CD41 expression defines the onset of primitive and definitive hematopoiesis in the murine embryo

The platelet glycoprotein IIb (alpha(IIb); CD41) constitutes the alpha subunit of a highly expressed platelet surface integrin protein. We demonstrate that CD41 serves as the earliest marker of primitive erythroid progenitor cells in the embryonic day 7 (E7.0) yolk sac and high-level expression identifies essentially all E8.25 yolk sac definitive hematopoietic progenitors. Some definitive hematopoietic progenitor cells in the fetal liver and bone marrow also express CD41. Hematopoietic stem cell competitive repopulating ability is present in CD41(dim) and CD41(lo/-) cells isolated from bone marrow and fetal liver cells, however, activity is enriched in the CD41(lo/-) cells. CD41(bright) yolk sac definitive progenitor cells co-express CD61 and bind fibrinogen, demonstrating receptor function. Thus, CD41 expression marks the onset of primitive and definitive hematopoiesis in the murine embryo and persists as a marker of some stem and progenitor cell populations in the fetal liver and adult marrow, suggesting novel roles for this integrin.     

Common antigen of oval and biliary epithelial cells (A6) is a differentiation marker of epithelial and erythroid cell lineages in early development of the mouse

Abstract. The A6 antigen - a surface-exposed component shared by mouse oval and biliary epithelial cells - was examined during prenatal development of mouse in order to elucidate its relation to liver progenitor cells. Immunohistochemical demonstration of the antigen was performed at the light and electron microscopy level beginning from the 9.5 day of gestation (26–28 somite pairs).
Up to the 11.5 day of gestation A6 antigen is found only in the visceral endoderm of yolk sac and gut epithelium, while liver diverticulum and liver are A6-negative. In the liver epithelial lineages A6 antigen behaves as a strong and reliable marker of biliary epithelial cells where it is found beginning from their emergence on the 15th day of gestation. It was not revealed in immature hepato-cytes beginning from the 16th day of gestation. However weak expression of the antigen was observed in hepato-blasts on 12–15 days of gestation possibly reflecting their ability to differentiate along either hepatocyte or biliary epithelial cell lineages.
Surprisingly, A6 antigen turned out to be a peculiar marker of the crythroid lineage: in mouse fetuses it distinguished A6 positive liver and spleen erythroblasts from A6 negative early hemopoietic cells of yolk sac origin. Moreover in the liver, A6 antigen probably distinguishes two waves of erythropoiesis: it is found on the erythroblasts from the 11.5 day of gestation onward while first extravascular erythroblasts appear in the liver on the 10th day of gestation. Both fetal and adult erythrocytes are A6-negative.
In the process of organogenesis A6 antigen was revealed in various mouse fetal organs. Usually it was found on plasma membranes of mucosal or ductular epithelial cells. Investigation of A6 antigen's physiological function would probably explain such specific localization.     

Embryonic macrophages of early rat yolk sac: Immunohistochemistry and ultrastructure with reference to endodermal cell layer

Macrophages are multifunctional cells that participate in numerous biological processes; they actively phagocytose foreign particles and cell debris. Embryonic tissue macrophages are present at early stages of mammalian development; their ontogeny and function is still under investigation. Our study used immunohistochemistry and electron microscopy to investigate early rat yolk sac macrophages using mouse antirat macrophage monoclonal antibodies (mAb) Mar 1 and Mar 3 produced by our laboratory. Mar 3 mAb revealed the first emergence of immature macrophages in the rat yolk sac at fetal day nine coinciding with the beginning of yolk sac haemopoiesis that consisted mainly of erythropoiesis, while Mar 1 mAb detected specifically rat yolk sac macrophages at about the 13th to 14th day of gestation. Immunoreactivity against Mar mAbs was mainly located in the yolk sac endodermal cell layer, which may signify endodermal origin of the yolk sac macrophages. Ultrastructurally mature yolk sac macrophages contained numerous endocytic vesicles or vacuoles, well-developed Golgi saccules and many electron dense granules in their cytoplasm and a number of microvillous projections from the cell surface. After establishment of the circulation between yolk sac and embryo, Mar 3 positive cells were also demonstrated inside fetal undifferentiated mesenchymal tissue at fetal day 12. The study demonstrated the first emergence of immature yolk sac macrophages being among the earliest haemopoietic cells formed in mammalian development. Thus, Mar mAbs managed to detect macrophage differentiation antigens through their development early in the rat yolk sac.     

