GI: Histology I

Describe the muscularis externa layer. What is it responsible for?

It is outside the submucosa layer. It usually consists of an inner circular muscle layer and an outer longitudinal muscle layer. Between these two muscle layers lies the Auerbach's plexus. These muscles contract to generate peristaltic waves that move the food through the GI tract.

Describe the cell type lining of the surface of the stomach and their function.

Lining the surface of the stomach are tall columnar surface mucus cells. These cells have mucin granules and extend into the gastric pit. These cells contain mucin granules in their apical cytoplasm that is released to provide a protective coat over the surface of the stomach. These lining cells are continuously being sloughed off and are being replaced by the cells deeper in the gastric pit every 3-4 days

What are the two intrinsic nerve plexuses of the GI tract? Where are they located?

Meissner's plexus in the submucosa and Auerbach's plexus between the two layers of muscle in the muscularis extena

What is the structure that connects the serosa layer to the posterior abdominal wall?

Mesentery

What is unique about the muscularis externa of the esophagus?

The upper third of the esophagus has a muscularis externa composed of skeletal muscle. This muscle is innervated by the autonomic system and is NOT under voluntary control. The middle third of the esophagus has both skeletal and smooth muscle, while the lower third only contains smooth muscle.

What type of epithelium covers the esophagus?

non-keratinized stratified squamous epithelium

What three layers make up the lining mucus membrane?

1. Lining epithelium 2. Lamina Propria 3. Muscularis Mucosa

Glands that lie outside the GI tract and send their contents into the GI lumen by means of ducts are located in which organs?

1. Liver and gallbladder (common hepatic + cystic duct-->common bile duct) 2. Pancreas-->pancreatic duct (exocrine pancreas)

What are the main roles of the small intestine? What morphological features aid the SI in accomplishing these tasks?

Digestion and absorption. For digestion: have input from the liver and gallbladder (bile and buffer) as well as from the pancreas (buffer and enzymes) draining into the 2nd part of the duodenum. This draining is controlled by the Sphincter of Oddi and occurs at the ampulla of Vater. In addition, the small intestine has brush border enzymes for further digestion of proteins and carbohydrates For absorption: Increased surface are through the formation of large folds (plicae circularis) which involve the lining mucosa and the submucosal layers. On top of the pliacae circularis, the lining mucosa folds to form intestinal villi. The intestinal absorptive cells lining the intestinal villi have microvilli to form the brush border.

What are the three divisions of the small intestine and approximately how long are they?

Duodenum: 10 inches Jejunum: 2/5 remaining length Ilieum: 3/5 remaining length The SI ranges in size depending on the person and the gender but is usually around 22-23ft long

Where are the enteroendocrine cells located in the stomach? Where are their products secreted? What is another name for these cells?

Enteroendocrine cells are located on the sides of the gastric glands. Unlike the other cell types, their endocrine secretions are deposited into the blood capillaries in the surrounding lamina propria rather than into the lumen of the stomach. Because these cells have a high affinity for silver, they are called argentaffin cells

Which cell type secretes acetic mucin in the stomach? Alkali mucin in the duodenum?

Gastric neck mucosal cells; Brunner's glandular cells

How can you distinguish the duodenum from the ileum?

In the first portion of the duodenum, there is a prominent population of mucus glands in the submucosal layer called Brunners glands. These glands secreted alkali mucin that neutralizes the acidic chyme from the stomach In the ileum there is a very prominent population of lymphatic nodules known as Peyer's patches. This are an identifying feature of the ileum

Where are the crypts of Lieberkuhn found?

In the small and in the large intestine

What two cell types are found on the surface of the intestinal villi? Describe each.

Intestinal absorptive cell: tall and columnar with dense microvillus brush border containing glycocalyx (sugar rich coat) Goblet cells: contain mucin granules and secrete a protective mucous coat over the surface lining epithelium

What is the function of the serosa layer in the GI?

It is a slippery membrane that allows organs to slide past one another without damaging each other.

Where is the appendix located. What does the appendix contain?

It is attached to the first portion of the colon called the cecum. It contains debris and a large amount of lymphatics

List the ways in which the large intestine differs from the small intestine.

1. There are no villi in the large intestine 2. The intestinal glands (crypts of Lieberkhan) are deeper in the large intestine 3. Large intestine has more goblet cells relative to the intestinal absorptive cells 4. There are few, if any, Paneth cells and enteroendocrine cells in the crypts in the large intestine 5. In the small intestine, the muscularis extena has the normal arrangement. In the large intestine, there is a modified muscularis externa where the outer longitudinal layer is broken up into three discrete bands called teniae coli

Name the layers of the esophagus going from inside to outside.

1. non-keratinized stratified squamous epithelium 2. lamina propria 3. muscularis mucosa 4. submucosa 5. muscularis externa 6. adventitia

What is the rate of turnover for the intestinal villi cells?

2-3 days

How many glands empty into a single gastric pit?

3-7

What is found in the center of the villus?

A central lymphatic lacteal, a capillary plexus, and smooth muscle

What layer lies outside the muscularis externa?

