Stem cell niche

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Stem cell niche is the environment of stem cells formed mainly by surrounding cells. Niche is not mere passive physical support, stem cells fate (eg differentiation versus renewal) and proliferation is actively regulated by the niche. Stem cells are generally kept undifferentiated by the niche and the process of exit from the niche itself can sometimes be enough to cause differentiation of the stem cells. Though generally the surrounding cells are the main niche component, however the extracellular matrix, and physiochemical nature of the environment including the pH, ionic strength (eg Ca2+ concentration, metabolites like ATP are also part of the niche. The stem cells and niche may be involved in induction of each other during development and reciprocally signal to maintain each other during steady state or adulthood.

A Nature Insight review defines niche as follows:

"Stem-cell populations are established in 'niches' — specific anatomic locations that regulate how they participate in tissue generation, maintenance and repair. The niche saves stem cells from depletion, while protecting the host from over-exuberant stem-cell proliferation. It constitutes a basic unit of tissue physiology, integrating signals that mediate the balanced response of stem cells to the needs of organisms. Yet the niche may also induce pathologies by imposing aberrant function on stem cells or other targets. The interplay between stem cells and their niche creates the dynamic system necessary for sustaining tissues, and for the ultimate design of stem-cell therapeutics...The simple location of stem cells is not sufficient to define a niche. The niche must have both anatomic and functional dimensions"

—David T. Scadden, The stem-cell niche as an entity of action, Nature, 441 (7097), 1075-1079 (29 June 2006)

1 History
2 Cellular composition of the niche

[edit] History

Though the concept of stem cell niche was prevailoing in vertebrates, the first characterization of stem cell niche in vivo was worked out in Drosophila germinal development.

[edit] Cellular composition of the niche

[edit] Drosophila germarium

In drosophila ovary the niche comprizes of terminal filament cells, cap cells and inner sheath cells. Themain niche in drosophila testis is the hub cell.

[edit] ES Cell niche

For vertebrate embryonic stem cells in vivo the niche is formed by the trophoblasts.

[edit] Vertebrate Adult stem cell niches:

[edit] A. Hematopoietic stem cell niche

Vertebrate hematopoietic stem cells niche in the bone marrow is formed by cells subendoosteal osteoblasts, sinusoidal endothelial cells and bone marrow stromal (also sometimes called reticular) cells which includes a mix of fibroblastoid, monocytic and adipocytic cells.

[edit] B. Hair follicle stem cell niche

The bulge area at the junction of arectores pili muscle to the hair follicle seath has been shown to host the skin stem cells with maximum span of developmental potential. There cells are maintained by signaling in concert with niche cells - signals include paracrine (eg sohic hedgehog), autocrine and juxtacrine signals.