PAROTID GLAND

The parotid gland is the largest salivary gland. It is a branched tubuloalveolar gland surrounded by a connective tissue capsule with septa—representing a component of the stroma, the supporting tissue of the gland. Adipose cells are frequently found in the stroma.

Septa divide the gland into lobes and lobules (see Figure 17-1). Septa also provide support to blood vessels, lymphatics, and nerves gaining access to the acini, the main components of the parenchyma—the functional constituent of the gland. Acini are surrounded by reticular connective tissue, a rich capillary network, plasma cells, and lymphocytes. Acini consist mainly of serous secretory cells and, therefore, are classified as serous acini.

Each serous acinus is lined by pyramidal cells with a basally located nucleus. Similar to all protein-producing cells, a prominent rough endoplasmic reticulum system occupies the cell basal region. Secretory granules are visible in the apical region (Figure 17-4).

Figure 17-4 Histologic aspects of the major salivary glands

The lumen of the acinus collects the secretory products, which are transported by long intercalated ducts to the less abundant striated ducts (Figure 17-5). The secretory product of the serous acini is modified by the secretion of the striated duct and then transported by the oral cavity by a main excretory duct (Stensen’s duct).

Figure 17-5 Structure of a mixed acinus and its striated duct

Clinical significance: Mumps, rabies, and tumors

In addition to its role in the production of saliva, the parotid gland is the primary target of the rabies and mumps virus transmitted in saliva containing the virus. The mumps virus causes transient swelling of the parotid gland and confers immunity.

Two complications of mumps are orchitis and meningitis. Bilateral orchitis caused by the mumps virus can result in sterility.

The parotid gland is the most frequent site for slow-growing benign salivary gland tumors. Its surgical removal is complicated by the need to protect the facial nerve running through the parotid gland.

SUBMANDIBULAR (SUBMAXILLARY) GLAND

The submandibular gland is a branched tubuloalveolar gland surrounded by a connective tissue capsule. Septa derived from the capsule divide the parenchyma of the gland into lobes and lobules.

Although both serous and mucous cells are present in the secretory units, the serous cells are the predominant component (see Figure 17-4). Mucous cell–containing acini are capped by serous demilunes. The intercalated ducts are shorter and the striated ducts are longer than those in the parotid gland. Adipocytes are not frequently seen in the submandibular gland.

The main excretory duct of the submandibular gland (Wharton’s duct) opens near the frenulum of the tongue.

SUBLINGUAL GLAND

Contrasting with the parotid and submandibular glands, which are surrounded by a dense connective tissue capsule, the sublingual gland does not have a defined capsule. However, connective tissue septa divide the glandular parenchyma into small lobes. The sublingual gland is a branched tubuloalveolar gland with both serous and mucous cells (see Figure 17-4), although most of the secretory units contain mucous cells. The intercalated and striated ducts are poorly developed. Usually each lobe has its own excretory duct that opens beneath the tongue.

EXOCRINE PANCREAS

The pancreas is a combined endocrine and exocrine gland. The endocrine component is the islet of Langerhans and represents about 2% of the pancreas volume. The main function of the endocrine pancreas is the regulation of glucose metabolism by hormones secreted into the bloodstream (see discussion of the islet of Langerhans in Chapter 19, Endocrine System).

The exocrine pancreas is a branched tubuloacinar gland organized into four anatomic components: (1) ahead, lying in the concavity of the second and third parts of the duodenum; (2) a neck, in contact with the portal vein; (3) a body, placed anterior to the aorta; and (4) a tail, ending near the hilum of the spleen.

The pancreas lies close to the posterior abdominal wall in the upper abdomen, and therefore it is protected from severe trauma. Blood is provided by vessels derived from the celiac artery, the superior mesenteric artery, and the splenic artery. The venous drainage flows into the portal venous system and the splenic vein. Efferent innervation is through the vagus and splanchnic nerves.

The main pancreatic duct (of Wirsung) runs straight through the tail and the body, collecting secretions from ductal tributaries. It turns downward when it reaches the head of the pancreas and drains directly into the duodenum at the ampulla of Vater, after joining the common bile duct. A circular smooth muscle sphincter (of Oddi) is seen where the common pancreatic and bile duct cross the wall of the duodenum.

The pancreas has structural similarities to the salivary glands: (1) It is surrounded by connective tissue but does not have a capsule proper. (2) Lobules are separated by connective tissue septa containing blood vessels, lymphatics, nerves, and excretory ducts.

The functional histologic unit of the exocrine pancreas is the acinus (Figures 17-6 to 17-8). The lumen of the acinus is the initiation of the secretory-excretory duct system and contains centroacinar cells that are unique to the pancreas. Centroacinar cells are continuous with the low cuboidal epithelial lining of the intercalated duct. The exocrine pancreas lacks striated ducts and myoepithelial cells. Intercalated ducts converge to form interlobular ducts lined by a columnar epithelium with a few goblet cells and occasional enteroendocrine cells.

