The human embryo, but not its yolk sac, generates lympho-myeloid stem cells: mapping multipotent hematopoietic cell fate in intraembryonic mesoderm

We have traced emerging hematopoietic cells along human early ontogeny by culturing embryonic tissue rudiments in the presence of stromal cells that promote myeloid and B cell differentiation, and by assaying T cell potential in the NOD-SCID mouse thymus. Hematogenous potential was present inside the embryo as early as day 19 of development in the absence of detectable CD34+ hematopoietic cells, and spanned both lymphoid and myeloid lineages from day 24 in the splanchnopleural mesoderm and derived aorta where CD34+ progenitors appear at day 27. By contrast, hematopoietic cells arising in the third week yolk sac, as well as their progeny at later stages, were restricted to myelopoiesis and therefore are unlikely to contribute to definitive hematopoiesis in man.

Functionally defined CD164 epitopes are expressed on CD34(+) cells throughout ontogeny but display distinct distribution patterns in adult hematopoietic and nonhematopoietic tissues

Three distinct classes of epitopes on human CD164 have been identified. Two of these, recognized by the monoclonal antibodies 105A5 and 103B2/9E10, are the CD164 class I and class II functionally defined epitopes, which cooperate to regulate adhesion and proliferation of CD34(+) cell subsets. In this article, we demonstrate that these 2 CD164 epitopes are expressed on CD34(+) cells throughout ontogeny, in particular on CD34(+ )cell clusters associated with the ventral floor of the dorsal aorta in the developing embryo and on CD34(+) hematopoietic precursor cells in fetal liver, cord blood, and adult bone marrow. While higher levels of expression of these CD164 epitopes occur on the more primitive AC133(hi)CD34(hi)CD38(lo/-) cell population, they also occur on most cord blood Lin(-)CD34(lo/-)CD38(lo/- )cells, which are potential precursors for the AC133(hi)CD34(hi)CD38(lo/-) subset. In direct contrast to these common patterns of expression on hematopoietic precursor cells, notable differences in expression of the CD164 epitopes were observed in postnatal lymphoid and nonhematopoietic tissues, with the class I and class II CD164 epitopes generally exhibiting differential and often reciprocal cellular distribution patterns. This is particularly striking in the colon, where infiltrating lymphoid cells are CD164 class I-positive but class II-negative, while epithelia are weakly CD164 class II-positive. Similarly, in certain lymphoid tissues, high endothelial venules and basal and subcapsular epithelia are CD164 class II-positive, while lymphoid cells are CD164 class I-positive. It therefore seems highly likely that these CD164 class I and II epitopes will mediate reciprocal homing functions in these tissue types.

The human AC133 hematopoietic stem cell antigen is also expressed in epithelial cells and targeted to plasma membrane protrusions

The human AC133 antigen and mouse prominin are structurally related plasma membrane proteins. However, their tissue distribution is distinct, with the AC133 antigen being found on hematopoietic stem and progenitor cells and prominin on various epithelial cells. To determine whether the human AC133 antigen and mouse prominin are orthologues or distinct members of a protein family, we examined the human epithelial cell line Caco-2 for the possible expression of the AC133 antigen. By both immunofluorescence and immunoprecipitation, the AC133 antigen was found to be expressed on the surface of Caco-2 cells. Interestingly, immunoreactivity for the AC133 antigen, but not its mRNA level, was down-regulated upon differentiation of Caco-2 cells. The AC133 antigen was specifically located at the apical rather than basolateral plasma membrane. An apical localization of the AC133 antigen was also observed in various human embryonic epithelia including the neural tube, gut, and kidney. Electron microscopy revealed that, within the apical plasma membrane of Caco-2 cells, the AC133 antigen was confined to microvilli and absent from the planar, intermicrovillar regions. This specific subcellular localization did not depend on an epithelial phenotype, because the AC133 antigen on hematopoietic stem cells, as well as that ectopically expressed in fibroblasts, was selectively found in plasma membrane protrusions. Hence, the human AC133 antigen shows the features characteristic of mouse prominin in epithelial and transfected non-epithelial cells, i.e. a selective association with apical microvilli and plasma membrane protrusions, respectively. Conversely, flow cytometry of murine CD34(+) bone marrow progenitors revealed the cell surface expression of prominin. Taken together, the data strongly suggest that the AC133 antigen is the human orthologue of prominin.

Expression of CXCR4, the receptor for stromal cell-derived factor-1 on fetal and adult human lympho-hematopoietic progenitors

Stromal cell-derived factor-1 (SDF-1) is a CXC chemokine produced by stromal cells that acts as a chemoattractant for human CD34+ progenitor cells. We investigated the expression of CXCR4, the receptor for SDF-1, on CD34+ cells from different hematopoietic sites and developmental stages. CXCR4 was detected by flow cytometry on 37 % of fetal bone marrow (BM) [gestation weeks (gw) 14-23] and 40% of adult BM CD34+ cells. Interestingly, in fetal liver CD34+ cells, CXCR4 was expressed at lower levels at later stages (9%, gw 20-23) compared to early stages of development (39%, gw 7.5-18), suggesting a development-related change in the migratory capacity of progenitors. CXCR4 was detected at similar levels on both phenotypically primitive and committed progenitors from fetal and adult sites. However, B cell lineage progenitor and precursor cells expressed CXCR4 at the highest density (80% of BM CD34+/CD10+ pro-B cells are CXCR4+). CXCR4 was also expressed in the fetal thymus in early T cell precursors and found to be down-regulated during T cell maturation. Finally, we found that stem cell factor, alone or in combination with other cytokines, can up-modulate CXCR4 expression on CD34+ cells by three- to fourfold. In conclusion, our results suggest that CXCR4 may play an important role in the local and systemic trafficking of human CD34+ cells as well as in human B lymphopoiesis and that its expression can be modulated by cytokines.

