APR - WS: BLMW Changing Function through the GI Tract

Question 1

What are the 5 major stages that digestion is split into?

Answer 1

1. Ingestion
2. Fragmentation
3. Digestion
4. Absorption
5. Elimination

Question 2

What are the four basic layers of the GI tract?

Answer 2

The basic layout is 4 layers:

1. Mucosa
2. Submucosa
3. Muscularis propria (externa)
4. Adventitia or Serosa

Question 3

Describe the mucosa of the GI tract.

Answer 3

Mucosa consists of three layers:

1. Specialised surface epithelium: resting on a basal lamina – it is in contact with the luminal content
2. Lamina propria: consisting of loose connective tissue with blood and lymph vessels, nerve fibres and smooth muscle. It is supporting surface epithelium.
3. Muscularis mucosae: a thin double layer of smooth muscle (delimits mucosa from submucosa), controlled by parasympathetic Meissner’s (submucosal) plexus.

Question 4

Describe how the epithelium of mucosa is modified according to its function in each part of the GI tract.

Answer 4

PROTECTIVE:

• oral cavity
• pharynx
• oesophagus

SECRETORY:
- stomach

ABSORPTIVE:
- small intestine

ABSORPTIVE + PROTECTIVE:
- large intestine

PROTECTIVE:
- anal canal

Question 5

Describe the submucosa of the GI tract.

Answer 5

The submucosa consists of loose connective tissue with:

• Blood vessels
• Nerves which form the plexus of the submucosa (Meissner’s plexus).
• Leukocytes
• Variable amounts of lipid
• Glands (in oesphagus/duodenum)

Question 6

Describe the muscularis externa (propria) of the GI tract.

Answer 6

The muscularis externa (propria) consists of two thick layer of smooth muscle (except the stomach):

• Inner circular (helical with a low pitch)
• Outer longitudinal (helical with a steep pitch)

Between the inner and outer layers of muscularis externa is a plexus of nerves: Auerbach’s plexus or myenteric plexus.

It provides motor innervation to muscularis externa and has both parasympathetic and sympathetic input.

Question 7

Describe the adventitia/serosa of the GI tract.

Answer 7

It is a thin layer of connective tissue continuous with surrounding organs.

It is termed serosa in more mobile parts.

It is bound by simple, squamous epithelium - termed mesothelium in the peritoneal cavity.

Question 8

What type of cells is the mucosa of the oesophagus lined with?

What properties do those cells have?

Answer 8

The mucosa is lined with stratified (squamous) epithelium cells.

They provide physical protection in the oral cavity, pharynx, oesphagus,
anal canal, uterine cervix and vagina.

Those sites are subject to mechanical abrasion but are kept moist by glandular secretions.

The cells remain nucleated.

Question 9

How does the layer composition change down the oesophagus to help with swallowing?

Answer 9

The muscularis externa has skeletal muscle at top of the oesophagus, as the initiation of swallowing is voluntary.

It then transitions and is replaced entirely by smooth muscle.

Question 10

How can we tell that it is the gastro-oesophageal junction?

Answer 10

There is an abrupt transition from protective stratified squamous epithelium of the oesophagus to the glandular secretory mucosa of the cardia of stomach.

Question 11

What is the function of the stomach?

Answer 11

It is the mechanical and biological/chemical (pepsin/HCl) breakdown of food to soft fluid (chyme).

it also has limited absorption e.g. water, alcohol, drugs.

Question 12

What are gastric rugae?

Answer 12

They are ridges on the stomach wall produced by
folding of internal
surface of stomach.

It allows the stomach to
expand to accommodate food by unfolding the rugae. The rugae then reform as
the stomach empties.

Question 13

Describe the Gi layers in the stomach.

Answer 13

1. secretory mucosa with gastric pits (foveoli) and gastric glands
2. muscularis mucosae
3. submucosa (no glands)
4. muscularis externa (propria) with 3 layers for strong churning action for mechanical breakdown and mixing to make chyme, also respond to hormone gastrin. It is made up of:
   - Oblique (O) layer
   - Circular (C) layer
   - Longitudinal (L) layer

Question 14

Describe how the gastric pits/glands vary in secretion and divide the stomach into 4 main regions.

Answer 14

Cardia Mucosa:
Glands are sparse, mainly mucus secreting

Fundus/Body Mucosa:
Glands are densely packed: producing mucus and gastric juice (acid and enzymes)

Pyloric Mucosa:
Deep gastric pits, branching, coiled glands mucus secreting.
Endocrine cells secrete Gastrin

Question 15

What type of cell is the mucosa of the stomach lines with?

