Organs Associated With The Digestive Tract

Chapter 16 Organs associated with the digestive tract

General outline Components: salivary glands, the pancreas, the liver, and the gallbladder. These organs are located outside the digestive tract and have ducts open into the digestive tract. (exocrine gland) Functions: Ø Salivary gland: produce saliva Ø Pancreas: produce digestive enzymes and hormones Ø Liver: produce bile and synthesize blood proteins Ø Gallbladder: store and concentrate the bile.

Formation of glands from covering epithelia. Epithelial cells proliferate and penetrate connective tissue. They may or may not maintain contact with the surface. When contact is maintained, exocrine glands are formed; without contact, endocrine glands are formed. The cells of endocrine glands can be arranged in cords or in follicles.

Principal types of exocrine glands. The part of the gland formed by secretory cells is shown in black; the remainder shows the ducts. The compound glands have branching ducts.

1.Salivary glands  Include parotid, submandibular and

sublingual glands

General structure A capsule and septa (CT)

lobules

parenchyma (secretory end pieces and a duct system) Septa

(1) Secretory end pieces

①.Serous cell and serous acinus LM: Usually pyramidal in shape; cytoplasm is deep stained  nucleus are spherical in shape and near the base. EM:  rich in RER, Golgi complex dark secretory granules. Function: Protein-secreting cells

Serous acinus

Lumen G

RER

RER

②.Mucous cell and mucous tubule LM: Usually cuboidal to columnar in shape; cytoplasm are light stained  nucleus are flattened ovoid in shape and close to the base. EM: RER, Golgi complex, light secretory granules Function: secrete mucoproteins

Mucous tubule

G

③Serous demilune and mixed acinus The mucous cells form tubules, their end are capped by serous cells, which constitute the serous demilunes. Mixed acinus: composed of both serous and mucous cells, mainly mucous tubule with a serous demilune.

④.Myoepithelial cell Situation: are found between the basal lamina and the basal plasma membrane of the cells forming secretory end pieces and intercalated ducts. Stellate (basket cells) or spindle shaped . Establish intercellular junctions among themselves and with secretory cells. Function: Фcontraction: helping to squeeze out secretion from acini. Фprevention: prevent the end pieces distention .

(2) The duct system

①.Intercalated ducts The initial portion of the duct system. Simple cuboidal epithelium Have the ability to divide and differentiate into secretory cells or ductal cells.

②. Striated duct

?

LM: simple columnar epithelium; the nucleus is near the cell apex; cytoplasm is acidophilic; has basal striations. EM: the basal striations created by membrane infolding and mitochondia Function: reabsorbing sodium and excreting potassium; transport water and ions

*Intralobular duct: include intercalated and striated ducts. ③.Interlobular duct Location: CT septa Wall: pseudostratified or stratified cuboidal epithelium

④.Main duct Location: CT septa Wall : stratified columnar epithelium nonkeratinized-stratified squamous epithelium

(3)Three types of salivary glands

①.Parotid gland A branched acinar gland. entirely serous. longer intercalated duct and striated duct. secrete products contain more salivary amylase.

②.Submandibular gland A branched tubuloacinar gland. mixed gland. more serous acini, less mixed or mucous acini short intercalated duct, longer striated duct secrete products contain less salivary amylase, more mucus

③.Sublingual gland A branched tubuloacinar gland . Mixed gland, mainly mucous acini. without intercalated duct, rare striated ducts. secrete products are mainly composed of mucus.

2. Pancreas ＊

A mixed exocrine-endocrine gland;

A capsule and septa (CT)

lobules

Exocrine portion parenchyma Endocrine portion

A.Exocrine portion ( compound tubuloacinar gland) 1.Acini a. entirely serous--basophilia of basal cytoplasm(RER); superanuclear Golgi complex; Zymogen granules in apical cytoplasm. b. centroacinar cells—small, pale stained cells in the lumen of acini, beginning of the intercalated ducts. c. absence of myoepithelial cells

2.Ducts: a. Long intercalated ducts (squamous epithelium), no striated ducts. b. Intralobular ducts (large, cuboidal) → interlobular ducts (columnar) → main pancreatic duct (high columnar with goblet cells).

3.The functions of exocrine portion ФSecret pancreatic juice containing water, ions, and several proteases (trypsinogen, chymotrypsinogen), amylase, lipase, etc. (The majority of the enzymes are stored as proenzymes in the secretory granules of acinar cells.)

Фsynthesize the protease inhibitors. In acute pancreatitis, the proenzymes may be activated and digest the whole pancreas, leading to very serious complications. Possible causes are alcoholism, gallstones, metabolic factors, infection, and drugs.

B. Endocrine portion (islet of Langerhans): 1. They appear as rounded clusters of cells embedded within exocrine portion, more abundant in the tail of pancreas. 2.Cells are polygonal or rounded, lightly stained, arranged in cords separated by a network of capillaries( fenestrated). 3. Trichrome stain allow the recognition of acidophils(α) and basophils(β).

4.Using immunocytochemical methods, four types of cells –A, B, D, and F—have been recognized.

B cells

A cells

D cells

Endocrine portion Cell type

Quantity

Distribution

Hormone produced

Hormone function

A cells

~20%

Peripheral

Glucagon

Acts on several tissue to make energy stored in glycogen and fat. Increases blood glucose content

B cells

~70%

center

Insulin

Acts on several tissue to cause entry of glucose into cells and promotes decrease of blood glucose content

D cells

<5%

Variable

Somatostatin

Inhibit release of other islet cell hormones

F cells

rare

variable

Pancreatic polypeptide

Control of gastric secretion? Control of secretion of the exocrine pancreas?

