title: Recent advances in defining the hematopoietic stem cell niche.
abstract: OBJECTIVE
  Hematopoietic stem cells are thought to reside in discrete cellular spaces termed "niches." The cellular elements and matrix surrounding the stem cell within the niche constitute the microenvironment. The purpose of this review is to discuss recent reports that have begun to elucidate the geographic location, key cellular type, and molecular mechanisms operating in stem cell niches.
  
  
  RESULTS
  Studies that have revealed the osteoblast as the key in vivo cellular element of the adult stem cell niche are the most significant recent findings. Additional studies have highlighted the importance of the Notch and Wingless (Wnt) signaling pathways in the hematopoietic microenvironment. Genomewide expression screens have been used to perform molecular profiling of stromal cell lines that serve as surrogate stem cell niches. These profiles have revealed novel regulatory molecules and have reinforced the roles of classic developmental morphogens in the niche space. The transcriptional profiling from these screens suggests that it is highly unlikely that a single factor or signal transduction pathway will control stem cell properties.
  
  
  CONCLUSIONS
  This review highlights the recent advances made toward elucidating the cellular and molecular attributes of the hematopoietic stem cell niche. Complete knowledge of the cellular architecture and molecular mechanisms in stem cell niches is essential to understanding the basic stem cell behaviors of self-renewal and differentiation.
authors.affiliation: Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA. kmoore@molbio.princeton.edu
authors.forename: Kateri A
authors.initials: KA
authors.lastname: Moore
mesh_terms: D000818:Animals; D002454:Cell Differentiation; D002455:Cell Division; D006412:Hematopoietic Stem Cells / Q000502:physiology*; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000502:physiology*; D015398:Signal Transduction; D051153:Wnt Proteins
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D051153:Wnt Proteins