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HISTOLOGY & CELL BIOLOGY Mr. BABATUNDE, D.E DIGESTIVE

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HISTOLOGY & CELL BIOLOGY Mr. BABATUNDE, D.E DIGESTIVE HISTOLOGY & CELL BIOLOGY Mr. BABATUNDE, D.E DIGESTIVE GLANDS I EXTRINSIC GLANDS ❖ Of the digestive system are located outside the wall of the alimentary canal and deliver their secretion into the lumen via a system of ducts. ❖ Provide enzymes, buffers, emulsifiers, and lubricants for the digestive tract, as well as hormones, proteins, globulins, and numerous additional products for the remainder of the body. ❖ Are the salivary glands (parotid, sublingual, and submandibular), the pancreas, and the liver. MAJOR SALIVARY GLANDS Parotid Gland Classification ❖ Purely serous, compound, tubuloalveolar gland. Capsule ❖ Formed by the continuation of the superficial cervical fascia, is mostly collagenous in nature. ❖ Forms broad bands of trabeculae (septa) that subdivide the gland into lobes and lobules. Trabeculae ❖ Convey blood and lymph vessels, ducts, and nerves into the substance of the gland. ❖ Often contain fat cells in older individuals. Acini ❖ Although are said to be composed of purely serous cells, they are actually seromucous in character. ❖ Are surrounded by myoepithelial cells. ❖ Their center contains the lumen. Parotid gland : only contains serous secretory units Figure 16—2. Photomicrograph of a parotid gland. Its secretory portion consists of serous amylase—producing cells that store this enzyme in secretory granules. Intralobular (intercalated and striated) ducts are also present. Pararosaniline— toluidine blue (PT) stain. Medium magnification. Acinar Cells ❖ Pyramidal cells, whose apical aspects usually contain secretory granules. ❖ Nucleus is round and basally located. ❖ Cytoplasm contains extensive rough endoplasmic reticulum, a well-developed Golgi apparatus, and numerous mitochondria. Myoepithelial Cells ❖ Are stellate shaped. ❖ Their cytoplasm is difficult to discern with the light microscope. ❖ Electron micrographs demonstrate that they resemble smooth muscle cells and are contractile. Ducts ❖ Are of various diameters. Stensen’s Duct ❖ Is the largest (main parotid duct). ❖ Opens into the oral vestibule at the parotid papilla. ❖ Is lined by a simple columnar or pseudostratified epithelium. Striated Ducts ❖ Are lined by high cuboidal cells. ❖ Have basal striations that parallel the longitudinal axis of the cell. ❖ Striations are due to deep infoldings of the basal plasma membrane, which compartmentalize mitochondria. Intercalated Ducts ❖ Are the smallest ducts. ❖ Are lined by a simple cuboidal epithelium and lie interposed among the acini, where they constitute the initial part of the duct system. Sublingual Gland Classification ❖ Mixed, compound, tubuloalveolar gland. Capsule ❖ See preceding discussion of parotid gland. Acini ❖ Are mostly mucous but capped with serous demilunes. Occasionally, purely mucous acini are present, but purely serous units are not present. Acinar Cells ❖ The mucous cells are pyramidal shaped, with their nuclei flattened against the basal plasma membrane. ❖ Light micrographs display pale blue cells with a “frothy” appearance (with hematoxylin and eosin). ❖ Electron micrographs demonstrate that the apical region of the cell contains a large number of mucinogen granules. ❖ Organelles of the cell (Golgi apparatus, rough endoplasmic reticulum, and mitochondria) are located between the granules and the nucleus. Mucous Acinar Cells Mucous gland Mixed gland Serous Demilune Cells ❖ Are located peripheral to the mucous acini and deliver their Mucous cells secretions into their spaces Demilume between the ( serous cells) neighboring mucus-producing cells. Ducts ❖ Short intercalated and striated ducts are present, but are few in number. Figure 16—5. Photomicrograph of a sublingual gland showing the predominance of mucous cells. H&E stain. Low magnification. Sublingual gland : contains both pure mucous & mixed secretory units Mucous Mixed secretory secretory units unit Submandibular Gland Classification ❖ Mixed, compound tubuloalveolar gland. Capsule ❖ See preceding discussions of parotid and sublingual glands. Acini ❖ Composed mostly of serous acini; however, groups of mucous acini with serous demilunes are also present. Submandibular gland : contains both pure serous & mixed secretory units serous secretory units mixed secretory units Figure 16—4. Photomicrograph of a submandibular gland. Note the presence of dense serous cells forming demilunes and pale-staining mucous cells grouped along the tubular portion of this tubuloacinar gland. Medium magnification. Acinar Cells ❖ See discussions of parotid and sublingual glands. Myoepithelial Cells ❖ See discussions of parotid and sublingual glands. Ducts ❖ Intercalated ducts are short, but present; striated ducts are long and clearly evident. Saliva Volume ❖ Average daily production is 1.0 to 1.2 liters. Function ❖ Protects the oral cavity by lubrication, control of bacterial flora (by the presence of lysozyme, lactoferrin, and IgA), and its cleansing action. ❖ Assists in taste sensation. ❖ Initiates digestion of carbohydrates via action of its enzyme, salivary amylase. Primary Secretion ❖ Saliva secreted by the acini, prior to modification by the system of ducts, resembles extracellular fluid. Final Secretion ❖ Intercalated ducts may deliver bicarbonate ions into the saliva. ❖ Striated ducts (via their sodium pump) remove sodium and chloride ions from the luminal fluid and actively pump potassium ions into it. Saliva ❖ Is a hypotonic solution PANCREAS ❖ Is a retroperitoneal gland that produces both exocrine and endocrine secretions. ❖ Is subdivided into the following regions: head, uncinate process, neck, body, and tail. Tail contains the highest concentration of endocrine tissue (islets of Langerhans). Figure 16—6. Photomicrograph of a pancreas showing the exocrine portion (acini) and the endocrine portion (islet of Langerhans). The acini contain secretory cells with basophilic cytoplasm. Different types of endocrine cells are seen in the islet. PT stain. Medium magnification. Capsule ❖ Is composed of delicate connective tissue that subdivides the gland into lobules by forming numerous septa. Septa convey blood and lymph vessels, nerves and ducts in and out of the gland. Exocrine Pancreas Classification ❖ A purely serous, compound tubuloalveolar gland. Exocrine Pancreas Acini ❖ Only serous, whose centers contain centroacinar cells, which form the beginning of the duct system in the pancreas. Figure 16—7. Schematic drawing of the structure of pancreatic acini. Acinar cells are pyramidal, with granules at their apex and rough endoplasmic reticulum at their base. The intercalated duct partly penetrates the acini. These duct cells are known as centroacinar cells. Note the absence of myoepithelial cells. Intercellular Canaliculi ❖ Lead from between acinar cells to the lumen of the acinus. Acinar Cells ❖ Are pyramidal in shape and have a round nucleus that is basally located. ❖ Basal region is strongly basophilic. ❖ Apical region is densely packed with secretory granules (zymogen granules). Figure 16—9. Electron micrograph of an acinar cell from a rat pancreas. Note the nucleus (N) surrounded by numerous cisternae of rough endoplasmic reticulum (RER) near the base of the cell. The Golgi complex (G) is situated at the apical pole of the nucleus and is associated with several condensing vacuoles (C) and numerous mature secretory (zymogen) granules (S). The lumen (L) contains proteins recently released from the cell by exocytosis. x8000. ❖ Electron micrographs demonstrate abundant rough endoplasmic reticulum, an extensive Golgi apparatus, numerous mitochondria, and many free ribosomes. ❖ Secretory granules are all membrane bound and are occasionally noted to be in the process of being released in “chains” that extend to the lumen of the acinus. Figure 16—10. Electron micrograph of the apex of 2 Acinar lumen pancreatic acinar cells (A) and a centroacinar cell (C) from a rat pancreas. Note the lack of secretory granules and the very scant rough endoplasmic reticulum in the centroacinar cell as compared with the acinar cell. L, acinar lumen. x30,000. Myoepithelial cells ❖ are not present in the pancreas Ducts ❖ Extend from within the acini. Centroacinar Cells ❖ Are low cuboidal in shape, and they constitute the initial part of the intercalated duct. Figure 16—7. Schematic drawing of the structure of pancreatic acini. Acinar cells are pyramidal, with granules at their apex and rough endoplasmic reticulum at their base. The intercalated duct partly penetrates the acini. These duct cells are known as centroacinar cells. Note the absence of myoepithelial cells. Centroacinar cell Centroacinar cell Intercalated Ducts ❖ Lead into a small number of intralobular ducts, which in turn empty into the large interlobular ducts. Interlobular Ducts ❖ Are located between lobules in extensive connective tissue and empty into the main (or accessory) pancreatic duct. Pancreatic Duct ❖ The main pancreatic duct, delivers the pancreatic secretions into the duodenum at the papilla of Vater. Histophysiology ❖ Two hormones, released by APUD cells of the digestive tract, control the release of pancreatic secretions. Cholecystokinin (pancreozymin) ❖ Induces the acinar cells to release pancreatic juices rich in digestive enzymes. ❖ The enzymes are trypsin, chymotrypsin, peptidase, pancreatic amylase and lipase, and ribo- and deoxyribonucleases Secretin ❖ Induces the intercalated ducts to secrete large quantities of an enzyme-poor, alkaline fluid that probably functions in neutralizing the acidic chyme that enters the duodenum. Endocrine Pancreas Islets of Langerhans ❖ Are spherical clumps
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