Histology and Organ Structure

Question 1

Other than O2 entering through the lungs, where do all the other atoms entering the blood stream come from?

Answer 1

• The digestive system
  
  • Cells use as building blocks
  • Nitrogen, carbon, hydrogen, oxygen, sulfur, iodine, calcium, etc

Question 2

What are the building blocks of proteins, carbohydratees and fats that the digestive system needs to pull out selectively?

Answer 2

• Proteins - amino acids
  
  • Carbohydrates - monosaccharides
  • Fats - partially digested lipids like fatty acids and monoglycerides

Question 3

What are the sphincters in place to prevent backward movement and to facilitate peristalsis?

Answer 3

• Upper esophageal
  
  • Lower esophageal pyloric
  • Ileocecal
  • Internal and external anal sphincters

Question 4

The esophagus and anus are unique in the digestive tube for what reason?

Answer 4

• They are not really covered by mucus.
  
  • The mucosae is a tissue type that has a layer of mucus at the surface for protective purposes
  • Also helps the digesting normal flora live as well

Question 5

What are the four definable layers of the digestive tube?

Answer 5

• Mucosa
  
  • Submucosa
  • Muscularis externa
  • Serosa/adventitia

Question 6

What is the muscularis externa?

Answer 6

• Comprised of an inner circularly and outer longitudinally arranged layers of smooth muscle and nerve plexes
  
  • Its main function is peristalsis and churning of the lumenal contents

Question 7

What is the submucosa?

Answer 7

• Contains connective tissue that is usually more dense than in the mucosa, larger blood vessels, nerve plexes, glands and lymphatic nodules
  
  • Lymphoid cells of various types are also scattered throughout

Question 8

What is the mucosa?

Answer 8

• Comprised of an epithelial layer plus the underlying loose, well-vascularized connective tissue called the lamina propria
  
  • Thin layer of smooth muscle underlying this is referred to as the muscularis mucosae
  • Lymphocytes, plasma cells and macrophages will be scattered throughout the lamina propria

• Mucosa. The mucosa is made up of three components: the epithelium , a supporting lamina propria and a thin smooth muscle layer, the muscularis mucosae , which produces local movement and folding of the mucosa. At four points along the tract, the mucosa undergoes abrupt transition from one form to another: the gastro-oesophageal junction, the gastroduodenal junction, the ileocaecal junction and the rectoanal junction.

Question 9

What is the serosa/adventitia?

Answer 9

• Serosa is an outer covering of squamous epithelial cells separated from the underlying muscular layers by a relatively thin layer of connective tissue
  
  • It is called the adventitia in the case of the esophagus above the diaphragm where the outer squamous layer is absent

• Adventitia. This outer layer of loose supporting tissue conducts the major vessels, nerves and contains variable adipose tissue. Where the gut lies within the abdominal cavity ( peritoneal cavity ), the adventitia is referred to as the serosa ( visceral peritoneum ) and is lined by a simple squamous epithelium ( mesothelium ). Elsewhere, the adventitial layer merges with retroperitoneal tissues.

Question 10

Describe the organization of the esophagus

Answer 10

• Muscular tube that is 2/3 1/3 split in the type of muscles that control the movements
  
  • Top 2/3 is under skelatal muscle control mostly with a mix of smooth muscle as well
  • Bottom 1/3 is all smooth muscle
  • No digestion happens here, it’s a muscular tube designed to transport food from the mouth to the stomach, thus it is missing the cornified epithelial digestive layer
  • The valve or sphincter to the stomach is not a true or complete one, but made up of tonic muscular contractions
  • Heartburn is gastric reflux through this partial sphincter which can be due to alterations in smooth muscle tone.

Question 11

How is the stomach histologically divided?

Answer 11

• Three histologically distinguishable zones
• Cardia, fundus and pyloris
• Cardia
○ Small area of mainly mucus-secreting glands around the entry of the esophagus
• Fundus
○ Main body of the stomach that secretes acid, peptic digestive products and mucus
• Pyloris
○ Secretes mainly mucus and has a relative preponderance of endocrine cells that secrete the stomach hormone gastrin

Question 12

What is special about the wall of the stomach?