Answer 15

The mucosa of the stomach is lined with simple columnar epithelium cells.

They are often found on absorptive surfaces such as small intestine, and secretory surfaces such as stomach.

Question 16

How do the simple columnar cells create the tubular gastric glands?

Answer 16

The simple columnar epithelium invaginates to form gastric pits
connected to 1-7 simple tubular gastric glands.

Question 17

What types of cells are found in the gastric pits/glands?

Answer 17

Surface mucous cells: secrete mucus and bicarbonate to protect cells from acidic and enzymatic properties of gastric juices and ingested food (pale staining).

Mucous Neck Cells: line upper parts of gastric glands and secrete mucous

Stem cells: found in the neck region, divide every 3 days approximately to replace all other cells types

Endocrine Cells: e.g. G Cells secrete peptide hormone gastrin. Others secrete
hormones including serotonin and somatostatin.

Question 18

Describe parietal cells (aka oxyntic).

Answer 18

• large rounded eosinophilic cells with many mitochondria and centrally located nucleus (appearance: pale pink fried egg)
• they are found throughout the gastric gland but are most numerous in the isthmus (top part)
• secrete HCl via canaliculi in response to gastrin
• secrete intrinsic factor for vitamin B12 absorption

Question 19

Describe chief cells (aka peptic or zymogenic cells).

Answer 19

• they are the main cell in the lower regions of the gastric glands
• release zymogen granules containing digestive enzymes (lipase) and proteases (pepsinogen) in response to gastrin
• acid environment activates pepsinogen to pepsin
• they are condensed cells, basically located nucleus and basophilic cytoplasm with many mitochondria and granules

Question 20

The small intestine has a large surface area. In what 4 ways is this achieved?

Answer 20

1. Duodenum, jejunum and ileum together are 4-6 m long.
2. Inner lining forms a series of ridges, the plicae circularis.
3. Plicae circularis have villi, finger-like projections of mucosa.
4. Enterocytes of villi have brush border of microvilli.

Question 21

What are the plicae circulares?

Answer 21

Pilca circulares are transverse envagination/ridges of mucosa and part of the submucosa lined by villi.

They begin in the duodenum; they are the most prominent in jejunum; they become progressively further apart in ileum.

Question 22

What are the villi in the GI tract?

Answer 22

They are finger-like projections of mucosa.

They are found on the surface of simple columnar epithelium of absorptive cells (enterocytes), goblet cells and endocrine cells with a core of loose connective tissue (lamina propria).

They have a core with blood vessels to convey digested components to the liver via hepatic portal vein, lacteals (lymphatic vessels) for delivering digested fats to the blood), nerve fibres, smooth muscle and lymphocytes (for antibodies in lumen and lamina propria).

Question 23

What are the microvilli in the GI tract?

Answer 23

They are 0.5-1 μm long and their shape is maintained by actin filaments anchored to cell membrane. The base of the actin filaments form part of terminal web linked to intermediate filaments (T and F).

Microvilli (MV) form a brush border on apical side of enterocyte with glycocalyx.

Glycocalyx is a coating of glycoproteins and glycolipids projecting from plasma membrane. It protects from autodigestion and the site of adsorption of pancreatic enzymes for final digestion stage.

Question 24

List the type of epithelial cells found in the small intestine.

Answer 24

1. Enterocytes
2. Goblet cells

(the following are located in crypts)
3. Paneth cells

4. Endorcine cells
5. Stem cells

Question 25

Describe enterocytes of the small intestine.

Answer 25

• they are tall columnar absorptive cells with basely located nuclei
• they have between 2000-3000 microvilli with glycocalyx

Question 26

Describe goblet cells of the small intestine.

Answer 26

• they produce mucus for protection and lubrication

- they are as numerous as enterocytes in the small intestine

Question 27

Describe Paneth cells of the small intestine.

Answer 27

• they are ;ocated in the lower third of crypts
• they have basal nuclei and prominent granules
• they release protective enzymes such as lysozyme and phospholipase A and peptides e.g. defensins
• their secretions help destroy parasites and bacteria

Question 28

Describe the endocrine cells of the small intestine.

Answer 28

• they are usually in the lower third of the crypts
• they have eosinophilic cyto granules (subnuclear)
• they secrete hormones (one type per cell)
  e. g. secretin, somatostatin, serotonin etc that regulate water and electrolyte metabolism and enzyme secretion, regulate gastrointestinal motility and mucosal growth, stimulate the release of other hormones.

