Stomach: The stomach is the expanded continuation of the esophagus. -…
Stomach: The stomach is the expanded continuation of the esophagus.
Glandular vs. nonglandular regions of the stomach
Depending on the species, the stratified squamous epithelium of the esophagus may continue for a ways into the stomach (horses, ruminants). When this occurs, this region of the stomach is referred to as the
Humans and carnivores have a distinct ending of the stratified squamous epithelium of the esophagus at the entrance to the stomach. The regions of the stomach that are lined by simple columnar epithelium are collectively called the
There are 3 glandular regions in the stomach:
Cardiac gland region
- the glandular regions directly adjacent to the stratified squamous epithelium of the esophagus or nonglandular region of the stomach;
contains mucous-secreting cells and argentaffin cells
(secrete gastric hormones).
Fundic gland region
- the glandular region adjacent to the cardiac region;
contains mucous cells, chief cells and many parietal cells
Pyloric gland region
- between the fundic gland region and the duodenum;
similar histologically to the cardiac gland region
Gastric pits: When observing the lumen of the stomach grossly, you will see that the mucosa is thrown into folds called gastric folds or rugae. Since they are greatly diminished or absent in a full stomach, it is thought that the rugae allow expansion of the stomach (versus contributing to increasing surface area).
The lining of the lumen itself appears smooth, i.e. there are no villi in the stomach. Instead, the epithelium is invaginated INTO the lamina propria forming structures called gastric pits.
Simple vs compound stomachs
Ruminants have 4-chambered stomachs called compound stomachs.
Humans, carnivores and horses have single-chambered stomachs referred to as simple stomachs.
Tunics of the stomach
Tunica submucosa - The tunica submucosa contains connective tissue and submucosal nervous plexi, but does not contain any glands.
Tunica muscularis - Though frequently described as having 2 or 3 layers of smooth muscle, the arrangement of muscle in the tunica muscularis of the stomach is not as distinct as the layers of the tunica muscularis in the esophagus and intestines. It may be difficult to distinguish these layers one from the other.
Lamina propria mucosae - similar to that found in other locations, with loose connective tissue filled with variable amounts of lymphatic cells and macrophages
Lamina epithelialis mucosae - There is an abrupt change from the stratified squamous epithelium of the esophagus or nonglandular region of the stomach (left) to the simple columnar epithelium lining the stomach and the rest of the digestive tract (arrow).
The simple columnar epithelium of the gastric pits in the cardiac and pyloric gland regions contains mucous-secreting cells. In fact, the base of the gastric pit takes on the appearance of a tubular gland. In the fundic gland gastric pit (shown here), there are several other cells types (more below). In any case, the gastric pits do not go deeper than the lamina muscularis mucosae.
This is a section from the pyloric gland region (which is very similar in appearance to the cardiac gland region). Note that the cells have the typical appearance of mucous-secreting cells.
The lamina epithelialis mucosae of the fundic gland region is distinct. The gastric pits contain several cells:
Parietal cells - bright pink, large, round cells; produce hydrochloric acid (HCl) and intrinsic factor (helps to absorb vitamin B12 in intestines).
Mucous neck cells - mucous-secreting cells found closer to the lumen than the deeper parts of the gastric pit.
Chief cells - basophilic cells found in the deeper parts of the gastric pits; secrete pepsinogen and lipase (ultimately a digestive enzyme).
Chief cells are difficult to tell apart from mucous neck cells. They are deeper in the gastric pit and have a more centrally placed nucleus than the mucous neck cells (though it may be displaced).
Enteroendocrine cells - small cells found throughout the gastric pit; secrete gastrointestinal hormones including serotonin (increases gut motility).
Lamina muscularis mucosae - a distinct band of smooth muscle at the base of the gastric pits (arrows).
Tunica serosa - Since the stomach is completely suspended in the abdominal cavity, it is surrounded by a serous membrane, the tunica serosa (arrow).
: From the outside, the equine stomach is similar in appearance to a human or carnivore stomach. However, almost half of the stomach (the half closest to the esophagus) is lined with stratified squamous epithelium, making it nonglandular.
The line marking the transition from nonglandular parts of the stomach is easy to see grossly. This “line” is called the
Histologically, the transition is also abrupt and distinct. On the left of the slide of the margo plicatus is the stratified squamous epithelium of the nonglandular stomach, and to the right is the simple columnar epithelium of the cardiac gland region of the stomach (Note also the lymphatic nodule).
: The ruminant stomach is divided into
4 main compartments
and is specifically designed to allow the animal to obtain energy from the digestion of rough diets (although most of the digestion is actually performed by bacteria housed in the stomach).
The first 3 compartments are nonglandular and have multiple papillae, which are folds of the mucosa into the lumen
Reticulum – nonglandular; more of an outpocketing of the rumen; the papillae have a neat-looking honeycomb appearance
he papillae of the reticulum contain smooth muscle bundles from the lamina muscularis mucosae; however, there is no continuous line of smooth muscle separating the lamina propria from the tunica submucosa.
Omasum – nonglandular; contains numerous thin flat folds that resemble the pages of a book (why it’s sometimes called the “book stomach”); these folds in turn contain numerous papillae which look and feel like a rough sandpaper.
The papillae of the omasum contain smooth muscle from BOTH the lamina muscularis mucosae and the tunica muscularis. This corresponds with the function of the omasum as a very strong “grinder,” mashing rough food materials. Note the thickness of the smooth muscle of the papillae of the omasum. You can see the smooth muscle of the tunica muscularis actually projecting into the papillae.
Rumen - nonglandular, very large compartment; grossly, the papillae make the rumen look like shag carpet interrupted by smooth areas called pillars.
The rumen contains papillae, but lacks lamina muscularis mucosae; the papillae do not have any smooth muscle in them.
Note the histological appearance of these sections of rumen. There is condensed connective tissue (black arrow) visible in the right image at the base of the papillae.
Abomasum – the “true” stomach; this compartment is glandular and contains the cardiac, fundic and pyloric gland regions
the abomasum of the ruminant stomach is indistinguishable from the glandular regions from other stomachs.