IntroductionHematopoietic stem cells (HSCs), which give rise to erythroid, myeloid, and lymphoid lineages, can be identified on the basis of the expression of CD34 and the absence of lineage-specific markers (termed LinϪ). 1 Human umbilical cord blood (CB)-derived cells provide a rich source of HSCs, which are comparable to bone marrow-derived HSCs. [2][3][4][5][6][7][8][9][10][11] T cells of the ␣␤ lineage differentiate in the thymus via discrete developmentally regulated steps that involve a series of commitment events and developmental checkpoints including T-cell receptor (TCR) V(D)J rearrangement, TCR-␤-selection, and positive/negative selection of developing thymocytes. 12 The earliest intrathymic progenitors express high levels of CD34 and CD7, do not express CD1a, and are triplenegative (TN) for mature T-cell markers CD4, CD8, and CD3. 8 Commitment to the T-cell lineage is strongly associated with the expression of CD1a by CD7-expressing thymocytes. 12,13 Several studies have implicated the Notch pathway in promoting HSC expansion, self-renewal, 14 survival, 15,16 and the induction of T-cell-lineage commitment. 15,[17][18][19][20][21] In humans there are 4 Notch receptors, 22-26 which can pair with 2 serrate-like ligands (Jagged 1 and 2) 27,28 or 3 Delta-like ligands (Dll1, 3, and 4). 29,30 Notch signaling acts at multiple stages during T-cell differentiation, influencing the choice to become an ␣␤-versus ␥␦-T cell, 31-34 as well as the decision to become a CD4 versus CD8 T cell. [35][36][37] The strongest evidence for the role of Notch signaling in T-cell development comes from gain-of-function and loss-of-function studies, 17,20,[38][39][40][41][42] in which signaling through Notch-1 was shown to play a crucial role in determining B-versus T-cell-lineage choice. 20,21 HSCs express multiple Notch receptors, 22,43 but the expression patterns of the various Notch ligands have been reported to be distinct between bone marrow stromal cells 29,[44][45][46][47] and thymic epithelial cells. 48 Taken together, these results suggest that different Notch receptors and ligands may control different aspects of hematopoiesis, depending on the microenvironment, allowing for self-renewal in the bone marrow and influencing cell fate decisions in the thymus. 46 This led us to hypothesize that bone marrow stromal lines, such as OP9 cells, 49 that support B-cell differentiation may do so because the appropriate Notch ligand to induce T-cell commitment and differentiation is absent. We recently demonstrated that the OP9 bone marrow stromal cell line, which does not express the Delta-like 1 (Dll1) Notch ligand, when retrovirally transduced to express Dll1 (OP9-DL1), inhibited the development of B cells and rather favored the development of T cells from fetal liver-derived HSCs 50 or mouse embryonic stem (ES) cells. 51 Given the high level of homology between mouse and human Dll1 molecules and the observation that mouse stromal lines can support the differentiation of human HSCs, 29,52-54 we sought to determine whether human...

IntroductionThe prevalence of nonhealing wounds is predicted to increase due to the growing aging population. Despite the use of novel skin substitutes and wound dressings, poorly vascularized wound niches impair wound repair. Mesenchymal stem cells (MSCs) have been reported to provide paracrine signals to promote wound healing, but the effect of human Wharton’s jelly-derived MSCs (WJ-MSCs) has not yet been described in human normal skin.The aim of this study is to examine the effects of human WJ-MSC paracrine signaling on normal skin fibroblasts in vitro, and in an in vivo preclinical model.MethodsHuman WJ-MSCs and normal skin fibroblasts were isolated from donated umbilical cords and normal adult human skin. Fibroblasts were treated with WJ-MSC-conditioned medium (WJ-MSC-CM) or nonconditioned medium.ResultsExpression of genes involved in re-epithelialization (transforming growth factor-β2), neovascularization (hypoxia-inducible factor-1α) and fibroproliferation (plasminogen activator inhibitor-1) was upregulated in WJ-MSC-CM-treated fibroblasts (P ≤ 0.05). WJ-MSC-CM enhanced normal skin fibroblast proliferation (P ≤ 0.001) and migration (P ≤ 0.05), and promoted wound healing in an excisional full-thickness skin murine model.ConclusionsUnder our experimental conditions, WJ-MSCs enhanced skin wound healing in an in vivo mouse model.