Embryonic red blood cell formation

An extra-erythrocyte role of haemoglobin body in chondrocyte hypoxia adaption

Although haemoglobin is a known carrier of oxygen in erythrocytes that functions to transport oxygen over a long range, its physiological roles outside erythrocytes are largely elusive1,2. Here we found that chondrocytes produced massive amounts of haemoglobin to form eosin-positive bodies in their cytoplasm. The haemoglobin body (Hedy) is a membraneless condensate characterized by phase separation. Production of haemoglobin in chondrocytes is controlled by hypoxia and is dependent on KLF1 rather than the HIF1/2α pathway. Deletion of haemoglobin in chondrocytes leads to Hedy loss along with severe hypoxia, enhanced glycolysis and extensive cell death in the centre of cartilaginous tissue, which is attributed to the loss of the Hedy-controlled oxygen supply under hypoxic conditions. These results demonstrate an extra-erythrocyte role of haemoglobin in chondrocytes, and uncover a heretofore unrecognized mechanism in which chondrocytes survive a hypoxic environment through Hedy.

Not all red cells sickle the same: Contributions of the reticulocyte to disease pathology in sickle cell anemia

Sickle cell anemia (SCA) is associated with morbidity and early death. While the switch from fetal to sickle hemoglobin during the first months of life results in hemolytic anemia with reticulocytosis, the role of the reticulocyte in the pathophysiology and prognosis of SCA is not well-defined. Reticulocytes have unique cytoskeletal and membrane components that allow them to be distinguished from mature sickle erythrocytes in the circulation. Reticulocytes in patients with SCA are less dense than more mature and 'sickled' erythrocytes, and have increased adhesive properties. The circulating reticulocyte number in peripheral blood may assist in predicting disease severity in SCA; characterization of patient-specific reticulocyte properties during infancy and childhood may assist in predicting therapeutic response to therapies. Here, we review the biological and clinical data regarding reticulocytes and their potential impact on SCA pathophysiology and disease severity.

Microfluidic assay of the deformability of primitive erythroblasts

Primitive erythroblasts (precursors of red blood cells) enter vascular circulation during the embryonic period and mature while circulating. As a result, primitive erythroblasts constantly experience significant hemodynamic shear stress. Shear-induced deformation of primitive erythroblasts however, is poorly studied. In this work, we examined the deformability of primitive erythroblasts at physiologically relevant flow conditions in microfluidic channels and identified the regulatory roles of the maturation stage of primitive erythroblasts and cytoskeletal protein 4.1 R in shear-induced cell deformation. The results showed that the maturation stage affected the deformability of primitive erythroblasts significantly and that primitive erythroblasts at later maturational stages exhibited a better deformability due to a matured cytoskeletal structure in the cell membrane.

Heterogeneity of F cells in β-thalassemia

BACKGROUND

Fetal hemoglobin (HbF), which is largely replaced after birth by the adult Hb, is concentrated in a few "F cells." Their number significantly increases in certain physiologic and clinical situations, including in β-thalassemia (β-thal). Their quantification is used to detect fetal-maternal hemorrhage (FMH), where fetal cells enter the maternal circulation. We were confronted with a pregnant woman with β-thal who was suspected to have FMH. To establish the usefulness of a flow cytometric procedure to differentiate between fetal cells and the maternal F cells, we screened adult β-thal patients.

STUDY DESIGN AND METHODS

Blood samples were simultaneously stained with fluorescent antibodies to HbF and to carbonic anhydrase (CA) isotype II, which is specific to adult red blood cells (RBCs).

RESULTS

A heterogeneous distribution of RBCs with respect to HbF and CA expression was observed: adult non-F cells (CA+HbF-) and F cells (CA+HbF+/HbF++) as well as F cells with characteristics of fetal cells (CA-HbF++).

CONCLUSIONS

The presence of CA-HbF++ RBCs in nonpregnant women, and even men, with thal indicates that the CA/HbF method is inappropriate for detection of FMH. The coexistence of F cells carrying fetal or adult markers suggests that they originate from two types of stem cell, adult and fetal, lineages. Normally, the fetal lineage is insignificant, but in β-thal, as HbF-containing RBCs have a selective advantage, the "fetal" lineage gains significance.

