas is a large gland that lies in the upper abdominal cavity, behind the stomach. Its CT capsule forms septa, which subdivide the gland into lobules. The pancreas is both an exocrine and an endocrine gland. The endocrine (secrete hormones directly into the blood) components of the pancreas, Islets of Langerhans (2%), are scattered among the exocrine secretory acini (secrete substances by way of ducts into the duodenum).
- Pancreas is a dual organ: with exocrine and endocrine functions. Exocrine: It's making enzymes: trypsin, chymotrypsin, which are pushed in ducts into the duodenum. Endocrine: Producing hormones (insulin, glucagon and somatostatin), which are pushed straight into the bloodstream (no ducts).
- The exocrine pancreas produces about 1.2 L/day of a bicarbonate-rich fluid containing digestive proenzymes. Forty to fifty acinar cells form a round to oval acinus whose lumen is occupied by 3 to 4 centroacinar cells, the beginning of the duct system of the pancreas.
- Pancreas has acini (making digestive enzymes) and pancreatic islets (beta cells make insulin, alpha cells make glucagon
- Large, interlobular ducts lie between lobules
- Cells live in clusters (called acinus)
- Space in the acinus = centroacinar cell. It is the lining cell of the first duct, and lives in the centre of the acinus. They appear to be floating in the middle of the acinus, but they aren't really.
- The acinar cells are producing zymogens (in their zymogen granules, appear as speckling) - these are the precursors of the pancreatic enzymes
Each acinar cell has an apical cytoplasm containing zymogen secretory granules, which contain inactive enzyme precursors.
The duct system of the pancreas begins within the center of the acinus, with the terminus of the intercalated ducts, composed of low cuboidal centroacinar cells. Intercalated ducts join each other to form larger intralobular ducts, several of which converge to form interlobular ducts. These ducts deliver their contents into the main pancreatic duct, which joins the common bile duct before opening in the duodenum at the papilla of Vater. Smooth muscle fibres provide the sphincter (of Oddi) at the termination of the common duct. The sphincter permits intermittent flow of bile and pancreatic secretions into the lumen of the duodenum. The exocrine pancreas differs from the parotid gland by the absence of striated ducts.
Histophysiology of the exocrine pancreas
The acinar cells of the exocrine pancreas manufacture, store, and release a variety of enzymes such as trypsinogen which converts proteins to amino acids; amylase, which converts starch and glycogen into maltose, which is then converted to glucose by the enzyme maltase; and lipase, which breaks down fats into fatty acids and glycerol
Each Islet of Langerhans (up to 200µm in diameter) is a richly vascularised spherical conglomeration of approx 3000 cells. There are around 1 million Islets in the human pancreas interspersed among the exocrine cells. Each Islet is surrounded by reticular fibres, which also enter the substance of the Islet to encircle the network of fenestrated capillaries.
Islet of Langerhans (Pancreatic Islets) cells and their hormones
Alpha cells are large cells (20%) found mainly at the periphery of an Islet produce glucagon, a peptide hormone in response to low blood glucose levels.
Beta cells are the most numerous (70%) found mainly in the centre of an Islet. They produce insulin, which is released into the intercellular space in response to increased blood glucose levels, as occurs after consumption of a carbohydrate-rich meal. The plasma membranes of these cells have glucose transport proteins, glucose permease, which are activated to take up glucose, decreasing blood glucose levels.
Delta cells are the largest cells (5%) and manufacture somatostatin, which affects the smooth muscle cells of the alimentary tract and gallbladder, reducing the motility of these organs.
Gastrin, released by G cells, stimulates gastric release of HCl, gastric motility and emptying and the rate of cell division in gastric regenerative cells.
Pancreatic polypeptide, a hormone of unknown function, is produced by pancreatic polypeptide cells.
- Alpha cells live on the periphery, the beta cells are more central. Beta cells are the targets of type 2 diabetes.
- Scattered through the islet are the delta cells, which make somatostatin
- In the islet, you see a huge amount of vasculature (capillaries - for putting hormones into the bloodstream)
- M label is myoepithelial cells (squeeze enzymes out into ducts)