Exocrine Glands 2024 PDF

Microscopic Anatomy EXOCRINE GLANDS Dr. Vladimir Sirotkin 307 Weiskotten Hall Addition Phone: 464-8508 Email: [email protected] Learning Objectives: 1. Learn gland classification based on their structural organization. 2. Recognize different types of sec...

Microscopic Anatomy EXOCRINE GLANDS Dr. Vladimir Sirotkin 307 Weiskotten Hall Addition Phone: 464-8508 Email: [email protected] Learning Objectives: 1. Learn gland classification based on their structural organization. 2. Recognize different types of secretions and their mechanisms. 3. Examine histological organization of complex glands: salivary glands and exocrine pancreas. Suggested reading: Junqueira’s Basic Histology, 13th edition, pp 86-97, pp 323-329, pp 378-382 Diagram of the ultrastructure of a secreting serous cell.  Microscopic Anatomy EXOCRINE GLANDS I. Exocrine gland organization and classification. A. Exocrine glands develop as invaginations of surface epithelium. Induction by connective tissue. Exocrine glands retain a connection with the surface via ducts. B. Structural organization of glands (Figure 1). Secretory portion: acinar or tubular. Acinus (plural: acini) is a berry-like cluster of cells with a central lumen. Conducting portion: ducts. Note that in some glands, ducts also have secretory functions. Figure 1. Organization of exocrine glands (Figure Figure 4-20 of Junqueira).  Microscopic Anatomy C. Classification of glands (Figure 2). Simple glands – single unbranched duct. Secretory portion can be branched or unbranched. Examples: sweat glands and sebaceous glands. Compound glands – multiple branched ducts. Secretory portion can be acinar, tubular, or tubuloacinar. Examples: salivary glands and exocrine pancreas. Figure 2. Classification of exocrine glands (Table 4-4 of Junqueira). D. Types of secretion (Figure 3). Merocrine (called eccrine in sweat glands): Exocytosis of proteins or glycoproteins. Examples: sweat glands, salivary glands, exocrine pancreas. Apocrine: Shedding of apical cell segment filled with secretory product. Example: mammary glands. Holocrine: Lysis of cells filled with secretory product. Example: sebaceous glands of skin.  Microscopic Anatomy Figure 3. Different types of secretion (Figure 4-21 of Junqueira). E. Glandular epithelium in merocrine glands. Serous cells: Stain well with H&E. Secrete proteins. Abundant RER and Golgi in the basal portion of the cell. Secretory granules in the apical cytoplasm. Mucous cells: Secrete mucin, made of glycoproteins rich in complex carbohydrates. Mucins stain poorly with H&E but can be stained with periodic acid- Schiff (PAS) stain. Abundant RER and Golgi in the basal portion of the cell. Mucin-filled secretory granules in the apical cytoplasm. Upon secretion, mucin becomes mucus, a viscous, jelly-like protective lubricant.  Microscopic Anatomy Myoepithelial cells: Located within the same basal lamina as secretory or duct cells. Possess long actomyosin-rich processes that surround the epithelial cells. Contraction helps expel secretory product from the gland lumen. Present in sweat, salivary, and mammary glands. F. Ion and fluid transport across glandular epithelia. Ion channels and pumps. Example: Na+/K+ ATPase. Mitochondria provide source of energy. Membrane specializations increase surface area. Tight junctions seal off the apical surface from the basal surface. II. Unicellular glands. Goblet cells in the lining of the small intestine and in the respiratory epithelium of the trachea. III. Organization of Multicellular compound glands. Connective tissue capsule. Septa divide parenchyma into lobules. CT stroma contains: Capillary plexus. Nerves. Lymphocytes and plasma cells. Parenchyma contains: Secretory acini or tubules. Intercalated ducts. Intralobular ducts. Interlobular ducts are in the septa. IV. Multicellular Compound Glands: Salivary glands (Figure 4). A. Histological organization of the salivary glands. Functions and composition of saliva: Moisture and lubrication. Initiation of the digestion of carbohydrates: alpha-amylase. Immune defense: lysozyme, lactoferrin and sIgA. Secretion of calcium and phosphate to make acquired pellicle. Clinical correlations: Reduced function of salivary glands due to disease or radiotherapy leads to speech difficulties, oral mucosa atrophy, and dental caries. Pleomorphic adenomas: mostly benign tumors characterized by deposition of cartilaginous material by myoepithelial cells. Infectious diseases (mumps and rabies) targeting salivary glands.  Microscopic Anatomy Figure 4. Organization of the submandibular gland (Figure 16-2 of Junqueira). Structure and organization: Connective tissue capsule. Septa – extensions of the capsule that divide parenchyma into lobules. Connective tissue contains: Capillary plexus that surrounds secretory and ductal components. Nerves that control secretion: parasympathetic stimulation and sympathetic inhibition. Lymphocytes and plasma cells. Parenchyma organization: Acini drain into intercalated ducts. Intercalated ducts drain into intralobular ducts. Intralobular ducts drain into interlobular ducts located in the septa. Acini: Types: mucous, serous or mixed. Serous cells: Pyramidal in shape. Secrete proteins including sIgA. Also secrete bicarbonate and absorb chloride. Mucous cells: Cuboidal to columnar in shape. Secrete mucin (glycoproteins rich in complex carbohydrates).  