Coexpression of the C-MYC protooncogene with α-fetoprotein and albumin in fetal mouse liver

The level of mRNAs for the c-myc protooncogene and the serum proteins alpha-fetoprotein (AFP) and albumin in liver, visceral yolk sac and gut between day 9 and day 19 of mouse gestation was studied by in situ hybridization employing single-stranded RNA probes. In the prehepatocyte population, c-myc was coexpressed with albumin and AFP. No heterogeneity was noted within this cell population with respect to the expression of these mRNAs up to day 15. AFP expression was high in the liver primordium and rose further until day 15. Albumin mRNA was expressed weakly but distinctly in the hepatic bud and increased throughout fetal life. C-myc expression in prehepatocytes exhibited a maximum around day 13 and a dramatic decline after day 15, but was much lower in other cell types of the fetal liver. In the visceral yolk sac, AFP was strongly expressed, with albumin expression first becoming detectable at day 13, while c-myc mRNA was detected up to day 9. In the endodermal gut epithelium, c-myc expression was high, albumin mRNA was not detected and AFP message was restricted to individual loops of the gut. These results suggest that a period of high c-myc expression in the developing liver may allow rapid expansion of the prehepatocyte population at a specific stage of differentiation.     

Of birds and mice: hematopoietic stem cell development

For many years it has been assumed that the ontogeny of the mammalian hematopoietic system involves sequential transfers of hematopoietic stem cells (HSCs) generated in the yolk sac blood islands, to successive hematopoietic organs as these become active in the embryo (fetal liver, thymus, spleen and eventually bone marrow). Very little was known about early events related to hematopoiesis that could take place during the 4.5 day gap separating the appearance of the yolk sac blood islands and the stage of a fully active fetal liver. Experiments performed in birds documented that the yolk sac only produce erythro-myeloid precursors that become extinct after the emergence of a second wave of intra-embryonic HSCs from the region neighbouring the dorsal aorta. The experimental approaches undertaken over the last ten years in the murine model, which are reviewed here, led to the conclusion that the rules governing avian hematopoietic development basically apply to higher vertebrates.     

In vitro development of murine T cells from prethymic and preliver embryonic yolk sac hematopoietic stem cells.

Mature T cells are derived from prethymic stem cells, which arise at one or more extrathymic sites and enter and differentiate in the thymus. The nature of these prethymic stem cells is a critical factor for the formation of the T-cell repertoire. Although the bone marrow of adult mice can provide such stem cells, their origin during murine embryogenesis is still undetermined. Among potential sites for these progenitor cells are the fetal liver and the embryonic yolk sac. Our studies focus on the yolk sac, both because the yolk sac appears earlier than any other proposed site, and because the mammalian yolk sac is the first site of hematopoiesis. Although it has been shown that the yolk sac in midgestation contains stem cells that can enter the thymic rudiment and differentiate toward T-cell lineage, our aim was to analyze the developmental potential of cells in the yolk sac from earlier stages, prior to the formation of the liver and any other internal organ. We show here that the yolk sac from 8- and 9-day embryos (2-9 and 13-19 somites, respectively) can reconstitute alymphoid congenic fetal thymuses and acquire mature T-cell-specific characteristics. Specifically, thymocytes derived from the early embryonic yolk sac can progress to the expression of mature T lymphocyte markers including CD3/T-cell receptor (TCR), CD4 and CD8. In contrast, we have been unable to document the presence of stem cells within the embryo itself at these early stages. These results support the hypothesis that the stem cells capable of populating the thymic rudiment originate in the yolk sac, and that their presence as early as at the 2- to 9-somite stage may indicate that prethymic stem cells found elsewhere in the embryo at later times may have been derived by migration from this extra-embryonic site. Our experimental design does not exclude the possibility of multiple origins of prethymic stem cells of which the yolk sac may provide the first wave of stem cells in addition to other later waves of cells.     

