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Intestinal epithelial stem cells reside in specific niches within glandular invaginations, called crypts, on the mucosal surface of the small intestine. Small intestinal stem cells are located in the base of the crypts just above the Paneth cells and there are about 4*10^6 of them.

Stem cells from several crypts provide the epithelium for each of the villi that project into the intestinal lumen. The number of stem cells per crypt has not been definitively established and estimates vary widely from 0.4% to 60% of the crypt cell population. The variation in estimates appear to depend on differences in opinion regarding what defines a stem cell and/or which species and stage of development is being studied.

Unique Feature

One of the remarkable features of the small intestinal stem cells is the fact that, in spite of the large numbers of stem cells present in the small intestine, these cells very rarely acquire the genetic defects associated with cancer induction or deterioration in functional competence with age. This feature is very desirable for reproducing small intestine using epithelial stem cells.

Organoid Unit

Intestinal organoid units are defined as villus structures with overlying epithelium and core of mesenchymal stromal cells. The average size of an organoid unit is 0.1 mm by 0.25 mm and there are in average of 40,000 units in the small intestine.

The optimal method for isolating small intestinal epithelium was using collagenase to digest the tissue. It allows clusters of crypts and villi to be released. The figure below represents the basic organoid unit after isolation.

The use of organoid unit for tissue engineered small intestine has been favored to date because it guarantees the capture of stem cells. The organoid unit is capable of producing four types of cells as well as providing growth factors such as hepatocytes growth factor for the differentiation of stem cells and develop into neoepithelium. However, these units have to be seeded on scaffolds to provide proper growth.