Microscopic Anatomy Myoepithelial cells (basket cells): Reside within the basal lamina. Nucleus triangular or elongated. Possess long contractile actomyosin-rich processes. Also present in ducts. Intercalated ducts: Small, 4-6 cells in circumference. Cuboidal cells that lack secretory granules. Connect the secretory portion to the intralobular duct. Intralobular ducts: More than 6 cells in circumference. Lined by simple columnar epithelium. Basal striations: basal membrane infoldings that house mitochondria. Active transport of ions: saliva contains 7x potassium, 3x bicarbonate, 1/10 sodium compared to blood plasma. Mucous glands lack striated ducts. Also secrete sIgA. Interlobular ducts: Large ducts located in septa. Lined with stratified cuboidal-to-columnar epithelium. Mechanism of sIgA secretion. Plasma cells secrete dimeric IgA. Serous cells and intralobular duct cells produce IgA receptor. IgA receptor mediates transcytosis of IgA into the gland lumen. Proteolysis of receptor produces sIgA, the IgA in complex with receptor fragment called secretory component. B. Comparison of parotid, submandibular, and sublingual glands. C. The parotid gland. Compound acinar gland. Serous acini. Secretes alpha-amylase and other proteins. Intercalated ducts are long. Intralobular ducts are striated. A diagnostic feature: the presence of abundant adipose tissue. D. The submandibular gland. Compound tubuloacinar gland.  Microscopic Anatomy Serous, mucous, and mixed acini. Mixed acini: Serous demilunes (Recent evidence suggests that these are fixation artifacts). Serous cells secrete proteins including lysozyme. Some intralobular ducts are striated. E. The sublingual gland. Compound tubuloacinar gland. Primarily mucous acini. Some mixed acini with serous demilunes. Intralobular ducts are not striated. V. Multicellular Compound Glands: The Exocrine Pancreas (Figure 5). Figure 5. Organization of a pancreatic acinus (Figure 16-9 of Junqueira). A. Structure and function. Compound acinar gland (similar to parotid). Lobules separated by connective tissue septa. Septa contain blood vessels, lymphatics, nerves, and ducts. Exocrine pancreas secretes: Digestive proenzymes including proteases, lipases, nucleases, and amylases. Regulated by cholecystokinin. Bicarbonate that is alkaline and neutralizes stomach acid, creating optimum pH for pancreatic enzymes. Regulated by secretin Pancreatic enzymes are inactive and are activated by enterokinase cleavage in the duodenum. Pancreas also produces protease inhibitors. Clinical correlation: necrotizing pancreatitis – enzyme-mediated inflammation.  Microscopic Anatomy B. Endocrine pancreas. The endocrine portion of the pancreas is represented by the islets of Langerhans. This is a specific diagnostic feature of the pancreas. C. Special features of the exocrine pancreas. Serous acini: Polarized serous cells surround small lumen. Abundant Golgi and RER. Zymogenic granules in the apical half of the cell (towards lumen). Lack myoepithelial cells. Secretion is stimulated by cholecystokinin, a hormone produced by enteroendocrine cells in duodenum. Intercalated ducts: Protrude into acini as centroacinar cells. Lack secretory granules. Produce bicarbonate. Secretin, another hormone produced by enteroendocrine cells in duodenum, stimulates bicarbonate release. Intralobular ducts: Lack striations. Also secrete bicarbonate-rich fluid. Interlobular ducts: Located in the connective tissue septa. Low columnar epithelium. VI. Medical applications. A. Adenocarcinomas, the malignant tumors of glandular epithelia, are some of the most common tumors in adults. B. Pleomorphic adenomas: mostly benign tumors of the salivary glands. C. Infectious diseases (mumps and rabies) targeting salivary glands. D. Reduced function of salivary glands due to disease or radiotherapy. E. Necrotizing pancreatitis: pancreatic enzyme-mediated inflammation due to gallstones, alcohol abuse, infections, drugs, or trauma.  Microscopic Anatomy Review questions for Exocrine Glands. 1) What are the three major mechanisms of secretion? Provide a specific example of a gland for each type. 2) What are the differences between simple and compound glands? 3) What is the function of myoepithelial cells? Where are these cells located? 4) What are the three types of acini found in the salivary glands? How do their secretions differ? 5) What is the serous demilune? Are the serous demilunes present in the living tissue? 6) What is the function of basal striations? In which glands and in what part of a gland are the basal striations found? 7) How can you differentiate the pancreas from the parotid gland on the histological preparations? 8) What is the content of zymogenic granules? 9) Which cells are responsible for producing bicarbonate in the pancreas? 10) Which hormones regulate bicarbonate and pro-enzyme secretion in the pancreas?