The ontogeny of expression of murine metallothionein: comparison with the alpha-fetoprotein gene

The ontogeny of expression of mouse metallothionein was studied by RNA dot and Northern blot hybridization using a cloned cDNA probe. In some instances the synthesis of metallothionein was analyzed by cell-free translation of RNA as well as pulse-labeling of proteins in short-term organ cultures followed by polyacrylamide gel electrophoresis. Interesting parallels between metallothionein and alpha-fetoprotein gene expression during development were noted. Like alpha-fetoprotein mRNA ( Dziadek and Andrews, 1983), metallothionein mRNA was found to be abundant in developing liver as well as in visceral yolk sac endoderm. In addition, metallothionein mRNA was abundant in parietal yolk sac. During liver development metallothionein and alpha-fetoprotein mRNAs were abundant by Day 12 of gestation, increasing to maximal levels on Day 16 and decreasing during late fetal and neonatal life to basal levels in adult. Metallothionein mRNA increased in maternal liver and was also abundant in certain hepatomas. Synthesis of metallothionein and levels of metallothionein mRNA in visceral yolk sac increased from Day 9 of gestation to maximal levels on Days 11-12 and then decreased abruptly after Day 15. RNA from differentiated teratocarcinoma cells with primitive, parietal or visceral endoderm characteristics each contained high levels of metallothionein mRNA, whereas, levels of this mRNA varied widely among embryonal carcinoma stem cell lines. alpha-Fetoprotein mRNA was not detected in embryonal carcinoma cells but was expressed in visceral endoderm-like differentiated cells. These results indicate that parietal and visceral endoderm cells actively express the metallothionein gene and further suggest that expression may be initiated at the earlier stage of primitive endoderm.     

On the kinetics of erythroid cell differentiation in fetal mice. I. Microspectrophotometric determination of the hemoglobin content in erythroid cells during gestation

In a microspectrophotometric study, photographic emulsions and a computer are used for measuring the hemoglobin content of a large number (about 50,000) of erythroid cells in fetal mice. Histograms of the hemoglobin content in erythroid cells illustrate the kinetics of erythropoiesis in yolk sac derived nucleated cells in the fetal peripheral blood, in fetal liver, and in fetal spleen. After the occasional extrusion of their nucleus, yolk sac derived erythrocytes remain as “macrocytes” in fetal circulation two or three days longer than the nucleated yolk sac derived erythrocytes do. Erythrocytes in fetal liver have a constant hemoglobin content of 28 pg ² until day 17 of gestation. During further erythropoiesis in liver and then in the spleen, this amount is gradually adapted to the normal hemoglobin content in red blood cells of 16 pg.     

On the kinetics of erythroid cell differentiation in fetal mice. II. DNA and hemoglobin measurements of individual erythroblasts during gestation

Microspectrophotometric absorption measurements were used to determine the hemoglobin content of erythroid cells derived from the yolk sac during gestation of fetal C3H mice, from day 9 to day 15. Using the DNA content as a marker for the mitotic state between 2C and 4C phase, five successive cell generations and their mean hemoglobin contents were distinguished: 12 pg (pg, picogram = 10^?12 gm). 22.2 pg, 37 pg, 50 pg and 56 pg. In the final state, nucleated erythrocytes contained 98 ± 22 pg hemoglobin. Erythroid cells derived from the liver were measured on day 15 of fetal gestation. The hemoglobin content of proerythroblasts was below 0.3 pg. The two cell generations in the basophilic state had 0.6 pg and 1.7 pg respectively. Polychromatic erythroblasts yielded a hemoglobin content of 5.1 pg in the first cell generation and 7.5 pg in the second one. Orthochromatic erythroblasts contained 8 pg, reticulocytes 12 pg and mature erythrocytes 28 ± 7 pg hemoglobin. Calculations based on these data suggest that the rate of total hemoglobin synthesis is similar in both yolk sac and liver erythropoiesis. The difference between the final hemoglobin content in nucleated erythrocytes of yolk sac origin and that in hepatic erythrocytes can be explained by the different cell generation times.     

