Digestive System Glands

BIO 408

Histology

Dr. D. L. Daley

Major Salivary Glands

n Three pairs - parotid, mandibular and sublingual

n Branched tubuloalveolar glands

n connective tissue capsule forms septa that subdivide the glands into lobes and lobules

Salivary Glands: Secretory Portions

n Secretory portions arranged in tubules and acini (alveoli)

n Types of cells

n Serous cells - seromucous cells - secrete both proteins and polysaccharides - pyramid shape

n Mucous cells - like serous cells in shape - abundant secretory granules in the apical portion of the cells

n Myoepithelial (basket) cells - share basal lamina with acinar cells - long processes that envelope the acini and intercalated ducts - facilitate release of secretory products

Salivary Glands: Duct Portions

n Ducts are highly branched

n Smallest branches - intercalated ducts - attached to the secretory acini

n Single layer of cuboidal cells & some myoepithelial cells

n Striated ducts - form from the union of several intercalated ducts

n Composed of a single layer of cuboidal or low columnar cells

Salivary Glands: Duct Portions

n Striated ducts join to form intralobular ducts - these unite to from interlobular ducts

n Interlobular ducts join to form intralobar and interlobar ducts

n The terminal or principal ducts delivers the saliva to the oral cavity

Parotid Gland

n While physically the largest gland only produces 30% of saliva

n Said to produce a purely serous secretion - but does have a mucous component

n High levels of salivary amylase (ptyalin) & IgA

Submandibular Gland

n Produces about 60% of the total saliva

n About 90% of the acini produce serous saliva

n Fatty infiltration of the connective tissue elements of the gland is common by midlife

Sublingual Gland

n Very small - almond shaped - produces only 5% of total salivary gland output

n Produce a mixed but mostly mucous saliva

n No dense secretory granules in electron micrographs of the acini - like observed in parotid and submandibular glands

Pancreas

n Situated on the posterior body wall, deep to the peritoneum

n Four regions

n 1. uncinate process

n 2. Head

n 3. Body

n 4. Tail

n 25 cm long and 5cm wide - 150 grams

n Produces both exocrine and endocrine (Islets of Langerhans) secretions

Exocrine Pancreas

n Compound tubuloacinar gland

n Produces about 1200 of a bicarbonate rich fluid containing digestive proenzymes

n 40 to 50 acinar cells form a round to oval acinus - whose lumen is occupied 3 or 4 centroacinar cells, the beginning of the duct system of the pancreas

n The presence of centroacinar cells in the center of the acinus is a distinguishing characteristic of pancreas

Pancreas: Secretory Portions

n Each acinar cell resembles a truncated pyramid

n The apex of each cell facing the lumen contains proenzyme filled secretory (zymogen) granules

n Basal cell membranes have receptors of cholecystokinin & acetylcholine

Pancreas: Duct Portions

n The duct system begins within the center of the acinus with the terminus of the intercalated ducts composed of low cuboidal cells centroacinar cells

n Centroacinar cells & intercalated ducts have receptors for secretin and ACh on the basal membranes

Pancreas: Duct Portions

n Intercalated ducts join to form larger intralobular ducts - these merge to form even larger interlobular ducts

n The interlobular duct converge onto the main pancreatic duct which joins with the common bile duct before opening in the duodenum at the papilla of Vater

Islets of Langerhans

n Five types of cells

n Beta cells - 70% - Insulin - dec blood glucose

n Alpha cells - 20% - glucagon - inc blood glucose

n Delta cells - 5% - somatostatin - paracrine - inhibits hormone release - endocrine - reduces contractions of the smooth muscle of the gut and gall bladder

n G cells - 1% - gastrin - stimulates production of HCl by stomach parietal cells

n PP cells - 1% - Pancreatic polypeptide - inhibits exocrine secretions of the pancreas

Liver

n 1500 grams - body’s largest gland

n Has exocrine and endocrine function

n Only one cell type - hepatocyte - responsible for exocrine secretion (bile) and endocrine secretions (somatomedins - stim cartilage cell div & growth)

n Approximately 100 functions such as detoxication of the blood, production of urea, production of glycogen, secretion of cholesterol in the bile, production of plasma proteins & etc.

Liver

n Hepatic plates - one or two hepatocytes thick

n Plates separates from each other by sinusoids (large capillary spaces)

n Hepatic portal system

n Products of digestion - absorbed by blood capillaries & delivered to liver

n Capillaries of digestive tract drain into the hepatic portal vein - then to liver

n Portal system - capillaries to vein to capillaries to vein

Blood Flow & Bile

n Classical conceptualization

n Hepatic plates arranged into functional units - Liver lobules

n Middle of each lobule is a central vein

n Periphery of lobule - is hepatic portal vein & hepatic artery

n Open into sinusoids - mixed

Flow of Blood & Bile

n Hepatocytes - secrete bile into bile canaliculi

n Located within each hepatic plate

n Drain to bile ducts then to hepatic ducts - away from liver

Hepatic Acinus

n More accurate view of liver function

n The acinus is roughly berry-shaped unit of liver parenchyma centered on a portal tract (terminal branches of the portal vein and hepatic artery together with bile ductules that carry bile eventually to the common bile duct to the duodenum

Bile Production

n Hepatocytes produce - 250 to 1500 ml of bile/day

n Bile

n Bile salts derived from cholesterol (from bile acids conjugated with glycine or taurine - form micelles, bile pigments (bilirubin), phospholipids (mainly lecithin), cholesterol & inorganic ions

Gallbladder

n Small, pear-shaped organ situated on the inferior aspect of the liver - 10 cm in length and 4 cm in diameter - stores about 70ml of bile

n Stores and concentrates the bile

n Mucosa of empty gallbladder is highly folded into tall, parallel ridges - these ridges disappear when the gallbladder is full and the mucosa is smooth

n Simple columnar epithelium - clear cells & infrequent brush cells

Gallbladder

n Lamina propria - vascularized connective tissue

n Smooth muscle layer is mostly obliquely oriented fibers among fewer number of longitudinally oriented fibers

n Serosa/adventitia of connective tissue