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Small Intestine Histology

The small intestine is the site where the digestive processes are completed and where the nutrients are absorbed by cells of the epithelial lining. The small intestine consists of three segments: the duodenum, jejunum, and ileum. Similar to the rest of the gastrointestinal 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. 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. Densely covering the entire mucosa of the small intestine are short mucosal outgrowths called villi that project into the lumen. These finger or leaflike projections are covered by a simple columnar epithelium of absorptive cells called enterocytes which are simple columnar cells, with many interspersed goblet cells which secrete mucus for lubrication and physical protection of the intestinal epithelium. Each villus has a core of loose connective tissue that extends from the lamina propria. 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