Antibody repertoire development in fetal and neonatal piglets. VI. B cell lymphogenesis occurs at multiple sites with differences in the frequency of in-frame rearrangements

B cell lymphogenesis in mammals occurs in various tissues during development but it is generally accepted that it operates by the same mechanism in all tissues. We show that in swine, the frequency of in-frame (IF) VDJ rearrangements differs among yolk sac, fetal liver, spleen, early thymus, bone marrow, and late thymus. All VDJ rearrangements recovered and analyzed on the 20th day of gestation (DG20) from the yolk sac were 100% IF. Those recovered at DG30 in the fetal liver were >90% IF, and this predominance of cells with apparently a single IF rearrangement continued in all organs until approximately DG45, which corresponds to the time when lymphopoiesis begins in the bone marrow. Thereafter, the proportion of IF rearrangements drops to approximately 71%, i.e., the value predicted whether VDJ rearrangement is random and both chromosomes were involved. Unlike other tissues, VDJs recovered from thymus after DG50 display a pattern suggesting no selection for IF rearrangements. Regardless of differences in the proportion of IF rearrangements, we observed no significant age- or tissue-dependent changes in CDR3 diversity, N region additions, or other characteristics of fetal VDJs during ontogeny. These findings indicate there are multiple sites of B cell lymphogenesis in fetal piglets and differences in the frequency of productive VDJ rearrangements at various sites. We propose the latter to result from differential selection or a developmentally dependent change in the intrinsic mechanism of VDJ rearrangement.     

Expression of Dnmt3b in mouse hematopoietic progenitor cells and spermatogonia at specific stages

Two de novo-type DNA methyltransferases, Dnmt3a and Dnmt3b, are responsible for the creation of DNA methylation patterns during development. Dnmt3b is specifically expressed in the totipotent cells of mouse early embryos and Dnmt3a, a longer form of the two isoforms, is ubiquitously expressed in mesenchyme cells after the 10 day embryo stage [Mech. Dev. 118 (2002) 187]. In the present study, we demonstrated that Dnmt3b was expressed in the nuclei of specific cells in certain tissues after the 10 day embryo stage. In fetal liver, dorsal aorta and portal vein, Dnmt3b was expressed in cells expressing CD34, indicating that the cells were hematopoietic progenitor cells. However, Dnmt3b was not expressed in the hematopoietic progenitor cells in yolk sac blood islands at 8 day embryo stage and in adult bone marrow cells. Dnmt3b was also expressed in type-A spermatogonia after birth. Dnmt3b was expressed not only in the totipotent stem cells but also in the progenitor cells the direction of differentiation of which had been already determined. On the other hand, the long form of Dnmt3a was not expressed in these hematopoietic progenitor cells in fetal liver or type-A spermatogonia, but was expressed in hepatocytes in fetal liver and type-B spermatogonia. While Dnmt3b was distributed in both the heterochromatin and euchromatin regions, Dnmt3a was specifically localized to the euchromatin region.     

Characteristics of murine yolk sac erythroid progenitors and their population expansion in liquid culture

Remarkable differences were found between late erythroid progenitors (CFU-e) in cultures of murine yolk sac cells and those of fetal liver cells with respect to frequency, erythropoietin responsiveness and colony size. Cultures of yolk sac on day 11 of gestation showed a CFU-e population of lower frequency, less sensitivity to erythropoietin and smaller colony size than those from cultures of day 14 fetal liver cells. As the proportion of CFU-e to BFU-e was much lower in yolk sac than that in fetal liver, 48-96 h liquid culture experiments were done with these cells to examine the capacity of their precursors to generate a certain amount of CFU-e subpopulations. The cultures of yolk sac cells produced large numbers of CFU-e which formed some large-sized colonies but those of fetal liver cells generated only a small amount of CFU-e.     

Primitive lymphohematopoietic precursor cell lines generated in culture from day 7 early-mid-primitive streak stage mouse embryo.

During mouse development, the first lymphohematopoietic precursor cells and myeloid or erythroid cell lineage-determined cells can be detected in the yolk sac at days 8-8.5 of gestation. The characteristics of the cells that give rise to these yolk sac primitive lymphohematopoietic cells and the molecular events controlling this process remain poorly defined. We show here that cell suspensions from day 7 early-mid-primitive streak stage embryo proper generated early immature PgP-1+ Joro 177+ Lin- hematopoietic cells and some Mac-1+ myeloid and TER 119+ erythroid cells after co-culture with the yolk sac-derived stromal cell line YS6 without addition of exogenous cytokines. Purified Lin- hematopoietic cells generated in these cultures did not express genes known to be transcribed at early stages of lymphoid, myeloid or erythroid cell differentiation and were able to give rise to T and B lymphocytes, myeloid cells and erythroid cells after appropriate further induction in vitro. Several cell lines were established in culture with a mixture of four cytokines from the PgP-1+ Joro 177+ Lin- cell population. The cell lines shared phenotypic and genotypic characteristics with the PgP-1+ Joro 177+ Lin- cell population generated in culture from day 7 embryo proper and they were able to reconstitute the lymphohematopoietic system of irradiated mice. Taken together these results support a model of lymphohematopoiesis in which cells from day 7 early-mid-primitive streak mouse embryo proper migrate and colonize the visceral yolk sac. There they generate primitive lymphohematopoietic precursor cells and the first erythroid and myeloid hematopoietic cells under the influence of yolk sac stromal cells like the YS6 cells described here.     