Regulation of hematopoiesis and the hematopoietic stem cell niche by Wnt signaling pathways

Hematopoietic stem cells (HSCs) are a rare population of cells that are responsible for life-long generation of blood cells of all lineages. In order to maintain their numbers, HSCs must establish a balance between the opposing cell fates of self-renewal (in which the ability to function as HSCs is retained) and initiation of hematopoietic differentiation. Multiple signaling pathways have been implicated in the regulation of HSC cell fate. One such set of pathways are those activated by the Wnt family of ligands. Wnt signaling pathways play a crucial role during embryogenesis and deregulation of these pathways has been implicated in the formation of solid tumors. Wnt signaling also plays a role in the regulation of stem cells from multiple tissues, such as embryonic, epidermal, and intestinal stem cells. However, the function of Wnt signaling in HSC biology is still controversial. In this review, we will discuss the basic characteristics of the adult HSC and its regulatory microenvironment, the "niche", focusing on the regulation of the HSC and its niche by the Wnt signaling pathways.

Hemoglobin switch in the newborn: a flow cytometry analysis

BACKGROUND

Hemoglobin (Hb) production switches upon birth from fetal Hb (HbF) to adult Hbs. HbF production is reactivated in sickle cell anemia and beta-thalassemia, where increased HbF ameliorates clinical symptoms and HbF-stimulating drugs are used for treatment. Understanding of the switch is therefore of basic interest and important for rational drug design. We studied the switch by determining changes after birth in the frequency and HbF-content of HbF-containing red blood cells (F-RBC) and reticulocytes (F-retics).

METHODS

Blood samples of neonatal cord, babies in their first year and adults were stained with both phycoerythrin-conjugated anti-human HbF antibodies and the nucleic acid dye thiazol-orange, and analyzed by flow-cytometry. RBC and retics were distinguished by their different thiazol orange-derived fluorescence and their HbF content was determined by the intensity of phycoerythrin-derived fluorescence.

RESULTS

The frequency of both F-RBC and F-retics decreases with time after birth. In most cases, the percentage of F-RBC was higher, but their HbF-content was lower than that of retics. The HbF content of the F-RBC and the F-retics showed a gradual decrease with age.

CONCLUSIONS

The results can be explained by two models: (A) A pool of homogenous stem cells that undergo gradual changes in their globin transcription pattern. (B) A pool of heterogeneous stem cells with varying HbF potentials that become active in an age-dependent manner: first, cells with a high HbF potential and then cells with a gradually lower HbF potential. In both models, these changes are induced by a biological clock and environmental factors.

Fetal hemoglobin silencing in humans

Interruption of the normal fetal-to-adult transition of hemoglobin expression should largely ameliorate sickle cell and beta-thalassemia syndromes. Achievement of this clinical goal requires a robust understanding of gamma-globin gene and protein silencing during human development. For this purpose, age-related changes in globin phenotypes of circulating human erythroid cells were examined from 5 umbilical cords, 99 infants, and 5 adult donors. Unexpectedly, an average of 95% of the cord blood erythrocytes and reticulocytes expressed HbA and the adult beta-globin gene, as well as HbF and the gamma-globin genes. The distribution of hemoglobin and globin gene expression then changed abruptly due to the expansion of cells lacking HbF or gamma-globin mRNA (silenced cells). In adult reticulocytes, less than 5% expressed gamma-globin mRNA. These data are consistent with a "switching" model in humans that initially results largely from gamma- and beta-globin gene coexpression and competition during fetal development. In contrast, early postnatal life is marked by the rapid accumulation of cells that possess undetectable gamma-globin mRNA and HbF. The silencing phenomenon is mediated by a mechanism of cellular replacement. This novel silencing pattern may be important for the development of HbF-enhancing therapies.

Murine serum obtained from bone marrow-transplanted mice promotes the proliferation of hematopoietic stem cells by co-culture with MS-5 murine stromal cells

