Hypoxia and visualization of the stem cell niche.

It is widely accepted that mammalian stem cells reside in a specialized cellular and a cellular microenvironment called the niche. The niche contrary to other tissues is characterized by a low partial Oxygen pressure (ppO2). This microenvironment protects stem cells from deleterious effects of O2 on proteins and DNA, through the production of reactive oxygen species (ROS). In addition there is now solid evidence that this physiological hypoxia helps stem cells maintaining their major characteristics: multipotency and ability to differentiate and migrate from the niche to specialized tissues in order to fulfill the needs of the organism. Immuno Histological techniques can stain stem cells in situ by specific Abs (such as against CD34 and CD45 for Hematopoietic Stem Cells HSC). However, a universal marker of hypoxia is Hypoxia-Inducible Factor-1, HIF-1, which is stabilized by low ppO2 and acts as a transcription factor to regulate a vast array of genes downstream. HIF-1, together with pimonidazole, a chemical compound interacting with proteins that are reduced in a hypoxic environment, are bona fide markers of the stem cell niche.