Answer 12

• Contains substantial longitudinal folds called rugae or plicae mucosae that disappear upon distension
  
  • Outer muscularis externa differs from the basic pattern in that a third oblique layer of smooth muscle is present just lumenally to the circular muscular layer

Question 13

The stomach differs from the rest of the GI in the architecture of it’s mucosa. How?

Answer 13

• Facing the open cavity of the stomach are surface mucous-secreting cells
  
  • These are arranged in a myriad of folds along with an underlying lamina propria
  • Between the epithelial folds are spaces or gastric pits
  • Beginning at the bottom of the mucous secreting pits the epithelium changes and continues deep into the mucosa as one or more tubular gastric glands
  • The gastric glands contain a number of differentiated epithelial cells that are crucial for the unique funciton of the stomach: to begin the digestion of food at an acidic pH
  • The lumen of the stomach essentially extends to the very bottom of the lumen of the gastric glands, although these glands secrete components outwards
  • Humans have 3-4 million of these

Question 14

How often are surface cells replaced in the gastric epithelium?

Answer 14

• Every 3-5 days.
  
  • Main point here is that they are super rapidly dividing and constantly replenishing themselves
  • Such a high stem cell reservoir as well as a high mitotic rate will make these cells super susceptible to certain mitotically inhibitory drugs

Question 15

The stomach’s stem cells have two different routes they can take upon differentiation. What are these?

Answer 15

• They can divide and rise upward to the mucous secreting cells
  
  • OR they can migrate downward to the various specialized cells in the gastric glands

Question 16

What do the surface mucous cells do?

Answer 16

• They contain large vesicles full of stomach mucins and bicarbonate
  
  • Discharge these vesicles locally onto the surface to provide a viscous protective layer that shelters the epithelial cells against stomach acid and abrasion from churning chyme
  • Directly covering the short microvilli of the these surface cells is a more structured layer of cell surface glycoproteins, the glycocalyx

Question 17

What are the chief cells and what do they do?

Answer 17

• These cells are involved in the beginning of digestion and play less of a role in GI protection
  
  • Chief cells are typical protein secretors with apical granules and an elaborate basal RER
  • The primary secretory product is pepsinogen which gets converted to an active protease, pepsin, in the presence of acid
  • Pepsin is a protease that functions optimally at low pH
  • Chief (or peptic) cells can divide or be derived directly from stem cells

Question 18

What are the different layers of a gastric pit?

Answer 18

• Pit
○ Contains the surface mucous cells
• Isthmus
○ Cell architecture is mixed
• Neck
○ Mucous neck cells, which make a different mix of protective proteins?
○ Parietal cells start popping up here
• Fundus
○ Not to be confused with the gross fundus of the stomach as an entire organ
○ Parietal cells and chief cells, with chief cells more represented the deeper you go

Question 19

What do the parietal cells do?

Answer 19

• Unique acid-producing cells specialized to pumping H+ ions into the lumen of the gastric glands against a pretty steep gradient
• Does this using a H+/K+ - ATPase
• The pH of the gastric juice thus secreted is 1-1.5, so production of acid is a vigorous, energy demanding activity
• These cells have an extensive set of microvilli bordering canaliculi that extend well into the cell so they have an enormous surface area for pumping protons into the lumen
• They also contain a large number of mitochondria, characteristic of cells partaking in major transport functions
• Parietal cells are stimulated to produce acid by secretion of the local hormone gastrin and histamine
• Parietal cells also secrete an important glycoprotein required for uptake of vitamin B12 - intrinsic factor
○ Super important for RBC production in bone marrow

Question 20

What is up with Zollinger-Ellison syndrome?

Answer 20

• Excessive secretion of gastrin results in overproduction of HCl by parietal cells
  
  • It cannot be adequately neutralized in the small intestine and leads to duodenal ulcers and complications

Question 21

What are the enteroendocrine cells and what do they do?

Answer 21

• There are many flavors of these cells through the GI tract
• Members of a widely distributed population collectively referred to as APUD cells
○ Amine precursor uptake decarboxylation
• Many of these cells are in the GI tract, but they do not have to be
• STOMACH -
○ G cells that secrete gastrin (pyloris mostly)
○ A-cells that secrete glucagon
○ EC-cells that secrete serotonin
○ D-cells that secrete somatostatin (everywhere but middle portion of stomach)
• Cells are oriented towards vascular side, not lumenal side

Question 22

What do the A cells secrete and where are they?

