GI

Question 1

What do you know about the histology of the Lip?

Answer 1

The skin on the outer surface of the lip is a lightly keratinized, stratified squamous epithelium (Fig. 23a). The epithelial layer of the oral mucosa on the inside of the lip is thicker than that of the skin and is highly keratinized (Fig. 23a).
The ‘free margin’ of the lip is known as the vermilion border. This region looks red in a living person because it is highly vascularized.

Question 2

What do you know about oral mucosa histologically?

Answer 2

The mouth is lined by the oral mucosa (Fig. 23b), which consists of: • a thick stratified squamous epithelium, which protects against the large amount of wear and tear that the mouth receives;
• an underlying layer of loose, vascularized connective tissue (lamina propria).
The submucosa lies underneath the oral mucosa. This is a layer of dense irregular connective tissue, rich in collagen, containing salivary glands, larger blood vessels, nerves, and lymphatics. This layer is thin in regions overlying bone.

Question 3

What divides the tongue into anterior and posterior sections?

Answer 3

Sulcus terminals (V-shaped line)

Question 4

What do you know about the tongue histologically?

Answer 4

The tongue (Fig. 23e,f) is a mass of striated muscle covered in oral mucosa. It is divided into an anterior two-thirds and a posterior one-third by a V-shaped line, the sulcus terminalis.
The mucosa covering the upper (dorsal) surface of the tongue is thrown into numerous projections called papillae (Fig. 23e,f). The epithelium of the oral mucosa is a stratified non-keratinizing squa- mous epithelium, and an underlying layer of lamina propria sup- ports it.

Question 5

What are the three main type of papillae on your tongue?

Answer 5

• Filiform papillae (thread-like) are short whitish bristles. They are the commonest, appear white because they are keratinized, and contain very few taste buds.
• Fungiform papillae (mushroom-like) are small, globular, and appear red because they are not keratinized and are highly vascu- larized. They contain a few taste buds.
• Circumvallate papillae (wall-like) are the largest of the papillae. They are mostly found in a row just in front of the sulcus termi- nalis. Most of the taste buds are found in the circumvallate papillae in the walls of the clefts or furrows either side of the bud (Fig. 23h). Taste receptor cells in the taste buds only last about 10–14 days.

Question 6

What are the layer of the gut in general and do these change?

Answer 6

Layers of the gut
Mucosa:
Epithelium Lamina propria Muscularis mucosa

Submucosa

Muscularis externa

Adventitia (serosa)

These three layers are present throughout the gut. The structure of the different layers varies in different regions. This variation is related to the function in each region.

Question 7

What are the different parts of the mucosa and what are their principal functions?

Answer 7

• Epithelium: The type of epithelium varies between different regions of the gut (Fig. 24b). The epithelium can invaginate into the lamina propria to form mucosal glands, and into the submu- cosa to form submucosal glands.
• Lamina propria: This is a supporting layer of loose connective tissue that contains the blood and nerve supply for the epithelium, as well as lymphatic aggregations.
• Muscularis mucosae: This is a thin layer of smooth muscle, which lies underneath the lamina propria, and contracts the epithelial layer.

Question 8

What does the submucosa do?

Answer 8

The submucosa is a layer of supporting dense connective tissue, which contains the major blood vessels, lymphatics, and nerves.

Question 9

What does the muscular external layer do? What are some of its features?

Answer 9

Muscularis externa
This is the outer layer of smooth muscle. It contains two layers. In most regions of the gut, the smooth muscle fibers are arranged circularly in the inner layer, and their contraction reduces the size of the gut lumen. In the outer layer, the smooth muscle fibers are arranged longitudinally, and their contraction shortens the length of the gut tube.

Question 10

What is the difference between the adventitia and serosa?

Answer 10

This is the outermost layer, and contains connective tissue. In some regions of the gut, the adventitia is covered by a simple squamous epithelium (mesothelium), and in these regions, the outer layer is called the serosa

Question 11

What two nerve plexus are found in the gut tube and at what location?