Emergence of intraembryonic hematopoietic precursors in the pre-liver human embryo

Hepatic hematopoiesis in the mouse embryo is preceded by two hematopoietic waves, one in the yolk sac, and the other in the paraaortic splanchnopleura, the presumptive aorta-gonad-mesonephros region that gives rise to prenatal and postnatal blood stem cells. An homologous intraembryonic site of stem cell emergence was previously identified at 5 weeks of human gestation, when hundreds of CD34(++)Lin- high-proliferative potential hematopoietic cells border the aortic endothelium in the preumbilical region. In the present study, we have combined immunohistochemistry, semithin section histology, fluorescence-activated cell sorting and blood cell culture in an integrated study of incipient hematopoiesis in the human yolk sac, truncal arteries and embryonic liver from 21 to 58 days of development. The chronology of blood precursor cell emergence in these distinct tissues suggests a pivotal role in the settlement of liver hematopoiesis of endothelium-associated stem cell clusters, which emerge not only in the dorsal aorta but also in the vitelline artery. Anatomic features and in vitro functionality indicate that stem cells develop intrinsically to embryonic artery walls from a presumptive territory whose blood-forming potential exists from at least 24 days of gestation.

Expression of the cell adhesion proteins BEN/SC1/DM-GRASP and TAG-1 defines early steps of axonogenesis in the human spinal cord

We have studied the expression pattern of two cell adhesion proteins of the immunoglobin (Ig) superfamily, BEN/SC1/DM-GRASP (BEN) and the transient axonal glycoprotein TAG-1, during the development of the human nervous system. This study was performed by immunocytochemistry on sections of human embryos ranging from 4 to 13 weeks postconception. The overall distribution of the two proteins during development is very similar to that reported in other vertebrate species, but several important differences have been observed. Both proteins exhibit a transient expression on selected neuronal populations, which include the motor and the sensory neurons. In addition, BEN was also detected on virtually all neurons derived from the neural crest as well as in nonneuronal tissues. A major difference of expression with the chick embryo is that, in the motor neurons, BEN expression was not observed at early stages of development, thus arguing against a role of this molecule in pathfinding and fasciculation. BEN was observed to be restricted to subsets of motor neurons, such as the medial column at the upper limb level. Expression was also detected in a laterodorsal population of the ventral horn cells, which are likely to correspond to migrating preganglionic neurons that originate from the motor pool at the thoracic level. TAG-1 was found on commissural neurons and weakly on the sympathetic neurons; it was also detected on restricted nonneuronal populations. In addition, we observed TAG-1 expression in fibers that could correspond either to subsets of dorsal root ganglia (DRGs) central afferences (including the Ia fibers) or to the axons of association interneurons and in scattered motoneurons likely to correspond either to preganglionic neurons, to gamma-motoneurons, or to late-born motoneurons. Therefore, our results indicate that the molecular strategies used to establish the axonal scaffolding of the nervous system in humans are extremely conserved among the different vertebrates.

Early ontogeny of the human marrow from long bones: an immunohistochemical study of hematopoiesis and its microenvironment