Answer 22

• Cell
		○ A cell
	• Location
		○ Stomach and small intestine
	• Hormone produced
		○ glucagon
	• Hormone action
		○ Stimulates glycogenolysis by hepatocytes, thus elevating blood glucose levels

Question 23

What do the D cells secrete and where are they?

Answer 23

• Cell
		○ D cell
	• Location
		○ Stomach, small and large intestine
	• Hormone produced
		○ somatostatin
	• Hormone action
		○ Inhibits release of hormones by enteroendocrine cells in the vicinity

Question 24

What do the EC cells secrete and where are they?

Answer 24

• Cell
		○ EC cell
	• Location
		○ Stomach, small and large intestines
	• Hormone produced
		○ Serotonin
		○ Substance P
	• Hormone action
		○ Increases peristaltic movement

Question 25

What do the ECL cells secrete and where are they?

Answer 25

``` • Cell ○ ECL cell • Location ○ Stomach only • Hormone produced ○ histamine • Hormone action ○ Stimulates HCl secretion ```

Question 26

What do the G cells secrete and where are they?

Answer 26

``` • Cell ○ G cell • Location ○ Stomach and small intestine • Hormone produced ○ gastrin • Hormone action ○ Stimulates HCL secretion ○ Stimulates gastric motility especially contraction of pyloric region and relaxation of pyloric sphincter to regulate stomach emptying ○ Proliferation of regenerative cells in the body of the stomach ```

Question 27

What do the GL cells secrete and where are they?

Answer 27

``` • Cell ○ GL cell • Location ○ Stomach, small and large intestines • Hormone produced ○ glicentin • Hormone action ○ Stimulates hepatocyte glycogenolysis ○ Elevates blood glucose levels ```

Question 28

What do the I cells secrete and where are they?

Answer 28

``` • Cell ○ I cell • Location ○ Small intestine only • Hormone produced ○ cholecystokinin • Hormone action ○ Stimulates the release of pancreateic hormone and contraction of the gall bladder ```

Question 29

What do the K cells secrete and where are they?

Answer 29

``` • Cell ○ K cell • Location ○ Small intestine only • Hormone produced ○ Gastric inhibitory peptide • Hormone action ○ Inhibit HC secretion ```

Question 30

What do the Mo cells secrete and where are they?

Answer 30

``` • Cell ○ Mo cell • Location ○ Small intestine only • Hormone produced ○ motilin • Hormone action ○ Increases intestinal peristalsis ```

Question 31

What do the N cells secrete and where are they?

Answer 31

``` • Cell ○ N cell • Location ○ Small intestine only • Hormone produced ○ neurotensin • Hormone action ○ Increases blood flow to ileum and decreases peristaltic action of small and large intestines ```

Question 32

What do the PP cells secrete and where are they?

Answer 32

``` • Cell ○ PP cell or F cell • Location ○ Stomach and large intestine • Hormone produced ○ Pancreatic polypeptide • Hormone action ○ unknown ```

Question 33

What do the S cells secrete and where are they?

