The small intestine is the longest portion of the gastrointestinal or GI tract.
It’s approximately 6 meters long in adults and it’s the main site for both digestion and absorption of food in the body.
Although the majority of the small intestine shares similar histological structures, it’s still divided into three main segments.
The duodenum, jejunum, and ileum.
The most proximal segment of the small intestine is the duodenum, and it’s also the shortest segment as well.
The duodenum connects with the jejunum at the duodenojejunal junction.
The jejunum is approximately 2.5 meters long and will gradually transition to the Ileum, which is the last segment of the small intestine.
The ileum is usually about 3.5 meters long and leads to the cecum of the large intestine at the ileocecal junction.
Similar to the rest of the GI tract, the wall of the small intestine has 4 main layers: the mucosa, submucosa, muscularis propria, and in the small intestine the outermost layer is a layer of connective tissue called the serosa.
In these images, the serosa is only present in the image of the duodenum.
Although each segment of the small intestine shares similar overall structures, there are still quite a few differences between each segment that can be seen with light microscopy.
Even at low magnification, we can see that the finger-like projections or villi that extend into the lumen in both the duodenum and jejunum are very tall and slender when compared to the villi of the ileum are significantly shorter, broader, and their tips are flat in comparison to the duodenum and the jejunum.
Let’s take a closer look at the mucosa of the duodenum.
The villi of the entire small intestine are lined with enterocytes, which are simple columnar cells with microvilli.
The microvilli increase the surface area available to the enterocytes for absorption.
The pale cells within the villi are the goblet cells, which secrete mucus for lubrication and physical protection of the intestinal epithelium.
The lamina propria is the loose connective tissue that supports the enterocytes and forms the middle or core of each villus.
The crypts of Lieberkühn or intestinal crypts are glands found at the bases of the villi.
The crypts contain stem cells that slowly differentiate into the cells that form the epithelial lining of the small intestine, which include both the enterocytes and goblet cells.
The differentiated cells move up the villi as new cells are continually formed underneath them, and within a week, the cells will reach the tip of the villi where they’re shed into the lumen, similar to the separated epithelial cells seen at the top right of this image.
Some of the crypts in the small intestine have cells with very eosinophilic or pink granules.
These cells are called paneth cells, and they help provide an innate immune defence against intestinal pathogens.
And finally, underneath the villi, crypts, and lamina propria, is a thin supporting layer of smooth muscle called the muscularis mucosa.
If we look at a section of the jejunum, we can see that mucosa shares the same characteristics we saw in the duodenum’s mucosa.