T cell ontogeny: extrathymic and intrathymic development of embryonic lymphohemopoietic stem cells

Blood cell progenitors: Insights into the properties of stem cells

Hematopoiesis is a dynamic process in which eight lineages of mature blood cells are derived from a common stem cell. Great progress has been made in identifying the functionally disparate progenitors that emerge from the stem cell and in elucidating the molecules required for their growth and survival. Further work will be required to understand the molecular mechanisms that regulate commitment of stem and progenitor cells to each stage of progenitor cell development and ultimately into the mature blood cells.

Quantitative developmental anatomy of definitive haematopoietic stem cells/long-term repopulating units (HSC/RUs): role of the aorta-gonad-mesonephros (AGM) region and the yolk sac in colonisation of the mouse embryonic liver

In the developing mouse embryo the first definitive(transplantable-into-the-adult) haematopoietic stem cells/long-term repopulating units (HSC/RUs) emerge in the AGM region and umbilical vessels on 10-11 days post coitum (d.p.c.). Here, by limiting dilution analysis, we anatomically map the development of definitive HSC/RUs in different embryonic tissues during early colonisation of the liver. We show that by day 12 p.c. the mouse embryo contains about 66 definitive HSC/RUs (53 in the liver, 13 in other tissues), whereas on the previous day the total number of definitive HSC/RUs in the entire conceptus is only about 3. Owing to the length of the cell cycle this dramatic increase in the number of definitive HSC/RUs in only 24 hours is unlikely to be explained purely by cell division. Therefore,extensive maturation of pre-definitive HSCs to a state when they become definitive must take place in the day 11-12 embryo. Here we firstly identify the numbers of HSCs in various organs at 11-13 d.p.c. and secondly, using an organ culture approach, we quantitatively assess the potential of the aorta-gonadmesonephros (AGM) region and the yolk sac to produce/expand definitive HSC/RUs during days 11-12 of embryogenesis. We show that the capacity of the AGM region to generate definitive HSC/RUs is high on 11 d.p.c. but significantly reduced by 12 d.p.c. Conversely, at 12 d.p.c. the YS acquires the capacity to expand and/or generate definitive HSCs/RUs, whereas it is unable to do so on 11 d.p.c. Thus, the final steps in development of definitive HSC/RUs may occur not only within the AGM region, as was previously thought, but also in the yolk sac microenvironment. Our estimates indicate that the cumulative activity of the AGM region and the yolk sac is sufficient to provide the day 12 liver with a large number of definitive HSC/RUs,suggesting that the large pool of definitive HSC/RUs in day 12 foetal liver is formed predominantly by recruiting `ready-to-use' definitive HSC/RUs from extra-hepatic sources. In accordance with this we observe growing numbers of definitive HSC/RUs in the circulation during days 11-13 of gestation,suggesting a route via which these HSCs migrate.

In vivo repopulating hematopoietic stem cells are present in the murine yolk sac at day 9.0 postcoitus

The murine yolk sac, being the first site of embryonic blood cell production, has long been theorized to contain the migrating hematopoietic stem cells (HSC) that seed the liver and initiate hematopoiesis on day 10.0 postcoitus (pc). However, it remains controversial whether yolk sac cells isolated before day 11.0 pc possess any long-term repopulating HSC activity upon transplantation into adult recipient mice. We hypothesized that failure to demonstrate engraftment of day <11.0 yolk sac cells in adult hosts may result from an inability of yolk sac cells to home to the active adult hematopoietic sites (spleen and bone marrow). In the present studies, we transplanted yolk sac cells into conditioned newborn mice in whom the liver, as well as the spleen and bone marrow, concomitantly function as a site of blood cell formation. We report that yolk sac cells isolated from day 9.0 pc embryos provide long-term multilineage reconstitution for at least 11 months in primary conditioned newborn mice and for at least 6 months in secondary recipients. Donor yolk sac HSC progeny repopulated mature peripheral blood, thymus, spleen, and bone marrow lymphoid, myeloid, and erythroid compartments. Thus, day 9.0 pc yolk sac HSC can contribute to definitive multilineage hematopoiesis in transplanted recipients. Determination of HSC activity in the day 9.0 pc murine yolk sac suggests that yolk sac HSC are available to seed the liver on day 10.0 pc when definitive hematopoiesis is initiated.