Answer 33

``` • Cell ○ S cell • Location ○ Small intestine only • Hormone produced ○ secretin • Hormone action ○ Stimulates release of bicarbonate-rich fluid from pancreas ```

Question 34

What do the VIP cells secrete and where are they?

Answer 34

• Cell ○ VIP cell • Location ○ Stomach and small and large intestines • Hormone produced ○ Vasoactive intestinal peptide • Hormone action ○ Increases peristaltic action of small and large intestines ○ Stimulates elimination of water and ions by GI tract

Question 35

What are the three segments of the small intestine?

Answer 35

* Duodenum * Jejunum * Ileum

Question 36

Describe (general) the environment change between the stomach and duodenum

Answer 36

* Upon relaxation of the pyloric sphincter between the stomach and duodenum, chyme at a low pH passes into duodenum * Digestion continues in duodenum but way different pH * Digestion is from enzymes released by pancreas and enzymes present at the surface of the intestinal mucosa itself

Question 37

What are goblet mucous cells?

Answer 37

• Goblet mucous cells are scattered between absorptive/digestive cells ○ Produce mucous for protection/lubrication ○ Least abundant in duodenum

Question 38

What are plicae circulares

Answer 38

• Small intestine structures that help achieve the need for tons of surface area to facilitate absorption as well as digestion • Permanent transverse-oriented folds • Project into lumen about 1cm • Plicae are covered with villi that increase the surface area about 8 fold • The enterocytes themselves help to increase surface area even more with microvilli at the surface • Distributed over the microvillar surface is a glycocalyx into which glycoproteins of the plasma membrane extend • Digestive processes occur within the glycocalyx as digestive enzymes are found in this matrix • Goblet mucous cells are scattered between absorptive/digestive cells ○ Produce mucous for protection/lubrication ○ Least abundant in duodenum

Question 39

What are the different glands in the small intestine (early on) and what do they do?

Answer 39

• Crypts of lieberkuhn ○ Simple, tubular glands penetrate from the base of the villi deeper into the mucosa ○ Epithelium of the crypts is continuous with surface epithelium ○ Stemm cells are most abundant in lower 1/3 of crypts and replenish the other cells § Mucous cells § Enterocytes § Paneth cells ○ Paneth cells § Contain large esosinophilic granules which contain antibacterial peptides called defensins in addition to lysozyme and phospholipase • Brunner's glands ○ Only found in duodenum and release lots of bicarb into the crypts to neutralize the acidic chyme from the pyloris

Question 40

What are Brunner's glands?

Answer 40

• Brunner's glands | ○ Only found in duodenum and release lots of bicarb into the crypts to neutralize the acidic chyme from the pyloris

Question 41

What are the Crypts of lieberkuhn?

Answer 41

• Crypts of lieberkuhn ○ Simple, tubular glands penetrate from the base of the villi deeper into the mucosa ○ Epithelium of the crypts is continuous with surface epithelium ○ Stemm cells are most abundant in lower 1/3 of crypts and replenish the other cells § Mucous cells § Enterocytes § Paneth cells ○ Paneth cells § Contain large esosinophilic granules which contain antibacterial peptides called defensins in addition to lysozyme and phospholipase

Question 42

Describe the organization of the intestinal villi.

Answer 42

• Highly structured • Contain a loose lamina propria core within which small blood vessels and capillaries course • Also contain a scattering of lymphocytes • Also a series of small lymphatic spaces that join a larger lymphatic vessel in the center - lacteal • Lacteal can transport fluid entering from lumen of intestin • Lacteal also transports lipoprotein droplets called chylomicrons ○ Enterocytes suck up building blocks from lumen then pump di/triglycerides into lacteal • Blood from capillaries goes to hepatic portal system • Lacteal fluid to larger lymphatics and eventually thoracic duct

Question 43

Describe generally the work of the peyer's patches

Answer 43

• Lymphatic nodules and groups of nodules within the GI tube • Specialized epithelial cells - M cells - function as antigen-uptake cells ○ Phagocytose luminal contents and present antigens to underlying lymphocytes and macrophages • Plasma cells of the nodules release IgA class immunoglobulins • IgA binds to receptors on epithelial cells and are transcytosed to the lumenal surface where they serve as antibacterial neutralizers and opsinizers ○ Discourage inappropriate bacterial colonization

Question 44

Where are brunner's glands in the small intestine?

Answer 44

``` • Duodenum ○ present • Jejunum ○ absent • Ileum ○ absent ```

Question 45

Where are lymphatic tissue patches in the small intestine?

Answer 45

``` • Duodenum ○ A bit • Jejunum ○ more • Ileum ○ A TON ```

Question 46

Where are Goblet cells in the small intestine?

Answer 46

``` • Duodenum ○ A bit • Jejunum ○ more • Ileum ○ The most ```

Question 47

Where are plicae circularres in the small intestine?

Answer 47

``` • Duodenum ○ A bit • Jejunum ○ The most • Ileum ○ A bit ```

Question 48

Describe the distribution of villi in the small intestine?

Answer 48

``` • Duodenum ○ Most numerous • Jejunum ○ Decrease distally • ileum ○ Least abundant ```

Question 49

What are the major pancreatic enzymes that are zymogens?

