GI – Stomach

The stomach is composed of several anatomic zones. Moving proximal to distal, you pass:

1) the gastroesophageal junction, 2) the cardia, 3) the fundus and body, 4) the antrum and 5) the pylorus.

Histologically, the entire stomach is made up of simple tubular glands and foveolae (gastric pits) and there are essentially only 2 types of mucosa:

• Antral (cardia, antrum and pylorus)
• Oxyntic (fundus and body).

Glands: secrete various substances into the foveolae.

Foveolae (gastric pits): the part of the gland into which secretions flow.

We will start by looking at the histological structure of the fundus and body (as they are essentially the same).

Oxyntic mucosa is found in the fundus and body of the stomach, these are the digestive regions of the stomach. It is made up of tightly packed glands which occupy 3/4 of the mucosal thickness.

Let’s look at the glands at higher magnification;

Here we can see some of the cells lining the glands. The dark blue round structures are the nuclei. But notice the cytoplasm around the nuclei is basophilic (blue-staining). This tells us something about the activity of the cells, these are enzyme secreting cells, known as chief cells and they secrete pepsinogen.

The pepsinogen is secreted into the gastric pit and then into the lumen of the stomach where the low pH changes the configuration of the molecule, effectively changing it into pepsin. Pepsin is a highly active proteolytic enzyme that breaks down protein in the food.

There’s another type of cell in the gastric glands. Notice that their cytoplasm is eosinophilic (pink-staining). These are acid-secreting cells, known as parietal cells and they secrete hydrochloric acid.

The parietal cells also secrete another important substance required for the absorption of vitamin B12 in the terminal ileum, known as intrinsic factor.

There is a third type of cells in the gastric mucosa. We find it in the gastric pits, or foveolae, which are near the surface of the stomach.

The surface and pits of the stomach are lined by columnar mucinous epithelium called foveolar cells. These are tall and have pale eosinophilic cytoplasm. At the base of each cell is the nucleus. These cells produce a thick coating of mucus that protects the gastric mucosa from the enzymes and acid in the lumen.

Deeper in the pits and within the glands are the mucous neck cells, these have a similar function and mature up the glands to ultimately replace the surface mucous cells.

Endocrine cells are also present in the stomach, these occur singly in the glands. In the gastric body, they are mainly enterochromaffin-like (ECL) cells.

Now the glands aren’t the only thing in the mucosa of the stomach. Between the glands, we find connective tissue and some cells. This part of the mucosa is called the lamina propria.

The lamina propria consists mostly of loose connective tissue, but within it there are scattered lymphocytes and plasma cells.

In the gastric body the glands are so densely packed together there isn’t much room for the lamina propria.

Look back at the lower magnification view of the stomach to get your bearings again. Look beyond the gastric mucosa, here you see a thin layer of smooth muscle. This is seen here as the thin pink line, indicated by the green arrows. This is the muscularis mucosa.

Beyond the muscularis mucosa, we find the next layer, outlined by the yellow line in the picture below. Notice that its a lot thicker than the muscularis mucosa. Also, it looks paler. This is the submucosa.

The submucosa is made up of loose connective tissue; collagen, blood vessels and adipose tissue.

Beyond this layer is a thick, brightly eosinophilic layer, this is the muscularis propria. The muscularis propria in most of the GI tract consists of an inner circular layer and an outer longitudinal layer, but in the stomach there is a third layer -the oblique layer and it is the innermost layer.

External to the muscularis propria we have the final layer of the stomach. Here, the arrows indicate part of the muscularis propria, which we’ve just looked at. The blue line towards the bottom of the picture surrounds a layer of flattened cells. Let’s look at these at higher magnification.

At higher magnification, we see the layer of flattened mesothelial cells, outlined with a blue line. This is the serosa.

We’re now going to move on to look at the other parts of the stomach but, before we do, have a look again at the oxyntic mucosa. Note again the short gastric foveolae and the long glands, full of specialised cells. Fix this appearance in your mind’s eye. Let’s move on to compare the distal part of the stomach, the antrum.

Antral mucosa is found in the antrum, cardia and pylorus of the stomach, these are the border regions of the stomach. It is made up of loosely packed glands which occupy 1/2 of the mucosal thickness.

Here we’re looking at the antrum at higher magnification. Some of the antral glands are outlined by yellow lines. We see that many of the glands of the antrum are cut in cross-section rather than lengthwise, as in the fundus.

The antral glands are lined by mucinous cells, some of which are outlined here by green lines. Notice that the cells have clear cytoplasm. This is because they contain mucin.

The antral glands, however, also secrete something else: a proteolytic enzyme, which is secreted as its precursor, called a zymogen. They don’t secrete pepsinogen, which is the zymogen produced by the chief cells in the body of the stomach, but they do secrete another zymogen; Progastricsin.

Here we see a section of gastric antrum stained with an antibody to progastricsin, which shows up as brown granules in the cytoplasm of the cells lining the glands. Progastricsin is the zymogen precursor of the active enzyme gastricsin. Its optimal pH is 3.5, somewhat higher than that of pepsin. Progastricsin is secreted in gastric body, antrum and duodenum, whereas pepsinogen is secreted only in the gastric body.

As mentioned in the oxyntic mucosa, endocrine cells are also present in the antral mucosa. In the antrum, these are mostly mixed ECL, gastrin and somatostatin producing cells.

Apart from the body, fundus and antrum, there’s another part of the stomach that’s recognised. This is the upper part of the stomach, around the oesophagus.

In the cardia, there aren’t specialised elements like chief cells and parietal cells that we find in the fundus. Instead, the gastric glands are lined only by mucous neck cells.

So, in summary, the gastric mucosa has two types of mucosa, depending on its location in the stomach, but the general architecture of foveolae and glands is constant throughout.

Fundus and body: chief cells, parietal cells and mucous neck cells.

Antrum: mucous neck cells, some parietal cells.

Cardia: mucous neck cells.