Kit ligand has a critical role in mouse yolk sac and aorta–gonad–mesonephros hematopoiesis

Few studies report on the in vivo requirement for hematopoietic niche factors in the mammalian embryo. Here, we comprehensively analyze the requirement for Kit ligand (Kitl) in the yolk sac and aorta–gonad–mesonephros (AGM) niche. In‐depth analysis of loss‐of‐function and transgenic reporter mouse models show that Kitl‐deficient embryos harbor decreased numbers of yolk sac erythro‐myeloid progenitor (EMP) cells, resulting from a proliferation defect following their initial emergence. This EMP defect causes a dramatic decrease in fetal liver erythroid cells prior to the onset of hematopoietic stem cell (HSC)‐derived erythropoiesis, and a reduction in tissue‐resident macrophages. Pre‐HSCs in the AGM require Kitl for survival and maturation, but not proliferation. Although Kitl is expressed widely in all embryonic hematopoietic niches, conditional deletion in endothelial cells recapitulates germline loss‐of‐function phenotypes in AGM and yolk sac, with phenotypic HSCs but not EMPs remaining dependent on endothelial Kitl upon migration to the fetal liver. In conclusion, our data establish Kitl as a critical regulator in the in vivoAGM and yolk sac endothelial niche.     

Studies on the mechanism of trypan blue teratogenicity in the rat developing in vivo and in vitro

Trypan blue is a potent teratogen in vivo and in vitro in the rat. Many of the abnormalities produced by trypan blue--including swollen neural tube and pericardium, subectodermal blisters, hematomas, and generalized edema--may result from altered fluid balance in and around the embryo. The present study demonstrates relationships between changes in the fluid environment around the embryo and appearance of anomalies. Rat embryos were exposed in utero or in vitro to trypan blue during the early period of organogenesis. Both exposures resulted in defects that are typical of trypan blue treatment. Osmolality of exocoelomic fluid (ECF) was measured on gestation day 10 in vivo and day 12 in vitro, both after 48 hr of exposure to trypan blue. In both cases ECF osmolality was significantly lower than controls. This was correlated with the presence of edema-related anomalies in the embryo. On gestation day 11 in vivo, three days after maternal injection of trypan blue, ECF osmolalities were significantly higher than controls; however, there was tremendous variability in this parameter in day 11 treated embryos, and some had ECF osmolalities below the control range. Increased frequency of abnormalities was correlated with abnormal ECF osmolality, below and above the control range. Trypan blue probably exerts its teratogenic effects by disturbing the function of the visceral yolk sac. The movements of an amino acid and a monosaccharide across the visceral yolk sac were measured on gestation day 12 embryos in vitro. This aspect of yolk sac function was not altered by trypan blue exposure. Ultrastructure of the visceral yolk sac was observed after trypan blue exposure in vivo and in vitro. Endodermal cells in trypan blue-treated yolk sacs contained fewer large, electron dense lysosomes than controls. These were replaced by numerous small vacuoles, which may contain trypan blue. Trypan blue causes osmotic changes in the rat embryo in vivo and in vitro. These changes are correlated with embryonic malformations. Alterations in yolk sac ultrastructure indicate that trypan blue affects the function of this membrane.     