To examine whether serum obtained from bone marrow-transplanted mice can selectively expand hematopoietic stem cells (HSCs) among whole bone marrow cells in vitro, whole bone marrow cells were cultured with or without MS-5 murine stromal cells in the presence of serum obtained from transplanted mice on day 3 (day 3 serum) or serum from normal mice for 7 days. When whole bone marrow cells and MS-5 cells were co-cultured in day 3 serum for 7 days, the c-kit-positive, Sca-1-positive, lineage marker-negative cells (KSL cells) expanded approximately 25 times; however, when they were co-cultured in normal serum for 7 days, the KSL cells expanded approximately 1.3 times. Direct contact between the whole bone marrow cells and MS-5 cells was essential for expansion of KSL cells in the co-culture, and it upregulated the expression of some cytokines in MS-5. Above all, the day 3 serum specifically upregulated the expression of SCF, SDF-1 alpha, G-CSF, IL-11 and IL-6 in MS-5. The level of testosterone in the day 3 serum was higher than normal serum and the addition of the testosterone in the culture expanded the KSL cells among whole bone marrow cells on MS-5 cells and also upregulated the expression of SDF-1 alpha, IL-11 and IL-6 in MS-5. These data indicates that the serum of bone marrow-transplanted mice contains a factor(s) that induced changes in the expression levels of various cytokines in MS-5 stromal cells and enabled the MS-5 cells to expand the KSL cells among whole bone marrow cells.

"Maturational" globin switching in primary primitive erythroid cells

Mammals have 2 distinct erythroid lineages. The primitive erythroid lineage originates in the yolk sac and generates a cohort of large erythroblasts that terminally differentiate in the bloodstream. The definitive erythroid lineage generates smaller enucleated erythrocytes that become the predominant cell in fetal and postnatal circulation. These lineages also have distinct globin expression patterns. Our studies in primary murine primitive erythroid cells indicate that betaH1 is the predominant beta-globin transcript in the early yolk sac. Thus, unlike the human, murine beta-globin genes are not up-regulated in the order of their chromosomal arrangement. As primitive erythroblasts mature from proerythroblasts to reticulocytes, they undergo a betaH1- to epsilony-globin switch, up-regulate adult beta1- and beta2-globins, and down-regulate zeta-globin. These changes in transcript levels correlate with changes in RNA polymerase II density at their promoters and transcribed regions. Furthermore, the epsilony- and betaH1-globin genes in primitive erythroblasts reside within a single large hyperacetylated domain. These data suggest that this "maturational" betaH1- to epsilony-globin switch is dynamically regulated at the transcriptional level. Globin switching during ontogeny is due not only to the sequential appearance of primitive and definitive lineages but also to changes in globin expression as primitive erythroblasts mature in the bloodstream.

Control of globin gene expression during development and erythroid differentiation

Extensive studies during the last 30 years have led to considerable understanding of cellular and molecular control of hemoglobin switching. Cell biology studies in the 1970s defined the control of globin genes during erythroid differentiation and led to development of therapies for sickle cell disease. Molecular investigations of the last 20 years have delineated the two basic mechanisms that control globin gene activity during development--autonomous silencing and gene competition. Studies of hemoglobin switching have provided major insights on the control of gene loci by remote regulatory elements. Research in this field has an impact on understanding regulatory mechanisms in general and is of particular importance for eventual development of molecular cures for sickle cell disease and beta thalassemia.

Yolk sac-derived primitive erythroblasts enucleate during mammalian embryogenesis

The enucleated definitive erythrocytes of mammals are unique in the animal kingdom. The observation that yolk sac-derived primitive erythroid cells in mammals circulate as nucleated cells has led to the conjecture that they are related to the red cells of fish, amphibians, and birds that remain nucleated throughout their life span. In mice, primitive red cells express both embryonic and adult hemoglobins, whereas definitive erythroblasts accumulate only adult hemoglobins. We investigated the terminal differentiation of murine primitive red cells with use of antibodies raised to embryonic beta H1-globin. Primitive erythroblasts progressively enucleate between embryonic days 12.5 and 16.5, generating mature primitive erythrocytes that are similar in size to their nucleated counterparts. These enucleated primitive erythrocytes circulate as late as 5 days after birth. The enucleation of primitive red cells in the mouse embryo has not previously been well recognized because it coincides with the emergence of exponentially expanding numbers of definitive erythrocytes from the fetal liver. Our studies establish a new paradigm in the understanding of primitive erythropoiesis and support the concept that primitive erythropoiesis in mice shares many similarities with definitive erythropoiesis of mammals.

Haematology of foetal sheep

OBJECTIVE

To describe the ontogeny of blood cells throughout foetal development in sheep.

DESIGN

A haematological study on blood and bone marrow from 42 sheep foetuses aged between 19 days gestation and full term.

PROCEDURE

Virgin Merino ewes were mated and the developing foetuses removed surgically at different stages of gestation. Blood and bone marrow samples were collected, stained for cytological examination or processed for electron microscopy. Blood samples were also examined haematologically. Foetuses were incubated with 3H-thymidine and autoradiographed.