Engraftment of Embryonic Hematopoietic Cells in Conditioned Newborn Recipients

Yolk sac hematopoiesis is characterized by restricted hematopoietic cell differentiation. Although multipotent hematopoietic progenitor cells have been identified in the early yolk sac, long-term multilineage repopulating (LTMR) hematopoietic stem cell (HSC) activity has not been demonstrable before day 11 postcoitus (PC) using standard transplantation assays. In the present study, day-10 PC yolk sac hematopoietic cells were infused into myeloablated congenic newborn pups and donor cell engraftment and multilineage reconstitution of peripheral blood cells for at least 11 months in primary recipients was observed. In contrast, transplantation of day-10 PC yolk sac cells into congenic adult recipients did not result in engraftment despite pretransplant conditioning of the recipients or use of recipients that were genetically deficient in stem cells. Although fresh yolk sac cells were incapable of reconstitution when injected into adult recipient mice, yolk sac donor-derived cells residing in the bone marrow of primary newborn transplant recipients were capable of efficient reconstitution of conditioned secondary recipient adult mice. Primary newborn and secondary adult recipient animals engrafted with yolk sac cells were observed to have normal peripheral blood white blood cell counts. Lymphocyte subsets in peripheral blood, thymus, and spleen were also similar to control animals. The distribution and frequency of lineage-restricted progenitors derived from bone marrow of secondary transplant recipients were normal. These results indicate that day-10 PC yolk sac HSCs are capable of engrafting and reconstituting the hematopoietic system of conditioned newborn but not adult recipient animals. Furthermore, the ability of the yolk sac HSCs to differentiate into all hematopoietic lineages in these recipients strongly suggests that the local cellular microenvironment plays a prominent role in regulating yolk sac HSC differentiation.

Hematopoietic stem cells in the mouse embryonic yolk sac

The yolk sac is the first site of hematopoiesis during mammalian development. The yolk sac is also the first site of blood vessel development. Development of the blood islands in the yolk sac is an integrated process in which these two developmental events, hematopoiesis and vasculogenesis, proceed in concert. This review focuses on mouse yolk sac hematopoietic stem cells (YS-HSC), describing their differentiation in vitro and in vivo. YS-HSC go through a progressive series of changes prior to the initiation of lineage-specific differentiation. Experiments tracing their origins from postulated hemangioblasts, and the subsequent interaction between these stem cells and yolk sac endothelial cells are described. Differences between the extraembryonic YS-HSC and HSC found later within the embryo, perinatally or in adults, are described. YS-HSC have greater reproductive capability than HSC obtained from fetal liver, umbilical cord blood or adult bone marrow; they do not yet express major histocompatibility complex-associated antigens and they are able to reconstitute adult immunocompromised animals even when introduced in small numbers (< 100 cells/mouse). With recent results demonstrating the feasibility of expanding YS-HSC in vitro as well as of introducing new genes into these cells by transfection, the YS-HSC shows promise both as a means of achieving long-term restitution of hematopoiesis across histocompatibility barriers and as a self-renewing vehicle for gene transfer.

Murine fetal liver augments proliferation in an allogenic mixed lymphocyte culture: benzo(a)pyrene reduces augmentation

The effect of cells from the liver of C3H fetuses syngeneic to splenic responder cells on an allogeneic mixed lymphocyte response (MLR) was studied. After fractionation on ficoll-hypaque, interface cells from corn oil (vehicle for BP) or normal fetal liver (FL) controls (CO), obtained from 17-19 days gestation, enhanced proliferation in the mixed lymphocyte culture (> 2-fold), while cells from FLs transplacentally exposed to benzo(a)pyrene (BP) showed a decreased capacity for augmentation (> 2-fold less than CO). Unfractionated CO-FL cells at 0.5 x 10(6) did not augment proliferation, but at 0.25 x 10(6) enhancement with control FL cells was significantly higher than with BP-FL cells. Pelleted cells from BP- and CO-FLs were severely suppressive at the higher dose, while at 0.25 x 10(6) proliferation was augmented with CO-FL cells, but not affected with BP-FL cells. At doses of 0.1 x 10(6) or less, no effect was observed for either control or BP-FL cells. These data indicate that: a) FL cells syngeneic to responder cells of an allogenic mixed lymphocyte culture (MLC) have an augmenting but not suppressive capacity on cell proliferation; b) in utero insult with BP modifies the capacity of FL cells to augment proliferation in the MLC.

