These CD49fHCD41H FL cells respond to ADP and thrombin stimulation, rapidly XL765 order differentiating in vitro, as described for embryonic MEPs in semisolid assays.5, 6 Indeed, after 24 hours in culture, cytoplasmic elongations develop and proplatelets are generated in a process dependent upon the reorganization of the actin cytoskeleton, similar to that described in mature MKs.8, 22 Our observation that proplatelets are present in vivo under physiological conditions in isolated cellular FL preparations is particularly

relevant until, as recently, proplatelet development by MKs was demonstrated to occur in vivo after TPO treatment.23 The CD49fHCD41H MKPs found in the FL represent a potential source of pure MKPs that are readily isolated by FACS or immunomagnetic methods, in contrast to the BM clonogenic MK population that is relatively small.4, 16 As observed in adult BM MKs, c-KitDCD49fHCD41H cells from E11.5 FL express several integrin receptors. The engagement of α4β1, and not αVβ3, has been proposed to enhance TPO-induced megakaryopoiesis.24 In FL c-KitDCD49fH CD41H cells, we observed weaker

α4 expression in relation to the αV chain, which may be related to the TPO-independent maturation of these cells in vitro. This observation is consistent with learn more findings from c-Mpl-deficient mice, in which MK generation occurs in a TPO-independent manner before E9.5 in the YS and before E14.5 in the FL.6, 25 The α6 integrin chain (CD49f) associates with either β1 (CD29) or β4 (CD104) integrin chains to form receptors for laminin and kalininis, respectively, and has been implicated in adhesion and in vivo homing. This chain is expressed by HSCs and myeloid progenitors in E14.5 FL and BM,26 among other cells, and by MKs and platelets generated in vitro.27 However, to the best of our knowledge, CD49f has

not commonly been associated with MKs ex vivo. In contrast to the adult BM MK-lineage cells (including selleck inhibitor mature MKs) and other embryonic myeloid cells, embryonic CD49fHCD41H MKPs do not express the hematopoietic marker (CD45). Several of the cell-surface markers presented by the CD49fHCD41H MKPs present in the FL at E11.5 differ from those in other hematopoietic niches (manuscript in preparation). Specifically, these cells express endothelial and hepatoepithelial proteins, the latter probably being taken up by endocytosis in the liver (where they would be produced by the HeP), because CD49fHCD41H MKPs appear to only weakly or not at all express HNF-1, HNF-4α, and HNF-3β. Similarly, most CD49fHCD41H MKPs are Dlk−CD13−, indicating that they are not liver stem/progenitor cells. The possible relationship between the few CD49fHDlk+ cells and the more-abundant CD49fD Dlk+CD13+ cells will require further analysis.