RESULTS

During the first 4 weeks of development primitive erythroblast constituted the majority of the circulating blood cells. Definitive erythroid cells, originating in the liver, first appeared in the blood at around 27 days gestation and entirely replaced the primitive erythroblasts by 50 days gestation. Leukocyte numbers, especially lymphocyte count, increased rapidly after 49 days gestation. Erythropoiesis predominated in the marrow of all foetuses older than 70 days. In more marrow, myelopoiesis was the major activity and lymphopoiesis was not significant.

CONCLUSIONS

Red blood cell numbers and haemoglobin content progressively increases during foetal development. Primitive erythroblasts are not the precursors of the definitive erythroblasts. There are no significant differences in morphological features or maturation sequence between hepatic and bone marrow erythroblasts. Myelopoiesis is a major activity of bone marrow rather than of foetal liver.

Embryonic Hemoglobins Are Expressed in Definitive Cells

Human embryonic ζ and ɛ globin chains are synthesized in yolk sac–derived primitive erythroid cells, and decrease rapidly during definitive erythropoiesis. Examination of ζ and ɛ globin expression at the cellular level using dual-color immunofluorescence staining with specific monoclonal antibodies showed that embryonic globin proteins are present in definitive erythroid cells. More than half of fetal erythrocytes were positive for ζ and ∼5% for ɛ globin. Approximately one third of newborn red blood cells were ζ-positive and less than 1% ɛ-positive. Adult erythrocytes did not have embryonic globins. Erythroblasts that developed in liquid cultures also contained embryonic globin in amounts which declined with ontogenic age, and the proportion of positive cells in vitro was less than in the comparable erythrocytes that developed in vivo. Thus, embryonic globin chains are synthesized in definitive erythroid cells and decrease with ontogeny. Modulation of embryonic globin gene expression is not solely due to a switch from primitive to definitive erythropoiesis.

In vitro development of primitive and definitive erythrocytes from different precursors

During mouse embryogenesis the production of "primitive" erythrocytes (EryP) precedes the production of "definitive" erythrocytes (EryD) in parallel with the transition of the hematopoietic site from the yolk sac to the fetal liver. On a macrophage colony-stimulating factor-deficient stromal cell line OP9, mouse embryonic stem cells were shown to give rise to EryP and EryD sequentially with a time course similar to that seen in murine ontogeny. Studies of the different growth factor requirements and limiting dilution analysis of precursor frequencies indicate that most EryP and EryD probably developed from different precursors by way of distinct differentiation pathways.

Intraembryonic hematopoietic cell migration during vertebrate development

Vertebrate hematopoietic stem cells are derived from vental mesoderm, which is postulated to migrate to both extra- and intraembryonic positions during gastrula and neurula stages. Extraembryonic migration has previously been documented, but the origin and migration of intraembryonic hematopoietic cells have not been visualized. The zebrafish and most other teleosts do not form yolk sac blood islands during early embryogenesis, but instead hematopoiesis occurs solely in a dorsal location known as the intermediate cell mass (IM) or Oellacher. In this report, we have isolated cDNAs encoding zebrafish homologs of the hematopoietic transcription factors GATA-1 and GATA-2 and have used these markers to determine that the IM is formed from mesodermal cells in a posterior-lateral position on the yolk syncytial layer of the gastrula yolk sac. Surprisingly, cells of the IM then migrate anteriorly through most of the body length prior to the onset of active circulation and exit onto the yolk sac. These findings support a hypothesis in which the hematopoietic program of vertebrates is established by variations in homologous migration pathways of extra- and intraembryonic progenitors.