Diabetes A disease in which the body's production and use of insulin is impaired. As a result, the level of sugar in the blood is too high. There are many types of diabetes, but the most common are type Ⅰ, type Ⅱ and gestational. Insulin-dependent or type Ⅰdiabetes (juvenile diabetes) results from partial or total destruction of B cells due to autoimmune disease. Insulin-independent diabetes or type Ⅱ diabetes occurs at a later stage in life and is frequently associated with obesity.

Liver Largest digestive gland Nutrients are processed and stored for use by other parts of the body. Produce bile for lipid digestion Neutralize and eliminate toxic substance

Capsule: a thin connective tissue liver plates

Liver

Liver lobule Liver sinusoid Central vein Interlobular arteries Portal space Interlobular veins Interlobular bile ducts

Liver lobule: Dimensional aspect of the normal liver

A polygonal prism Central vein: occupies the centre of the liver lobule . liver plates: are composed of a single row of hepatocytes arranged in radial  liver sinusoid: situated between the liver plates, forming a complex network

Three-dimensional aspect of the normal liver. In the upper center is the central vein; in the lower center is the portal vein. Note the bile canaliculus, liver plates, Hering’s canal, Kupffer cells, sinusoid, fat-storing cell, and sinusoid endothelial cells.

Liver lobule: Plan A polygon Central vein: occupies the centre of the liver lobule . liver cord: are composed of a single row of hepatocytes arranged in radial  liver sinusoid: situated between the liver cords, forming a complex network

Pig liver

1. Central vein ●Terminal

hepatic venules with numerous pores in their very thin walls. ●Its

endothelial cells are continuous with those of the sinusoids.

2.Hepatocytes  polyhedral in shape, eosinophilic cytoplasm, one or two large rounded nuclei with one or 2 typical nucleoli  EM: Mi, RER, SER, Golgi complex, lysosomes, microbodies, inclusions.

Mitochondria provide the energy for the hepatocytes

Rough endoplasmic reticulum synthesize plasma proteins

Golgi complex participate in the formation of bile and lipoprotein

Smooth endoplasmic reticulum synthesize bile, triglyceride and LDL metabolism of the lipid, hormones and cholerythrin inactivate steroid hormone biotransformation of some materials detoxification of noxious substances

Lysosomes actively participate the metabolism of hepatocyte and renewal of organelles play a role in metabolism and transport of bilirubin storage of iron

Microbodies ＊ Detoxification: catalase and peroxidase; ＊ Reduce the hydrogen peroxide (H2O2) into H2O

Inclusions ＊ Include glycogen, lipid droplets, pigment etc; ＊ These contents vary according to physiologic state of human body

3.Bile canaliculi Formed by indented plasma membranes of opposing hepatocytes. ＊ Microvilli extend into canalicular lumen which contains bile, secretion of liver. ＊ Junctional complex including tight junctions around canaliculi isolate them from other portion of intercellular space, preventing bile from entering spaces of Disse(?). ＊

Bile canaliculi

Common bile duct duodenum

Cause jaundice (skin & sclera)

4.Liver sinusoid a. Large and irregular sinusoidal capillaries between hepatic plates, containing both venous and arterial blood. b. Lined with endothelium that is thin, discontinuous, fenestrated, no basal lamina. c. Have Kupffer cells, part of mononuclear phagocytic system. They are irregular, phagocytic, and contain a number of lysosomes.

5.Space of Disse a. The space between endothelial cells and hepatocytes. b. Contains fluid similar to plasma and microvilli from liver cells. c. Fat-storing cells are found in the space, which store vitamin A and produce type Ⅲ collagen fibers (reticular fibers). d. The site of exchange of materials between blood and hepatocytes.

Hepatic cirrhosis : In chronically diseased liver, fat-storing cells are activated by factors released by hepatocytes and Kupffer cells, proliferate, and aquire the features of myofibroblasts. It is characterized by the replacement of normal tissue with fibrous tissue and the loss of functional liver cells. It can result from alcohol abuse, or infection, especially by the hepatitis virus.

Portal space: ＊ connective tissue with portal triad at the corners among adjacent hepatic lobules. 1. Interlobular veins -- branches of the portal vein. 2. Interlobular arteries -- branches of the hepatic artery. 3. Interlobular bile ducts -- intrahepatic branches of the hepatic duct.

Human liver

pig liver

Blood Supply: portal V.

interlobular V.

（ functional vessel)

hepatic A. (nutrient vessel)

interlobular A.

sinusoids

central V. sublobular V. hepatic V. Inferior vena cava

Discharge way of bile: Bile canaliculi Hering’s canals Interlobular bile ducts

Hepatic duct Common bile duct duodenum

Cystic duct Gallbladder

Functions of liver:  Bile secretion  Synthesize: protein, glycogen, cholesterin  Neutralize and eliminate toxic substance (Bilirubin)  Defence  Hemopoiesis

Gallbladder Mucosa Simple columnar Epi. , no Goblet cells Laminar propria: C.T.

muscle layer composed of an inner circular, an outer longitudinal layers of smooth muscle

Adventitia most are fibrosa, some are serosa

What kinds of organs are these?

①

②

③

④

⑤

⑥

KEY POINTS General structure of salivary gland Structure and function of exocrine portion of pancreas Structure and function of endocrine portion of pancreas Structure and function of hepatic lobule Structure features of portal space