Answer 11

The gut is innervated by the autonomic nervous system (para- sympathetic and sympathetic). Interneurons connect nerves between sensory and motor neurons in a submucosal plexus (Meissner’s complex) and in the plexus of Auerbach (between the layers of circular and longitudinal muscle in the muscularis externa).

Question 12

What do you know about the mucosa of the oesophagus?

Answer 12

The epithelium of the esophagus is a protective stratified squamous non-keratinizing epithelium (Fig. 24d,e).
The basal layer contains dividing cells, which proliferate and move upwards, continuously replacing the lining of the epithelium.

Question 13

Submucosa of the oesophagus?

Answer 13

The submucosa contains loose connective tissue that contains both collagen and elastin fibers. It is highly vascular, and contains esophageal glands, which secrete mucus into the lumen to help ease the passage of swallowed food, and the nerve supply for the muscle layers and glands. The esophageal (submucosal) glands are tubu- loacinar glands, arranged in lobules, and drained by a single duct.

Question 14

How do the muscles differ in each part of the oesophagus ?

Answer 14

This muscular layer, lying underneath the submucosa (Fig. 24d), consists of an inner circular and an outer longitudinal layer of muscle.
In the top third of the esophagus, the muscle is striated; in the middle, there is a mixture of smooth and striated muscle; and in the bottom third, the muscle is entirely smooth.
The two layers allow contraction across and along the tube.
There is a sphincter at the top and bottom of the esophagus. The upper sphincter helps to initiate swallowing, and the lower to prevent reflux of stomach contents into the esophagus.

Question 15

What happens in Barrats Oesophagus?

Answer 15

Barrett’s esopha- geal disease, in which columnar/cuboidal cells replace the squa- mous protective lining, possibly as part of a healing response.

Question 16

What differentiates the different parts of the stomach histologically?

Answer 16

• Cardiac: closest to the esophagus. It contains mucous-secreting cardiac glands.
• Fundus: the body or largest part of the stomach. It contains gastric (fundic) glands (Fig. 25b).
• Pyloric: closest to the duodenum, ending at the pyloric sphincter (Fig. 25b). It secretes two types of mucus and the hormone gastrin. The pyloric sphincter relaxes when chyme formation is complete, squirting chyme into the duodenum. Also no parietal cells

Question 17

How does the pyloric region of the stomach differ from the fungus?

Answer 17

Pyloric region of stomach
This region of the stomach is very similar to the body of the stomach (fundus). However, the mucosal layer is reduced in size, there are no parietal cells, and the glands are mostly full of mucous- secreting cells, which extend into the submucosa (Fig. 25e).
The muscularis externa layer in this region thickens to form the pyloric sphincter

Question 18

Where are the stem cells of the stomach located? Why is this surprising?

Answer 18

They are located in the isthmus of the gastric gland as opposed to the bottom where you would find them conventionally throughout the tract.

Question 19

What are gastric glands?

Answer 19

In the stomach, the epithelium invaginates to form gastric glands (Fig. 25c,d) that extend into the lamina propria. The glands open out into the base of the gastric pits. Cells lining the glands synthe- size and secrete gastric juice.

Question 20

What cells are present in the gastric pit?

Answer 20

1. ) Neck mucous cells
2. ) Tall columnar mucous secreting cells
3. )Parietal
4. ) chief cells
5. ) Stem cells
6. ) G-cells

Question 21

What is special about the muscular external of the fundus?

Answer 21

In the stomach, this layer has three layers of muscle: an inner oblique layer, a central circular layer, and an external longitudinal layer. The contraction of these muscle layers help to break up the food mechanically.

Question 22

How does the muscularis externa differ between funds and pylorus?

Answer 22

The muscularis externa layer in this region thickens to form the pyloric sphincter.

Question 23

How do the SI achieve a larger SA for absorption?

Answer 23

• Plicae circulares are large circular folds (Fig. 26b), which are most numerous in the upper part of the small intestine.
• Folding of the mucosa into villi (Fig. 26a–c), small, finger-like mucosal projections, about 1 mm long (increase surface area by about ×10).
• Microvilli are very small, fine projections on the apical surface of the lining columnar epithelial cells (Fig. 26e). This surface layer is commonly known as the ‘brush border’, and it is covered by a surface coat/glycocalyx.