The developmental switch in embryonic rho-globin expression is correlated with erythroid lineage-specific differences in transcription factor levels

During chicken embryogenesis, the rho-globin gene is expressed only in the early developmental stages. We have examined the mechanisms that are responsible for this behavior. The transcription of the rho-globin gene is strongly correlated with the presence during development of primitive erythroid lineage cells, consistent with the idea that the expression of the rho-globin gene is restricted to that lineage. The “switching off” of rho-globin during development thus reflects the change from primitive to definitive cell lineages which occurs during erythropoiesis in chicken. We use transient expression assays in primary erythroid and other cells to show that the information for lineage- and tissue-specific expression of the rho-globin gene is contained in a 456 bp region upstream of the gene's translational start site. DNA-binding studies, coupled with analysis of the effect on expression of deletions and binding site mutations, were used to identify important control elements within this 456 bp region. We find that binding sites for the ubiquitous transcription factor Sp1, and the specific hematopoietic factor GATA-1, are crucial for expression of the gene in primitive erythroid cells. Quantitative analysis shows that nuclei of the primitive erythroid lineage contain 10-fold more of these factors than do the nuclei of definitive cells. We show that in principle these differences in factor concentration are sufficient to explain the lineage-specific behavior that we observe in our assays. We suggest that this may be an important part of the mechanism for lineage-restricted rho-globin expression during chicken erythroid development. Similar mechanisms may be involved in regulation of other (but not all) members of the globin family.

Individual stage selector element mutations lead to reciprocal changes in beta- vs. epsilon-globin gene transcription: genetic confirmation of promoter competition during globin gene switching

Biochemical and genetic analysis of the embryonic to adult beta-like globin gene switch in chickens has led to the hypothesis that competition between the promoters of the cis-linked epsilon- and beta-globin genes for interaction with a shared enhancer mediates the developmental changes in expression of beta-globin protein isotypes. To test specific predictions of this promoter competition model, a sensitive RNA/polymerase chain reaction assay has been used to investigate the effects of individual beta-globin promoter mutations on expression of the two linked genes in transiently transfected erythroid cells. Mutations that attenuated adult beta-globin transcription resulted concomitantly in a proportional increase in expression of the embryonic epsilon-globin gene. Consistent with the model, mutations disrupting the binding sites for either of two adult stage-specific transcription factors (NF-E4 and beta CTF) indicate that these sites are essential both for induction of beta-globin gene expression and for indirect suppression (through promoter competition) of epsilon-globin transcription in definitive (adult) erythroid cells. These results provide direct evidence that stage-specific transcription factors affect the equilibrium existing between multiple interacting globin cis-regulatory elements. We conclude that promoter competition is an important mechanism through which developmental regulation of chicken beta-globin gene switching is achieved and that such competitive interactions may prove to be generally applicable to the regulation of a variety of other temporally or spatially restricted gene expression patterns.

Evidence for the synthesis of embryonic globin chains in adult erythroid progenitor cells

Embryonic globin chains were found to be synthesized in vitro by the BFU-E colonies derived from adult sickle cell anemia (SS) patients, their heterozygous relatives as well as a few normal controls. In the absence of sufficient material for conducting direct structural analyses of these peptides, they were confirmed by evaluating the co-migration of the epsilon and zeta-chains with the corresponding structurally characterized globin chains obtained from K562 cell lysates on a reversed phase high performance liquid chromatogram. The presence of zeta-chain was also confirmed using an immunologic procedure. Furthermore, significant 35S-methionine incorporation peak was observed corresponding to the zeta-chain synthesized by the BFU-E-derived colonies implying an active synthesis of this embryonic globin chain in BFU-E cells obtained from hemopoietically adult persons.

Developmental programs of human erythroleukemia cells: globin gene expression and methylation

We investigated the programs of globin gene expression in three known (K562, HEL, and KMOE) and three novel (OCI-M1, OCI-M2, and HEL-R) human erythroleukemic cell lines of adult origin. RNAs from induced and uninduced cells were analyzed for epsilon-, gamma-, delta-, and beta-, zeta-globin-specific transcripts. While high-level gamma-globin expression was common, the lines differed in their expression of embryonic (epsilon, zeta) and adult (delta, beta) globin mRNAs. The patterns of globin gene methylation were generally consistent with their observed expression profiles, with many of the same correlations being seen in normal cells. Although the programs of globin gene expression and methylation displayed by the lines appeared to be diverse, they were not random; rather, they made developmental sense, mimicking defined globin gene programs observed during normal human development. The characteristics exhibited by several of these lines suggest that they may have been derived from the transformation of multi- or oligopotent hematopoietic progenitor cells. We speculate that the expression of fetal or embryonic globins in these adult erythroleukemic cell lines is not an aberration of neoplastic transformation but is indicative of a fetal or embryonic potential in normal adult hematopoietic progenitors.

