Small intestine histology

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Small intestine histology

USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A researcher is investigating the histological features of the gastrointestinal tract. In one section of the ileum, he finds a group of specialized cells in nodules in the submucosal layer, as shown in the image below (red circle). Which of the following is the function of this structure?  
Source: Wikimedia commons


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 small intestine is a long-narrow tube that is part of the digestive system, found between the stomach and the large intestine. It is the primary site of nutrient absorption, where the majority of the breakdown and absorption of food occurs.

The small intestine is composed of four main layers: the mucosa, submucosa, and muscularis propria, and in the small intestine, the outermost layer is a layer of connective tissue called the serosa. The mucosa is the innermost layer and is composed of three layers: the epithelium, the lamina propria, and the muscularis mucosae.

The epithelium is made up of enterocytes, goblet cells, enteroendocrine cells, and Paneth cells. The submucosa contains blood vessels, lymphatic vessels, and nerves. The muscularis externa consists of smooth muscle cells arranged in an inner circular layer and an outer longitudinal layer. The serosa is a thin layer of loose connective tissue that allows the small intestine to move more freely within the abdomen and contains blood vessels, nerves, and lymphatic vessels as well.


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