Question 24

What is a distinct feature of the duodenum?

Answer 24

Brunners glands (tubuloacinar glands). These are generally submucosal but may extend through the muscularis mucosa into the mucosa.

Question 25

What do you know about the mucosa of the duodenum?

Answer 25

• The pH of their mucous secretions is about 9, which neutralizes the acid chyme entering the duodenum from the stomach.
• The villi in the duodenum are shorter and broader than else- where in the small intestine, and have a leaf-like shape.
• The epithelium is made up of a simple columnar epithelium with microvilli and is rich in goblet cells, which secrete alkaline mucus that help to neutralize the chyme (Fig. 26f).
Contains Brunners glands not found anywhere else in the GI tract.

Question 26

What are the two types of cells in the mucosa of the jejunum?

Answer 26

The villi in the jejunum are long and thin.
The epithelium contains two types of cells (Fig. 26e,f): tall
columnar absorptive cells (enterocytes) and goblet cells, which secrete mucus, for lubrication of the intestinal contents, and pro- tection of the epithelium

Question 27

What are the different types of cells found in the crypts of lieberkuhn?

Answer 27

Crypts of Lieberkuhn lie between the villi. These are simple tubular glands that contain the following.
• Paneth cells: defensive cells found at the base of the crypts. They secrete antimicrobial peptides (defensins), lysozyme and tumor necrosis factor α (pro-inflammatory). They stain dark pink with eosin in H&E.
• Endocrine cells: secrete the hormones secretin, somatostatin, enteroglucagon, and serotonin, and stain strongly with eosin.
• Stem cells: at the base of the crypts. They divide to replace all of the above cells, including enterocytes.

Question 28

What differentiates the submucosa of the jejunum from the duodenum?

Answer 28

The jejunum contains lacteals in its submucosa alongside the traditional blood vessels and nerves.
Also presence of peers patches

Question 29

Difference between the muscularis external of the stomach and the SI?

Answer 29

SI has 2 layers of muscles rather than the third oblique layer. This contracts to produce the peristaltic layer of the SI.

Question 30

How can you tell the difference between the jejunum and ileum?

Answer 30

This is the final region of the small intestine. It is similar to the jejunum, but has shorter villi, is richer in goblet cells and contains many more Peyer’s patches (see Chapter 43).

Question 31

What are the characteristics of the mucosa in LI?

Answer 31

The epithelium is folded to form tightly packed, straight tubular glands (crypts of Lieberkuhn; Fig. 27b).
The epithelium contains simple columnar mucous absorptive cells (Fig. 27c), which have short apical microvilli. These cells secrete a protective glycocalyx, which lines the epithelium, and absorb water, etc., (as outlined above). The epithelium also contains endo- crine cells, basal stem cells, and numerous goblet cells. Paneth cells may be found in the cecum.
Goblet cells are found in the crypts and the columnar absorptive cells on the luminal surface.
Lamina propriety does not contain lymphatics: Slow spreading cancer as a result.

Question 32

Submucosa of LI?

Answer 32

The lamina propria and submucosa both contain aggregations of leucocytes (Fig. 27b) (gut-associated lymphoid tissue or GALT; see Chapter 43), but these do not form the dome-shaped structures of Peyer’s patches (see Chapter 43).
The submucosa does not contain any glands.

Question 33

What is special about the muscularis externa of the LI?

Answer 33

Forms Taenia coli.
The muscularis externa contains two layers of smooth muscle (inner circular and outer longitudinal). The outer longitudinal layer is arranged in three longitudinal bands that fuse together in a structure called the taenia coli (Fig. 27a).
At the anus, the circular muscle forms the internal anal sphincter.

Question 34

How do you know if a histological slide is an appendix?

Answer 34

Large amounts of lymphoid tissue in the mucosa and submucosa are arranged in follicles with pale germinal centers (Fig. 27d), similar to Peyer’s patches (see Chapter 43). In adults, this structure is commonly lost, and the appendix is filled with scar tissue.