Developmental programs of human erythroleukemia cells: globin gene expression and methylation

We investigated the programs of globin gene expression in three known (K562, HEL, and KMOE) and three novel (OCI-M1, OCI-M2, and HEL-R) human erythroleukemic cell lines of adult origin. RNAs from induced and uninduced cells were analyzed for epsilon-, gamma-, delta-, and beta-, zeta-globin-specific transcripts. While high-level gamma-globin expression was common, the lines differed in their expression of embryonic (epsilon, zeta) and adult (delta, beta) globin mRNAs. The patterns of globin gene methylation were generally consistent with their observed expression profiles, with many of the same correlations being seen in normal cells. Although the programs of globin gene expression and methylation displayed by the lines appeared to be diverse, they were not random; rather, they made developmental sense, mimicking defined globin gene programs observed during normal human development. The characteristics exhibited by several of these lines suggest that they may have been derived from the transformation of multi- or oligopotent hematopoietic progenitor cells. We speculate that the expression of fetal or embryonic globins in these adult erythroleukemic cell lines is not an aberration of neoplastic transformation but is indicative of a fetal or embryonic potential in normal adult hematopoietic progenitors.

Developmental regulation of beta-globin gene switching

A chicken erythroid cell-specific enhancer is located in the intergenic region between the adult beta- and embryonic epsilon-globin genes. In this paper we show that the beta-globin enhancer stimulates transcription of both genes. epsilon-Globin is, however, inappropriately regulated since it is expressed in both embryonic and adult red blood cells. Appropriate stage-specific regulation is observed for both genes when they are present on one plasmid. By analysis of deletion and substitution mutants, we conclude that beta-globin tissue- and developmental stage-specific regulation is mediated by interaction of the beta-globin enhancer with a positive regulatory element within the adult beta-globin promoter, the developmental stage selector element (SSE).

Coexpression of embryonic, fetal, and adult globins in erythroid cells of human embryos: relevance to the cell-lineage models of globin switching

The cellular control of the switch from embryonic to fetal globin formation in man was investigated with studies of globin expression in erythroid cells of 35- to 56-day-old embryos. Analyses of globins synthesized in vivo and in cultures of erythroid progenitors (burst-forming units, BFUe) showed that cells of the yolk sac (primitive) erythropoiesis, in addition to embryonic chains, produced fetal and adult globins and that cells of the definitive (liver) erythropoiesis, in addition to fetal and adult globins, produce embryonic globins. That embryonic, fetal, and adult globins were coexpressed by cells of the same lineage was documented by analysis of globin chains in single BFUe colonies: all 67 yolk sac-origin BFUe colonies and 42 of 43 liver-origin BFUe colonies synthesized epsilon-, gamma-, and beta-chains. These data showed that during the switch from embryonic to adult globin formation, embryonic and definitive globin chains are coexpressed in the primitive, as well as in the definitive, erythroid cells. Such results are compatible with the postulate that the switch from embryonic to fetal globin synthesis represents a time-dependent change in programs of progenitor cells rather than a change in hemopoietic cell lineages.

Studies on avian erythrocyte metabolism. XVI. Accumulation of 2,3-bisphosphoglycerate with shifts in oxygen affinity of chicken erythrocytes

The ability of the chicken erythrocyte to accumulate 2,3-bisphosphoglycerate (2,3-P2-glycerate) and its effect upon the oxygen affinity (P50) of the cell suspensions have been determined. Erythrocytes from chick embryos, which contain 4-6 mM 2,3-P2-glycerate, and from chickens at various ages, which contain 3-4 mM inositol pentakisphosphate but no 2,3-P2-glycerate, were incubated with inosine, pyruvate, and inorganic phosphate. Red blood cells from 20-day chick embryos incubated in Krebs-Ringer, pH 7.45, containing 20 mM inosine and 20 mM pyruvate had an increase in 2,3-P2-glycerate from 4.3 to 11.9 mM after 4 h. Importantly, as 2,3-P2-glycerate concentration increased there was a corresponding increase in P50 of the cell suspension. Further, erythrocytes from 9- and 11-week, and 7-, 14-, 24-, and 28-month-old chickens when incubated similarly with inosine and pyruvate accumulated 2,3-P2-glycerate with corresponding increases in P50 of the cell suspensions. The ability of the red cell to accumulate this compound under the incubation conditions used apparently decreases with age of the bird (e.g., 11.9 mM in the 20-day embryo to 1.1 mM in the 28-month-old chicken after 4 h incubation). Despite the presence of significant amounts of inositol-P5, the accumulation of 2,3-P2-glycerate markedly decreases oxygen affinity of the cell suspensions. The delta P50/mumol increase in 2,3-P2-glycerate in the red cells of the 20-day chick embryo after 4 h incubation is 1.5 Torr; conversely, the delta P50/mumol decrease in 2,3-P2-glycerate in the red cells of the 17-day embryo after 6 h incubation in the presence of sodium bisulfite is 2.8 Torr. The demonstrated ability of the chicken erythrocyte to accumulate 2,3-P2-glycerate in response to certain substrates suggests that regulation of concentration of this compound could contribute significantly to regulation of blood oxygen affinity in birds.