A connective tissue layer that is continuous with the CT of surrounding tissue (adventitia) OR more commonly, a serosa layer that is a thin slippery membrane consisting of a single layer of squamous/cuboidal cells (mesothelium). Under this serosa layer, there is a variable amount of connective tissue

What lies in between the intestinal villi?

Crypts of Lieberkuhn/intestinal glands, which are tubular glands that appear as invaginations into the lamina propria

Describe the characteristics of parietal cells. What are their functions?

Parietal cells are unique cell type found in the gastric pits of the stomach. They are round and contain a lot of mitochondria. They have deep invaginations called canaliculi that are lined with microvilli. Due to the high membrane content of the cell, it is highly esonophilic. The parietal cell is responsible for secreting HCl and intrinsic factor. HCl is responsible for gross digestion and protection from bacteria. IF is needed to bind Vitamin B12 to protect it from degradation from proteases.

What is the main function of the large intestine?

Reabsorption of water and solidification of the by-products of digestion

What are the folds of the stomach called? Which layers do they involve?

Rugae; they involve the lining mucosa and the submucosal region

Which tissue type forms the outer layer of the stomach?

Serosa

Describe the characteristics of the anal canal. What clinical significance does the change in epithelial lining that occurs as you move from the upper anal canal to the lower anal canal have?

The anal canal has large longitudinal folds called rectal columns that involve the lining mucosa and the submucosa. It is lined by a simple columnar epithelium followed by a non-keratinized stratified squamous epithelium, which is continuous with the keratinized squamous epithelium of the skin. This transition region of the epithelium from one type to another is a common site for cancers.

Where are the chief cells found? What is their function?

The chief cells are the major cell type found in the gastric glands. They are usually located at the base of the gland. They are filled with RER and golgi and they are active zymogenic cells. They actively secrete pepsinogen and gastric lipase into the lumen of the stomach for the digestion of proteins and fats respectively. Pepsinogen is activated by the acidic environment of the stomach

Describe the function of the gastric pits.

The gastric pits are the structures through which the digestive juices (pepsinogen and HCl) are secreted into the lumen of the stomach. These pits are continuous with the gastric glands that lie deep in the lamina propria but DO NOT go into the submucosa

Describe the pyloric and cardiac glands. How can you tell them apart morphologically?

The glands of these two regions are lined by cells similar to the mucus-neck cells of the gastric pits. Parietal and chief cells are rare in these regions if not absent. These glands do contain enteroendocrine cells. To tell them apart: Cardiac gastric pits are shallower and the glands are straight. The pyloric gastric pits are deep and the glands are coiled.

List how the intestinal villi and goblet cell concentration change as you go down the small intestine

The intestinal villi are more prominent in the first portion of the small intestine where they are broad and leaf shaped. As you move down the tract, the villi become more slender and less numerous. The relative number of goblet cells to intestinal absorptive cells increases as one passes through the length of the small intestine

Glands that lie within the GI tract usually are located in which tissue layers?

The lamina propria and the submucosal layers

What cell type is found in the neck of the gastric gland? How are they unique?

The mucus neck cells are located in the neck of the gastric glands often squeezed between two parietal cells. They secrete acetic mucin which differs from the neutral mucin of the lining mucus cells.

How is the muscularis externa unique in the stomach region?

The muscularis externa has an inner layer of oblique muscle in addition to the inner circular and outer longitudinal muscle layers. This makes the muscularis externa much thicker in the stomach and facilitates the churning action of the stomach

What role does the smooth muscle play in the intestinal villus?

The smooth muscle contracts to move the villus up and down to facilitate the movement of fat from the central lacteal into the lymphatics

What layer is adjacent to the lining mucus membrane? What is present in this layer?

The submucosal connective tissue layer. This layer contains Meissner's nerve plexus and can contain glands. It is often involved in folding of the luminal tissue like rugae in the stomach and plicae in the jejunum

Where are the "gastric glands" located in the stomach?

The term "gastric glands" refers to the more complex glands that are found in the fundus and the body of the stomach. The cardiac and pyloric glands differ from these gastric glands in several ways.

How many types of glands are in the esophagus? Where are they located? What is their function?

There are two types of glands in the esophagus. The first type is the submucosal glands located in the submucosal layer throughout the length of the esophagus. These secrete mucin to lubricate the esophagus to protect it from abrasion . The second population of glands are called the cardiac glands and are located in the lamina propria at two sites: junction of the esophagus and the stomach and between the cricoid cartilage and the 5th tracheal ring. These glands also secrete protective mucin

Describe the cell types located in the crypts of Lieberkhan

These intestinal glands are lined by goblet cells and intestinal absorptive cells. Enteroendocrine cells are found along their lateral borders. At the bottom of the gland are Paneth cells (zymogenic cells). The paneth cells contain large granules in their apical cytoplasm which contain zinc and lysozyme.

What tissue type surrounds the large intestine? The small intestine?

They are both surrounded by a serosa

In which region of the plicae circularis most prominent?

They begin towards the end of the duodenum, are most prominent in the jejunum and disappear about halfway through the ileum

How long is the large intestine?

about 5 feet

What substances are absorbed in the stomach?

water, alcohol and drugs