Answer 49

* Zymogens are enzymes that require some break-down or modulation event to become active * Trypsin * Chymotrypsin * Elastase * Carboxypeptidase * Triacylglycerol lipase

Question 50

How is the acinar pancreas organized? | • Acini are clusters of pancreatic acinar cells that are arranged at the end of a common duct

Answer 50

○ Cul-de-sac houses at the end of a dead-end street • Basal portion of these cells is full of rough endoplasmic reticulum that synthesizes protein for secretion • The apical portion is full of secretory granules galled zymogen granules which contain the packaged product for secretion into the ducts

Question 51

What are centroacinar cells?

Answer 51

* Found in the acini, which represent the beginning of the duct system * Duct cells secrete much of the volume of the pancreatic juice, including water and bicarbonate to help the brunner's glands neutrallize the acid arriving from stomach * Secretion is under control of both secretin and cholycystokinin * 98% of the pancreas is dedicated to exocrine function

Question 52

What is the purpose of the whole zymogen thing?

Answer 52

* We want these enzymes active in the lumen of the GI, not while they are en route to the lumen * If these enzymes were active in the pancreas they'd create problems by digesting the organ itself

Question 53

What are the two pancreatic enzymes that are not zymogens but are active at the time of secretion?

Answer 53

• Amylase ○ Degrades starch to glucose and maltose • Ribonuclease ○ Cleaves RNA

Question 54

How does trypsin become active?

Answer 54

• Trypsinogen is a pancreatic enzyme and is activated by a proteolytic cleavage by enterokinase ○ Enterokinase is not pancreatic ○ It's on the surface of dutodenal digestive/absorptic cells § Enterocytes • Trypsin then activates the other pancreatic zymogens by proteolysis

Question 55

Give two examples why it behooves the GI tract to have the last stages of breakdown happen within the glycocalyx just on the surface of the enterocytes?

Answer 55

• Examples of amylase function and glucose absorption • Amylase ○ Makes maltose and isomaltose which are broken down by membrane anchored enzymes in the apical plasma membrane of enterocytes ○ Glucose is the result and now the enterocytes get all the glucose and the bacteria don't • Lactase is also an example ○ If people are deficient in MEMBRANE-ASSOCIATED lactase then the bacteria get all the lactose and they get gassy and bloated

Question 56

The externa, the muscular layer of the small intestine, is innervated by what?

Answer 56

* Efferent fibers of the myenteric nerve plexus * Also the plexus of Auerbach * Clusters of ganglion cell bodies are present between the two layers of the externa

Question 57

What structures of the small intestine are not present in the large intestine?

Answer 57

* Plicae and villi | * The large intestine is much more smooth

Question 58

What are columns of morgagni?

Answer 58

• Longitudinal structural folds of the mucosa

Question 59

Describe the organization of the large intestine

Answer 59

• Grossly the cecum, appendix, and the different parts of the colon, then the rectum • Contains numberous straight tubular glands or crypts and the epithelial layer has two cell types, abundant mucous-producing cells and absorptive cells • Main function is recovery of water and salt during conentration of fecal material • 2/3 of large intestine wall is muscular with a large band of circular smooth muscle • Muscular specializations are observed in the longitudinal layer ○ Most of this consists of bands called taeniae ○ Segmented contraction of the taeniae causes sacculation of the bowel which compresses and segments fecal material ○ At the anus, the circular layer is thickened to form the internal anal sphincter, and downstream is circular striated muscle, the external anal sphincter

Question 60

What are the three main salivary glands?

Answer 60

* Submandibular glands * Sublingual gland (singular) * Parotid glands * Serous, mucous or mixed secretions * Parotid - serous secretor * Submandibular - mixed * Sublingual - mucous * Glands are organized based on ancinar design * Contraction of myoepithelial cells serve to propel salivary secretions from the acini * Serous secretions are watery and contain enzymes like amylase, Rnase and dnase * Mucous secretions are lubricative and protective * Serous epithelial cells also transport IgA class immunoglobulins that, together with lysozyme and peroxidase, provide antibacterial action * Duct cells modify the ionic contents of the saliva as the contents move toward major ducts