Figure 17-6 Exocrine pancreas

Figure 17-7 Pancreatic acinus

Figure 17-8 Pancreatic acinus

Clinical significance: Carcinoma of the pancreas

The pancreatic duct–bile duct anatomic relationship is of clinical significance in carcinoma of the pancreas localized in the head region, because compression of the bile duct causes obstructive jaundice. The close association of the pancreas with large blood vessels, the extensive and diffuse abdominal drainage to lymph nodes, and the frequent spread of carcinoma cells to the liver via the portal vein are factors contributing to the ineffectiveness of surgical removal of pancreatic tumors.

Functions of the pancreatic acinus

The pancreatic acinus is lined by pyramidal cells joined to each other by apical junctional complexes (see Figure 17-8), which prevent the reflux of secreted products from the ducts into the intercellular spaces. The basal domain of an acinar pancreatic cell is associated with a basal lamina and contains the nucleus and a well-developed rough endoplasmic reticulum. The apical domain displays numerous zymogen granules (see Figure 17-8) and the Golgi apparatus.

The concentration of about 20 different pancreatic enzymes in the zymogen granules varies with the dietary intake. For example, an increase in the synthesis of proteases is associated with a protein-rich diet. A carbohydrate-rich diet results in the selective synthesis of amylases and a decrease in the synthesis of proteases. Amylase gene expression is regulated by insulin, an event that stresses the significance of the insuloacinar portal system.

The administration of a cholinergic drug or of the gastrointestinal hormones cholecystokinin and secretin increases the flow of pancreatic fluid (about 1.5 to 3.0 L/day).

The polypeptide hormone cholecystokinin, produced in enteroendocrine cells of the duodenal mucosa, binds to specific receptors of acinar cells and stimulates the release of zymogen (Figure 17-9).

Figure 17-9 Functions of the exocrine pancreas

Secretin is released when acid chyme enters the duodenum. Secretin is produced in the duodenum, binds to receptors on the surface of intercalated ductal cells, and triggers the release of bicarbonate ions and water into the pancreatic ducts. HCO₃⁻ ions and the alkaline secretion of Brunner’s glands, present in the submucosa of the duodenum, neutralize the acidic gastric chyme in the duodenal lumen and activate the pancreatic digestive enzymes.

Clinical significance: Acute pancreatitis and cystic fibrosis

Zymogen granules contain inactive proenzymes that are activated within the duodenal environment. A premature activation of pancreatic enzymes, in particular trypsinogen to trypsin, and the inactivation of trypsin inhibitor (tightly bound to the active site of trypsin), result in the autodigestion of pancreatic acini. This condition—known to occur in acute hemorrhagic pancreatitis—usually follows heavy meals or excessive alcohol ingestion. The clinical features of acute pancreatitis (severe abdominal pain, nausea, and vomiting) and rapid elevation of amylase and lipase in serum (within 24 to 72 hours) are typical diagnostic features.

Cystic fibrosis is an inherited, autosomal recessive disease affecting the function of mucus-secreting tissues of the respiratory (see Chapter 13, Respiratory System), intestinal, and reproductive systems; the sweat glands of the skin (see Chapter 11, Integumentary System); and the exocrine pancreas in children and young adults. A thick sticky mucus obstructs the duct passages of the airways, pancreatic and biliary ducts, and intestine, followed by bacterial infections and damage of the functional tissues. A large number of patients (85%) have chronic pancreatitis characterized by a loss of acini and dilation of the pancreatic excretory ducts into cysts surrounded by extensive fibrosis (hence the designation cystic fibrosis of the pancreas). Insufficient exocrine pancreatic secretions cause the malabsorption of fat and protein, reflected by bulky and fatty stools (steatorrhea).

The lack of transport of Cl⁻ ions across epithelia is associated with a defective secretion of Na⁺ ions and water. A genetic defect in the chloride channel protein called cystic fibrosis transmembrane conductance regulator (CFTR) is responsible for cystic fibrosis. The disease is detected by the demonstration of increased concentration of NaCl in sweat. Children with cystic fibrosis “taste salty” after copious sweating.

LIVER

The liver, the largest gland in the human body, consists of four poorly defined lobes. The liver is surrounded by a collagen-elastic fiber–containing capsule (of Glisson) and is lined by the peritoneum.

Blood is supplied to the liver by two blood vessels (Figure 17-10): (1) The portal vein (75% to 80% of the afferent blood volume) transports blood from the digestive tract, spleen, and pancreas. (2) The hepatic artery, a branch of the celiac trunk, supplies 20% to 25% of oxygenated blood to the liver by the interlobar artery and interlobular artery pathway before reaching the portal space.