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4.1: Crypts of Lieberkuhn and Enterocyte Maturation

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    20982
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    There are some additional anatomical and physiological features of the small intestine that are important to understand before defining uptake and absorption. Crypts of Lieberkuhn are pits between villi as pointed out by the green arrow in Figure \(\PageIndex{1}\).

    clipboard_ea6f8d79f4acdb348f3d5e37198c0acf2.png
    Figure \(\PageIndex{1}\): A crypt of Lieberkuhn is the pit between the villi in the small intestine as pointed out by the green arrow1

    The crypts of Lieberkuhn (often referred to simply as crypts) are similar to the gastric pits in the stomach. The crypts contain stem cells that can differentiate to produce a number of different cell types, including enterocytes2. From these stem cells in the crypt, immature enterocyte cells are formed that mature as they rise, or migrate, up the villi. Thus, the tips at the top of the villi are where the mature, fully functioning enterocytes are located, as represented by the purple cells in Figure \(\PageIndex{2}\)3.

    clipboard_e7129f5903dd34cd60da31167f0b48c13.png
    Figure \(\PageIndex{2}\): Crypts are represented by green arrows, fully mature enterocytes are represented by the purple cells at the top of the villi

    This maturation and migration is a continuous process. The life cycle of an enterocyte is 72 hours once it enters the villus from the crypt2. Once enterocytes have reached the top of the villus, they are sloughed off and are either digested (lipid and protein contents taken up by other enterocytes), or excreted in feces as depicted in Figure \(\PageIndex{3}\).

    clipboard_ed20045c7e0173fbb00123beb3dcc8d3a.png
    Figure \(\PageIndex{3}\): Enterocytes sloughed off the villus. Unless these cells are digested and their components are taken up by other enterocytes on the villus, they will be excreted in feces

    Thus, we define absorption as reaching body circulation, because compounds taken up into enterocytes might not make it into body circulation, and thus are not necessarily absorbed.

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    References

    1. digestive.niddk.nih.gov/ddiseases/pubs/celiac/
    2. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
    3. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.

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