title: Hematopoietic stem cells: challenges to expectations.
abstract: The past year provided a number of challenges to our expectations regarding hematopoietic stem cell (HSC) biology. Evidence has emerged that HSCs arise intraembryonically before they can be detected in the yolk sac. A number of genes that may regulate the formation, self-renewal, or differentiation of HSC have been identified. New markers for purifying HSCs have also been described. Although different groups have attributed different properties to HSCs, it now appears that the differences may be explained by variations in assay conditions rather than by differences in the HSCs themselves. Finally, insights have emerged into the complexity of the regulation of HSC proliferation and adhesion properties.
authors.affiliation: Division of Biology, California Institute of Technology, Pasadena 91125, USA. MorrisonS@starbase1.caltech.edu
authors.forename: S J
authors.initials: SJ
authors.lastname: Morrison
authors.forename: D E
authors.initials: DE
authors.lastname: Wright
authors.forename: S H
authors.initials: SH
authors.lastname: Cheshier
authors.forename: I L
authors.initials: IL
authors.lastname: Weissman
mesh_terms: D000818:Animals; D005786:Gene Expression Regulation; D006412:Hematopoietic Stem Cells / Q000502:physiology*; D006801:Humans; D051379:Mice