15.5:
Adult Stem Cells
Some adult tissues contain stem cells, unspecialized cells that divide to produce new stem cells as well as progeny that differentiate into mature cell types. Adult stem cells are usually multipotent. They can produce multiple types of differentiated cells but are typically limited to the kinds found in that particular tissue.
For example, in the small intestine, stem cells produce all the cells in the epithelium of the villi that protrude into the lumen, the interior space, including absorptive cells, which absorb nutrients, and different types of secretory cells, such as goblet, paneth, and enteroendocrine cells.
The stem cells are found at the base of the crypts that lie between the villi. They divide asymmetrically to produce a new stem cell, renewing the population, and a transit amplifying cell.
These cells divide rapidly, move up the villi, and differentiate into the mature, intestinal cell types with their specialized structures and functions. When they reach the tip of the villi, they undergo apoptosis, programmed cell death, and are shed into the lumen. The time between stem cell division and apoptosis is about three to five days, allowing the lining of the small intestine to be rapidly and continuously replaced.
15.5:
Adult Stem Cells
Stem cells are undifferentiated cells that divide and produce more stem cells or progenitor cells that differentiate into mature, specialized cell types. All the cells in the body are generated from stem cells in the early embryo, but small populations of stem cells are also present in many adult tissues including the bone marrow, brain, skin, and gut. These adult stem cells typically produce the various cell types found in that tissue—to replace cells that are damaged or to continuously renew the tissue.
The Small Intestine
The epithelium lining the small intestine is continuously renewed by adult stem cells. It is the most rapidly replaced tissue in the human body, with most cells being replaced within 3-5 days. The intestinal epithelium consists of thousands of villi that protrude into the interior of the small intestine—increasing its surface area to aid in the absorption of nutrients.
Intestinal stem cells are located at the base of invaginations called crypts that lie between the villi. They divide to produce new stem cells, as well as daughter cells (called transit amplifying cells) that divide rapidly, move up the villi and differentiate into all the cell types in the intestinal epithelium, including absorptive, goblet, enteroendocrine, and Paneth cells. These mature cells continue to move up the villi as they carry out their functions, except Paneth cells which move back down to reside at the base of the crypt.
When the mature cells reach the top of the villi, they undergo apoptosis—programmed cell death—and are shed. New cells continue to push up from below, continuously replacing the cells that are lost.
Suggested Reading
Montagnani, Stefania, Maria A. Rueger, Toru Hosoda, and Daria Nurzynska. “Adult Stem Cells in Tissue Maintenance and Regeneration.” Stem Cells International 2016 (2016). [Source]
Tweedell, Kenyon S. “The Adaptability of Somatic Stem Cells: A Review.” Journal of Stem Cells & Regenerative Medicine 13, no. 1 (May 30, 2017): 3–13. [Source]