181 research outputs found
Embryonic Origins of the Hematopoietic System: Hierarchies and Heterogeneity
The hierarchical framework of the adult blood system as we know it from current medical and hematology textbooks, displays a linear branching network of dividing and differentiated cells essential for the growth and maintenance of the healthy organism. This view of the hierarchy has evolved over the last 75 years. An amazing increase in cellular complexity has been realized; however, innovative single-cell technologies continue to uncover essential cell types and functions in animal models and the human blood system. The most potent cell of the hematopoietic hierarchy is the hematopoietic stem cell. Stem cells for adult tissues are the long-lived self-renewing cellular component, which ensure that differentiated tissue-specific cells are maintained and replaced through the entire adult lifespan. Although much blood research is focused on hematopoietic tissue homeostasis, replacement and regeneration during adult life, embryological studies have widened and enriched our understanding of additional developmental hierarchies and interacting cells of this life-sustaining tissue. Here, we review the current state of knowledge of the hierarchical organization and the vast heterogeneity of the hematopoietic system from embryonic to adult stages
The Ly-6A (Sca-1) GFP transgene is expressed in all adult mouse hematopoietic stem cells
The Sca-1 cell surface glycoprotein is used routinely as a marker of adult
hematopoietic stem cells (HSCs), allowing a >100-fold enrichment of these
rare cells from the bone marrow of the adult mouse. The Sca-1 protein is
encoded by the Ly-6A/E gene, a small 4-exon gene that is tightly
controlled in its expression in HSCs and several hematopoietic cell types.
For the ability to sort and localize HSCs directly from the mouse, we
initiated a transgenic approach in which we created Ly-6A (Sca-1) green
fluorescent protein (GFP) transgenic mice. We show here that a 14-kb Ly-6A
expression cassette directs the transcription of the GFP marker gene in
all functional repopulating HSCs in the adult bone marrow. A >100-fold
enrichment of HSCs occurred by sorting for the GFP-expressing cells.
Furthermore, as shown by fluorescence-activated cell sorting and
histologic analysis of several hematopoietic tissues, the GFP transgene
expression pattern generally corresponded to that of Sca-1. Thus, the
Ly-6A GFP transgene facilitates the enrichment of HSCs and presents the
likelihood of identifying HSCs in situ
Development of hematopoietic stem cell activity in the mouse embryo.
The precise time of appearance of the first hematopoietic stem cell activity in the developing mouse embryo is unknown. Recently the aorta-gonad-mesonephros region of the developing mouse embryo has been shown to possess hematopoietic colony-forming activity (CFU-S) in irradiated recipient mice. To determine whether the mouse embryo possesses definitive hematopoietic stem cell activity in the analogous AGM region and to determine the order of appearance of stem cells in the yolk sac, AGM region, and liver, we transferred these embryonic tissues into adult irradiated recipients. We report here the long-term, complete, and functional hematopoietic repopulation of primary and serial recipients with AGM-derived cells. We observe potent hematopoietic stem cell activity in the AGM region before the appearance of yolk sac and liver stem cell activity and discuss a model for the maturation of stem cell activity in mouse embryogenesis
The role of apoptosis in the development of AGM hematopoietic stem cells revealed by Bcl-2 overexpression
Apoptosis is an essential process in embryonic tissue remodeling and adult
tissue homeostasis. Within the adult hematopoietic system, it allows for
tight regulation of hematopoietic cell subsets. Previously, it was shown
that B-cell leukemia 2 (Bcl-2) overexpression in the adult increases the
viability and activity of hematopoietic cells under normal and/or
stressful conditions. However, a role for apoptosis in the embryonic
hematopoietic system has not yet been established. Since the first
hematopoietic stem cells (HSCs) are generated within the
aortagonad-mesonephros (AGM; an actively remodeling tissue) region
beginning at embryonic day 10.5, we examined this tissue for expression of
apoptosis-related genes and ongoing apoptosis. Here, we show expression of
several proapoptotic and antiapoptotic genes in the AGM. We also generated
transgenic mice overexpressing Bcl-2 under the control of the
transcriptional regulatory elements of the HSC marker stem cell antigen-1
(Sca-1), to test for the role of cell survival in the regulation of AGM
HSCs. We provide evidence for increased numbers and viability of Sca-1(+)
cells in the AGM and subdissected midgestation aortas, the site where HSCs
are localized. Most important, our in vivo transplantation data show that
Bcl-2 overexpression increases AGM and fetal liver HSC activity, strongly
suggesting that apoptosis plays a role in HSC development
CFU-S(11) activity does not localize solely with the aorta in the aorta-gonad-mesonephros region
The aorta-gonad-mesonephros (AGM) region is a potent hematopoietic site in
the midgestation mouse conceptus and first contains colony-forming
units-spleen day 11 (CFU-S(11)) at embryonic day 10 (E10). Because
CFU-S(11) activity is present in the AGM region before the onset of
hematopoietic stem cell (HSC) activity, CFU-S(11) activity in the complex
developing vascular and urogenital regions of the AGM was localized. From
E10 onward, CFU-S(11) activity is associated with the aortic vasculature,
and is found also in the urogenital ridges (UGRs). Together with data
obtained from organ explant cultures, in which up to a 16-fold increase in
CFU-S(11) activity was observed, it was determined that CFU-S(11) can be
increased autonomously both in vascular sites and in UGRs. Furthermore,
CFU-S(11) activity is present in vitelline and umbilical vessels. This,
together with the presence of CFU-S(11) in the UGRs 2 days before HSC
activity, suggests both temporally and spatially distinct emergent sources
of CFU-S(11). (Blood. 2000;96:2902-2904
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