An early pre-liver intraembryonic source of CFU-S in the developing mouse

It is widely accepted that during murine embryogenesis, totipotent haematopoietic stem cells first originate in the yolk sac, then migrate to the fetal liver and finally colonize the bone marrow shortly before birth. This view is based on in vitro studies showing that yolk sac cells can differentiate into various haematopoietic lineages and in vivo studies showing that yolk sac contains spleen colony-forming units (CFU-S) beginning at day 8 of gestation. However, some investigators have failed to find statistically significant numbers of CFU-S arising from day 9 yolk sac and, although one group reported that yolk sac could repopulate the haematopoietic system of W mutant mice, others have failed to confirm yolk sac-derived repopulation of adults. In the avian and amphibian systems, the yolk sac gives rise only to early, transitory haematopoiesis whereas the definite adult haematopoietic stem cells in these vertebrates are derived from the mesodermal region containing the dorsal aorta. Because this analogous area of the mouse embryo has not been previously examined for haematopoietic activity, we directly compared the CFU-S activity of the aorta, gonad, mesonephros (AGM) region with the yolk sac and fetal liver during embryogenesis. Here we report that this intra-embryonic AGM region contains CFU-S activity at a higher frequency than that in embryonic yolk sac and that such activity appears in the AGM region before the fetal liver.

Aging in the T lymphocyte compartment. A developmental view

A decline in the capacity of bone marrow cells to differentiate to T lymphocytes was found when cells from young and old donors were seeded onto an alymphoid fetal thymus. A step-by-step analysis of cell-cell interactions of the lymphohemopoietic cells and the thymic stroma indicated an effect of age on a variety of cell differentiation parameters. These included a decrease in the affinity of bone marrow cells to the stroma, and in their capacity to compete with the thymic lymphoid resident cells on colonization of the thymus. There was a significant decrease in the ability of cells of old donors to replicate sequentially within the thymic microenvironment. There was a reduced capacity of bone marrow cells from aging mice to express a developmental preference after seeding onto a syngeneic fetal thymus in a mixture with cells from allogeneic donors. We addressed the question whether the aging thymus contains increased levels of immature cells that fail to differentiate in the involuted thymic microenvironment by seeding thymocytes from young and old donors onto the fetal thymic stroma. The values of T cells that developed from the old donor inoculum were lower under these conditions. Our studies suggest that at least some of the manifestations of aging in the T cell compartment are related to developmentally programmed events in the lymphohemopoietic cell compartment.

Ontogeny of murine T cells: thymus-regulated development of T cell receptor-bearing cells derived from embryonic yolk sac

Pre-thymic stem cells which arise extrinsically to the thymus differentiate into mature T cells after colonizing the fetal thymus, but the origin of the stem cells during mammalian embryogenesis remains unknown. More than one potential site has been proposed, including the liver, the omentum and the embryonic yolk sac (YS). Since the murine YS appears earliest in gestation and is the first site of hematopoiesis, our studies focus on the mouse YS as the early major source of pre-thymic stem cells. Previous studies showed that freshly obtained YS cells expressing no known major T cell marker can develop into Thy-1+ cells in vitro either transiently or stably. However, the critical steps leading to functional maturation of T cells involve the rearrangement and expression of the T cell receptor (TcR) genes. We report here that YS cells grown in short-term cell culture show no detectable rearrangement or transcription at the TcR gamma gene locus, although the TcR gamma gene of in vitro transformed YS cells rearranges in the absence of the thymus. However, after seeding lymphocyte-depleted thymuses in vitro, YS cells can differentiate into various subsets of cells appearing in the thymus, including CD3/TcR alpha/beta- and gamma/delta-bearing cells. TcR V gamma 3 which is expressed mainly on early fetal thymocytes and on Thy-1+ dendritic cells can be detected on YS-derived cells. TcR V beta 8-expressing cells which appear later in thymic ontogeny are also detected. The kinetics for reconstituting the thymus varies among YS cells of different ages (day 10 to 13 gestation). These results indicate that a single wave of early mouse YS cells can undergo the molecular and cellular changes associated with T cell differentiation and acquire mature T cell characteristics in the absence of continuous inflow of other stem cells. The thymus acts in an inductive/regulatory role for that differentiation.