The development of rat alpha 2-macroglobulin. Studies in vivo and in cultured fetal rat hepatocytes

During inflammation and tissue injury, there is an increase in the plasma concentration of several proteins, the acute-phase proteins. The levels of some acute-phase proteins have been reported to increase in pregnant and tumour-bearing animals. Rat alpha 2-macroglobulin is classified as an acute-phase protein. In this study we report the expression of alpha 2-macroglobulin in various tissues during development of the rat embryo by analysis of mRNA. The tissues studied are liver, visceral yolk sac, placental labyrinth, decidua and trophoblast. In addition, the sites of alpha 2-macroglobulin expression are localized by in situ hybridization of cDNA for alpha 2-macroglobulin to mid-sagittal cryosections of rat embryos. The level of mRNA coding for alpha 2-macroglobulin is determined in the liver of rats aged between 12 days gestation and 2 days postnatal. alpha 2-Macroglobulin mRNA is first observed in fetal liver from 12 days of gestation and increases after day 17, reaching a maximum on day 20. At this time the level is greater than that found in the liver of an adult rat suffering from acute inflammation. alpha 2-Macroglobulin mRNA is detectable in the yolk sac, placental labyrinth, trophoblast tissue and decidua. In the decidua the alpha 2-macroglobulin message is first detected at 8 days of gestation, with high levels observed from 10 to 21 days of gestation. These observations are supported by in situ hybridization studies. Experiments using cultured hepatocytes show that cells derived from rats at 15 days and 19 days of gestation are capable of synthesizing and secreting alpha 2-macroglobulin. Both synthesis and secretion can be induced by the addition of dexamethasone to the culture medium.     

Of blood cells and the nervous system

Hematopoiesis is well-conserved between Drosophila and vertebrates. Similar as in vertebrates, the sites of hematopoiesis shift during Drosophila development. Blood cells (hemocytes) originate de novo during hematopoietic waves in the embryo and in the Drosophila lymph gland. In contrast, the hematopoietic wave in the larva is based on the colonization of resident hematopoietic sites by differentiated hemocytes that arise in the embryo, much like in vertebrates the colonization of peripheral tissues by primitive macrophages of the yolk sac, or the seeding of fetal liver, spleen and bone marrow by hematopoietic stem and progenitor cells. At the transition to the larval stage, Drosophila embryonic hemocytes retreat to hematopoietic “niches,” i.e., segmentally repeated hematopoietic pockets of the larval body wall that are jointly shared with sensory neurons and other cells of the peripheral nervous system (PNS). Hemocytes rely on the PNS for their localization and survival, and are induced to proliferate in these microenvironments, expanding to form the larval hematopoietic system. In this process, differentiated hemocytes from the embryo resume proliferation and self-renew, omitting the need for an undifferentiated prohemocyte progenitor. Larval hematopoiesis is the first Drosophila model for blood cell colonization and niche support by the PNS. It suggests an interface where innocuous or noxious sensory inputs regulate blood cell homeostasis or immune responses. The system adds to the growing concept of nervous system dependence of hematopoietic microenvironments and organ stem cell niches, which is being uncovered across phyla.     

Of blood cells and the nervous system: Hematopoiesis in the Drosophila larva

Hematopoiesis is well-conserved between Drosophila and vertebrates. Similar as in vertebrates, the sites of hematopoiesis shift during Drosophila development. Blood cells (hemocytes) originate de novo during hematopoietic waves in the embryo and in the Drosophila lymph gland. In contrast, the hematopoietic wave in the larva is based on the colonization of resident hematopoietic sites by differentiated hemocytes that arise in the embryo, much like in vertebrates the colonization of peripheral tissues by primitive macrophages of the yolk sac, or the seeding of fetal liver, spleen and bone marrow by hematopoietic stem and progenitor cells. At the transition to the larval stage, Drosophila embryonic hemocytes retreat to hematopoietic “niches,” i.e., segmentally repeated hematopoietic pockets of the larval body wall that are jointly shared with sensory neurons and other cells of the peripheral nervous system (PNS). Hemocytes rely on the PNS for their localization and survival, and are induced to proliferate in these microenvironments, expanding to form the larval hematopoietic system. In this process, differentiated hemocytes from the embryo resume proliferation and self-renew, omitting the need for an undifferentiated prohemocyte progenitor. Larval hematopoiesis is the first Drosophila model for blood cell colonization and niche support by the PNS. It suggests an interface where innocuous or noxious sensory inputs regulate blood cell homeostasis or immune responses. The system adds to the growing concept of nervous system dependence of hematopoietic microenvironments and organ stem cell niches, which is being uncovered across phyla.