Stem cell
A stem cell has certain properties that distinguish it from other cells:
- It is capable of self renewal.
- It can give rise to a range of differentiated progeny.
http://sciforums.spuriousmonkey.com/permanent/stem_cells.gif
Stem cell niche
Stem cells are often located in so-called stem cell niches, a structural arrangement of cells or tissues that regulate the proliferation and maintenance of stem cells and direct the differentiation of their progeny.
Bone Marrow
The bone marrow is the stem cell niche for Haematopoietic stem cells (HSCs). The HSCs form a subset of the cells in the bone marrow and have the capability for self-renewal and differentiation into different progeny. HSCs can freely circulate the blood stream but this doesn't mean they are functional everywhere.
The crypt
The crypt is the intestinal stem cell niche (Kyoung-Mee K, and Darryl S, 2002). There are millions of crypts per human colon and each on of them contains about 2000 cells. The crypt is maintained by a few cells at the base of the crypt. They give rise to progeny that differentiates and migrates upwards towards the lumen of the intestine. Cells at the lumen are constantly replaced.
The Cervical loop
The cervical loop is the epithelial stem cell niche in teeth. This structure is formed during cap stage of tooth development.
http://sciforums.spuriousmonkey.com/permanent/cervical-loop.png
It consists of
- 1. Dental follicle (mesenchyme)
- 2. Dental mesenchyme (mesenchyme)
- 3. Differentiating odontoblasts (mesenchyme)
- 4. Dentin (mesenchymal derived matrix)
- 5. Stellate Reticulum (putative epithelial stem cells)
- 6. Outer enamel epithelium
- 7. Inner enamel epithelium
- 8. differentiating ameloblasts (epithelium)
- 9. Enamel (epithelial derived matrix)
The inner core of loosely aggregated stellate reticulum forms the site of putative stem cells (Harada et al., 1999). This cell layer is lost during root development of non-continuously growing teeth, such as the mouse and human molars. The rodent incisor grows continuously and this stem cell niche is maintained in this tooth. The stem cells give rise to progeny which migrates into the inner enamel epithelium where the cells proliferate and form a zone of transit-amplifying cells. Away from the cervical loop these cells differentiate into ameloblasts which deposit the enamel. In continuously growing teeth the tip of the tooth is constantly worn down and replaced from the other end with cells and matrix produced by the stem cell niche.
http://spuriousmonkey.com/wiki/images/b/b3/Cervical-loop-incisor.jpg A histological picture of the cervical loop courtesy of spuriousmonkey
References
Harada H, Kettunen P, Jung HS, Mustonen T, Wang YA, Thesleff I (1999) Localization of putative stem cells in dental epithelium and their association with Notch and FGF signaling. J. Cell Biol. 147:105-20.
Kyoung-Mee K, and Darryl S (2002) Methylation reveals a niche: stem cell succession in human colon crypts. Nature 21:5441-5449.