Question 29

Describe stem cells of the small intestine.

Answer 29

• they are a source of new epithelial cells

Question 30

What hormonal activity occurs in the duodenum in response to chyme?

Answer 30

Semi-digested acidic chyme is pushed through the pyloric sphincter into the duodenum - this stimulates enteroendocrine cells in the mucosa to release hormones.

Cholecystokinin/pancreozymin (CCK or CCK-PZ) inhibits gastric emptying, and stimulates pancreatic acinar cells to secrete enzymes, and the gall bladder to release bile to emulsify fats via ducts into the duodenum.

Secretin stimulates Brunner’s glands and pancreatic centroacinar cells to release bicarbonate to neutralise acidic chyme, and stimulates the liver to make bile and parietal cells to reduce acid secretion.

The duodenal mucosa also releases enterokinase to activate trypsin from pancreas that, in turn, activates chymotrypsin.

Question 31

What are some histological differences between the duodenum, jejunum and ileum?

Answer 31

DUODENUM:

• the fistinctive feature of duodenum are Brunner’s glands located in submucosa
• mostly made up of mucous type and aid in neutralising acidic chyme
• the ducts of Brunner’s glands deliver secretion to Crypts of Lieberkuhn
• they secrete approximately 200 ml per day
• they diminish in number along length of duodenum

JEJUNUM:

• have the tallest villi
• have no Brunner glands
• have few lymphoid follicles
• have some enteroendocrine cells producing CCK and secretin

ILEUM:
- the ileum is easily identified due to its aggregations of lymphoid tissue is the form of Peyer’s patches

Question 32

Describe Peye’s Patches.

Answer 32

They are lymphoid follicles in the mucosa that bulge into the gut lumen, and are most prominent in the distal ileum.

Peyer’s patches are covered by follicle-associated epithelium: a few goblet cells and M cells with microfolds for transcytosis to take up antigens from the lumen and transport to the cells of Peyer’s.

The germinal centre is filled with proliferating/ maturing B lymphocytes, surrounded by T lymphocytes, dendritic cells and macrophages (particularly under the epithelium).

The immune cells that are responding to antigens migrate to the mesenteric lymph nodes where the immune response amplified, with activated lymphocytes returning to lamina propria and maturing into plasma cells.

Question 33

What is the large intestine made up of?

Answer 33

• Caecum
• Vermiform appendix
• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon
• Rectum
• Anal canal

Question 34

Describe the appendix.

Answer 34

It is a small blind-ended sack extending from the caecum. It’s structure resembles the rest of the large intestine.

The lamina propria of the mucosa and submucosa are heavily infiltrated with lymphocytes.

The lymphoid tissue can form follicles with germinal centres. Follicles can bulge into lumen (like Peyer’s patches in small intestine).

Question 35

What is the function of the colon?

Answer 35

Its function is to absorb water and electrolytes. thus desiccating the luminal content.

Question 36

How is the muscularis externa different in the colon?

Answer 36

The muscularis externa has the normal inner circular layer, but its outer longitudinal layer forms three bands - called teniae coli for powerful peristalsis to propel increasingly solid faeces to rectum.

Question 37

Describe the tubular glands of Lieberkuhn.

Answer 37

It is mucosa with straight tubular glands/crypts and no villi. They are on the absorptive surface of enterocytes, and take up water and electrolytes, thus desiccating luminal contents.

The glands/crypts contain many goblet cells that secrete mucus to protect/help passage of desiccated content, with stem cells at the base.

There are diffuse lymphocytes/plasma cells in the mucosa together with lymphoid aggregates/nodules: for defence against invading pathogens.

The muscularis mucosae prevents clogging of glands and mucus expulsion.

Question 38

Describe the rectum.

Answer 38

The rectum is the short dilated terminal portion of large intestine that stores faeces.

The rectal mucosa has tubular glands as per the large intestine, but has many more goblet cells.

It contains branched, tubular circumanal glands that secrete mucus to the anal orifice via longitudinal folds of mucosa called columns of Morgagni.

At the recto-anal junction, you have abrupt transition from simple columnar cells to stratified squamous epithelium cells of the anal canal.

Question 39

Describe the anal canal.

Answer 39

The anal canal is the final 2-3 cm of the GI tract, surrounded by voluntary skeletal muscle to form the external anal sphincter.

The inner circular muscle thickens to form the internal anal sphincter.

The stratified squamous epithelium of the anal canal gradually transitions to keratinised stratified squamous epithelium of the skin.