712Cell132,February22,2008©2008ElsevierInc.DOI10.1016/j.cell.2008.02.013Seeonlineversionforlegend,abbreviations,andreferences.SnapShot:HematopoiesisStuartH.OrkinandLeonardI.ZonHarvardMedicalSchool,Boston,MA02115,USASnapShot:HematopoiesisStuartH.OrkinandLeonardI.ZonHarvardMedicalSchool,Boston,MA02115,USA712.e1Cell132,February22,2008©2008ElsevierInc.DOI10.1016/j.cell.2008.02.013Theupperpaneldepictsthestagesofhematopoiesisinthemouse.Hematopoieticstemcells(HSCs)arederivedfromventralmesoderm,andthesequentialsitesofhe-matopoiesisduringdevelopmentincludetheyolksac,anareasurroundingthedorsalaortatermedtheaorta-gonadmesonephros(AGM)region,thefetalliver,placenta,andfinallythebonemarrow.ThepropertiesofHSCsineachsitediffer,presumablyreflectingdiversenichesthatsupportHSCexpansionand/ordifferentiationandintrinsiccharacteristicsofHSCsateachstage.Theinitialwaveofbloodproductioninthemammalianyolksacistermed“primitive.”Theprimaryfunctionforprimitivehematopoiesisisproductionofredbloodcells(RBCs)thatfacilitatetissueoxygenationastheembryoundergoesrapidgrowth.Thehemangioblastoftheyolksacisproposedtogiverisetobothbloodandendothelialcells(ECs).ThenextregionofhematopoiesisistheAGM.IthasbeenproposedthattheAGMformshemogenicECsintheventralwalloftheaortathatbudoffHSCs.SignificantnumbersofHSCsarealsofoundinthemouseplacenta.PlacentalHSCscouldarisethroughdenovogenerationorcolonizationuponcirculation,orboth.Therelativecon-tributionofeachoftheabovesitestothefinalpoolofadultHSCsremainslargelyunknown.Subsequentdefinitivehematopoiesisinvolvesthecolonizationofthefetalliver,thymus,spleen,andultimatelythebonemarrow.Indefinitivehematopoiesis,long-termHSCs(LT-HSCs)giverisetoshort-termHSCs(ST-HSCs).ST-HSCsproducecommonmyeloidprogenitors(CMPs)andcommonlymphoidprogenitors(CLPs).CLPsarethesourceofcommittedprecursorsofBandTlymphocytes,whereasCMPsgiverisetomegakaryocyte/erythroidprogenitors(MEPs)andgranulocyte/macrophageprogenitors(GMPs).GMPsgiverisetothecommittedprecursorsofmastcells,eosinophils,neutrophi...
