Also, new epithelium is formed here, which is important because the cells at this site are continuously worn away by the passing food. The basal (further from the intestinal lumen) portion of the crypt contains multipotent stem cells. During each mitosis, one of the two daughter cells remains in the crypt as a stem cell, while the other differentiates and migrates up the side of the crypt and eventually into the villus. These stem cells can differentiate into either an absorptive (enterocytes) or secretory (Goblet cells, Paneth cells, enteroendocrine cells) lineages.[3] Both Wnt and Notch signaling pathways play a large role in regulating cell proliferation and in intestinal morphogenesis and homeostasis.[4]
Loss of proliferation control in the crypts is thought to lead to colorectal cancer.
Intestinal juice
Intestinal juice (also called succus entericus[5]) refers to the clear to pale yellow watery secretions from the glands lining the small intestine walls. The Brunner's glands secrete large amounts of alkaline mucus in response to (1) tactile or irritating stimuli on the duodenal mucosa; (2) vagal stimulation, which increases Brunner's glands secretion concurrently with increase in stomach secretion; and (3) gastrointestinal hormones, especially secretin.[6]
Its function is to complete the process begun by pancreatic juice; the enzymetrypsin exists in pancreatic juice in the inactive form trypsinogen, it is activated by the intestinal enterokinase in intestinal juice. Trypsin can then activate other protease enzymes and catalyze the reaction pro-colipase → colipase. Colipase is necessary, along with bile salts, to enable lipase function.[citation needed]
Colonic crypts (intestinal glands) within four tissue sections. In panel A, the bar shows 100 µm and allows an estimate of the frequency of crypts in the colonic epithelium. Panel B includes three crypts in cross-section, each with one segment deficient for CCOI expression and at least one crypt, on the right side, undergoing fission into two crypts. Panel C shows, on the left side, a crypt fissioning into two crypts. Panel D shows typical small clusters of two and three CCOI deficient crypts (the bar shows 50 µm). The images were made from original photomicrographs, but panels A, B and D were also included in an article[7]
The intestinal glands in the colon are often referred to as colonic crypts. The epithelial inner surface of the colon is punctuated by invaginations, the colonic crypts. The colon crypts are shaped like microscopic thick-walled test tubes with a central hole down the length of the tube (the crypt lumen). Four tissue sections are shown here, two (A and B) cut across the long axes of the crypts and two (C and D) cut parallel to the long axes.
In these images the cells have been stained to show a brown-orange color if the cells produce a mitochondrial protein called cytochrome c oxidase subunit I (CCOI or COX-1). The nuclei of the cells (located at the outer edges of the cells lining the walls of the crypts) are stained blue-gray with haematoxylin. As seen in panels C and D, crypts are about 75 to about 110 cells long. The average crypt circumference is 23 cells.[8] From the images, an average is shown to be about 1,725 to 2530 cells per colonic crypt. Another measure was attained giving a range of 1500 to 4900 cells per colonic crypt.[9] Cells are produced at the crypt base and migrate upward along the crypt axis before being shed into the colonic lumen days later.[8] There are 5 to 6 stem cells at the bases of the crypts.[8]
As estimated from the image in panel A, there are about 100 colonic crypts per square millimeter of the colonic epithelium.[10] The length of the human colon is, on average 160.5 cm (measured from the bottom of the cecum to the colorectal junction) with a range of 80 cm to 313 cm.[11] The average inner circumference of the colon is 6.2 cm.[10] Thus, the inner surface epithelial area of the human colon has an area, on average, of about 995 cm2, which includes 9,950,000 (close to 10 million) crypts.
In the four tissue sections shown here, many of the intestinal glands have cells with a mitochondrial DNA mutation in the CCOI gene and appear mostly white, with their main color being the blue-gray staining of the nuclei. As seen in panel B, a portion of the stem cells of three crypts appear to have a mutation in CCOI, so that 40% to 50% of the cells arising from those stem cells form a white segment in the cross cut area.
Overall, the percentage of crypts deficient for CCOI is less than 1% before age 40, but then increases linearly with age.[7] Colonic crypts deficient for CCOI reaches, on average, 18% in women and 23% in men, by 80–84 years of age.[7]
Crypts of the colon can reproduce by fission, as seen in panel C, where a crypt is dividing to form two crypts, and in panel B where at least one crypt appears to be fissioning. Most crypts deficient in CCOI are in clusters of crypts (clones of crypts) with two or more CCOI-deficient crypts adjacent to each other (see panel D).[7]
The crypts of Lieberkühn are named after the eighteenth-century German anatomist Johann Nathanael Lieberkühn.
References
^Deakin, Barbara Young; et al. (2006). Wheater's functional histology : a text and colour atlas. drawings by Philip J. (5th ed.). [Edinburgh?]: Churchill Livingstone/Elsevier. ISBN978-0-4430-6-8508.
^Gonçalves, Carlos; Bairos, Vasco (2010). Histologia, Texto e Imagens (in Portuguese) (3rd ed.). Imprensa da Universidade de Coimbra. p. 261. ISBN9789892600703.
^Umar S. Intestinal stem cells. Curr Gastroenterol Rep. 2010;12(5):340-348. doi:10.1007/s11894-010-0130-3
^Fre S, Pallavi SK, Huyghe M, Laé M, Janssen KP, Robine S, Artavanis-Tsakonas S, Louvard D. Notch and Wnt signals cooperatively control cell proliferation and tumorigenesis in the intestine. Proc Natl Acad Sci U S A. 2009 Apr 14;106(15):6309-14. doi: 10.1073/pnas.0900427106