We examined long bones from 42 human embryos and fetuses whose gestational ages ranged from 6 to 28 weeks. Bone rudiment sections were stained using a panel of monoclonal antibodies directed at antigens expressed by hematopoietic cells, endothelial cells, smooth muscle cells, fibro-blasts, and stromal cells, to describe the events preceding and accompanying the onset of hematopoiesis in the diaphyseal region. Five distinct stages were identified. Stage I (6.6 to 8.5 gestational weeks [gw]) was that of entirely cartilaginous rudiments: chondrocytes were dilated and capillaries with CD34+ endothelial cells were observed in the perichondral limb mesenchyme. At stage II (8.5-9 gw) chondrolysis was actively proceeding; numerous CD68+ cells were observed, interspersed within the marrow cavity. Stage III (9 to 10.5 gw) was characterized by the development of the vascular bed in the absence of detectable hematopoiesis. At mid-diaphysis, specific structures that we named primary logettes were discernible; they consisted of small chambers of connective tissue, framed by a loose network of CD45-negative cells organized around an arteriole and limited from the surrounding sinus by a clearcut lining of CD34+ endothelial cells flanked on their abluminal side by alpha SM actin+ myoid cells. Stage IV (10.5-15 gw) was characterized by the onset of hematopoiesis. Hematopoietic cells were found exclusively in the primary logettes that had considerably increased in size. Logettes filled with hematopoietic cells were immersed within large and almost empty vascular sinuses. Logettes were attached by a short pedicle to connective tissue adjacent to bone/cartilage remaining formations; this tissue contained very rare hematopoietic cells. Logettes were few, usually less than 10 per long bone, and found solely in the diaphyseal area. Most hematopoietic cells found inside logettes were CD15+ myelocytes; rarely seen were glycophorin A+ immature erythroblasts and CD34+ nonendothelial cells. Hematopoietic cells within the logettes were in contact with alpha SM actin+ myoid cells and flattened endothelial-like (although consistently CD34-negative), aligned cells limiting small capillary lumina. Stage V (16 gw onward) was that of final organization of the long bones with areas of fully calcified bone and areas of dense hematopoiesis where logettes were no longer visible. This study shows three major features of incipient long bone hematopoiesis: 1) absence of CD34+ hematopoietic precursors before the onset of hematopoiesis and extreme rarity of those in the emerging blood-forming marrow, 2) predominance of granulopoiesis, and 3) exclusive development in specific structures organized by vascular cells. This study also suggests that CD68+ cells are instrumental in the chondrolysis process while vascular cells (endothelial and myoid cells) may be the critical microenvironment at the onset of hematopoiesis.

Aorta-associated CD34+ hematopoietic cells in the early human embryo

Hematopoiesis is established from circulating blood stem cells that seed the embryonic rudiments of blood-forming tissues, a basic notion in developmental hematology. However, the assumption that these stem cells originate from the extraembryonic mesoderm, where primitive hematopoiesis is initiated by intrinsic precursors, has been reconsidered after analysis of blood cell development in avian embryo chimeras: yolk-sac-derived stem cells do not contribute significantly to the definitive blood system, whose first forerunners develop independently along the ventral aspect of the embryonic aorta. Recently, the homologous intraembryonic tissues of the mouse have been submitted to sensitive in vivo and in vitro assays, which showed that they also harbor multipotential hematopoietic stem cells. We have now identified a dense population of hematogenous cells, marked by the surface expression of the CD34 glycoprotein, associated with the ventral endothelium of the aorta in the 5-week human embryo. Therefore, we extend to the human species the growing evidence that intraembryonic hematopoietic cells developing independently of the yolk sac might be the real stem of the whole blood system.

Immunoglobulin and T cell receptor gene rearrangements and in situ immunophenotyping in lymphoproliferative disorders

We investigated for rearrangements of the immunoglobulin (Ig) heavy and light chain genes and of the T cell receptor gamma (TCR gamma) and beta (TCR beta) genes 45 biopsy samples from a variety of lymphoproliferative disorders. They were diagnosed histopathologically and immunophenotypically as non-Hodgkin's lymphomas (NHLs) of the B cell type (19 cases), NHLs of the T cell type (3 cases), NHLs of "undetermined" cell type (3 cases), atypical lymphoid proliferation (1 case) and AIDS-related lymphadenopathies with florid polyclonal follicular hyperplasia (19 cases). A monoclonal proliferation of B cells was shown by DNA analysis in all 19 B cell NHLs. In two immunohistologically determined T cell NHLs (both diagnosed as mycosis fungoides) the cells had rearrangements of TCR beta gene, whereas in the third case (lymphoblastic NHL) the cells had rearrangements of Ig heavy chain and TCR gamma and TCR beta genes. None of the B cell NHLs exhibited TCR gamma and TCR beta gene rearrangement bands. All the "undetermined" cell NHLs demonstrated rearrangements of Ig heavy chain gene associated with the germ line TCR gamma and TCR beta genes; in two cases light chain gene rearrangements were also found. The atypical lymphoid proliferation, in which the differential diagnosis was between a reactive or malignant process, and two out of 19 cases of florid polyclonal follicular hyperplasia showed a clonal B cell population by DNA analysis. This study indicates that there was a strong correlation between the rearrangements of specific genes and the immunophenotype of the NHL; moreover, DNA analysis of tissue biopsy specimens from phenotypically "undetermined" cell NHLs and from equivocal lymphoid proliferation using Ig and TCR gene probes yielded an answer in the cases analyzed. The significance of clonal B cell expansions found in two AIDS-related lymphadenopathies should be interpreted with caution.

Nuclear oncogene amplification or rearrangement is not involved in human colorectal malignancies

We have examined 44 cases of human colonic and rectal carcinomas for structural rearrangement and amplification of c-myc, N-myc, L-myc, c-myb and p53 oncogenes. DNA hybridization showed evidence of c-myc amplification in only one of the samples tested. In addition, the same tumour also showed a rearrangement immediately 3' to the c-myc locus. No rearrangement could be found at the c-myc locus in the other 43 cases. Moreover, our molecular analysis of N-myc, L-myc, c-myb and p53 genes indicated no relevant alteration of the copy number and/or genomic structure of these nuclear oncogenes. Thus, at least in human colorectal malignancies, it is unlikely that nuclear oncogene structural alterations and/or amplification plays a major role in tumour induction or progression.