Differential expression of nuclear lamin proteins during chicken development

By immunocytochemistry, quantitative immunoblotting, and two-dimensional gel electrophoresis, we have analyzed the distribution of nuclear lamin proteins during chicken embryonic development. Whereas no qualitative differences in the patterns of expression of lamins A, B1, and B2 were observed during gametogenesis in either the female or the male germ line, profound changes in the composition of the nuclear lamina occurred during the development of somatic tissues. Most unexpectedly, early chicken embryos were found to contain little if any lamin A, although they contained substantial amounts of lamins B1 and B2. During embryonic development, lamin A became increasingly prominent, whereas the amounts of lamin B1 decreased in many tissues. Interestingly, the extent and the developmental timing of these changes displayed pronounced tissue-specific variations. Lamin B2 was expressed in fairly constant amounts in all cell types investigated (except for pachytene-stage germ cells). These results have implications for the purported functional specializations of individual lamin proteins. In addition, they suggest that alterations in the composition of the nuclear lamina may be important for the establishment of cell- or tissue-specific differences in nuclear architecture.

Analysis of human hemoglobin switching in MEL x human fetal erythroid cell hybrids

The switch from fetal to adult globin synthesis in man was studied using heterospecific cell hybrids between human fetal erythroblasts and mouse erythroleukemia cells. When erythroblasts from first trimester fetuses were used the hybrids expressed a fetal program of human globin expression. While in continuous culture, these hybrids switched from predominantly fetal to almost exclusively adult globin expression, providing direct evidence that switching can occur within a single cell lineage. Sequential studies of globin expression at a single cell level and subcloning experiments suggested that the switch reflects a progressive increase in the generation of beta + cells from gamma + cells. Hybrids formed with erythroblasts of second trimester fetuses switched faster than those produced with cells of first trimester fetuses. The findings suggest that the human gamma to beta switch is controlled by a developmental clock-like mechanism, which appears to be associated with chromosome 11.

Line-restricted hemoglobin synthesis in chick embryonic erythrocytes

The presence of embryonic hemoglobin in early definitive erythrocytes was checked by indirect immunofluorescence assay, using specific antibodies raised against embryonic Hb P. As positive control we used anti-Hb A which reacted with the alpha A chain shared by the minor embryonic Hb E and the adult Hb A. The assay was performed using blood smears from embryos between 6 and 15 days of incubation and yolk sac sections from embryos between 4 and 6 days. Hb P was never detected in the definitive line in circulating erythrocytes or in maturing erythroblasts still sequestered in the blood islands of the yolk sac. The expression of the 'specific' embryonic genes is thus restricted to the primitive line (as the 'specific' adult beta gene is restricted to the definitive line), and the hemoglobin switch is the result of the progressive substitution of the primitive line by the definitive one.

Erythropoiesis in the developing rainbow trout, Salmo gairdneri irideus: histochemical and immunochemical detection of erythropoietic organs

In the rainbow trout, Salmo gairdneri irideus, round, disc-like erythrocytes in the embryonic circulation (Ery L) are replaced by small, elliptical, disc-like erythrocytes (Ery ImA) after hatching. In the peripheral blood of alevins, Ery ImAs grow into mature, adult erythrocytes (Ery A) of large elliptical, disc-like shape (Iuchi, '73b; Yamamoto and Iuchi, '75). Ery L and Ery A have larval and adult hemoglobins, respectively (Iuchi, '73a). The ontogenetic sequence of hemoglobin switching and erythrocyte replacement during erythropoiesis was examined by o-dianisidine histochemistry as well as immunohistochemistry using FITC-antibody probe specific to adult hemoglobins. The first phase of embryonic erythropoiesis (for Ery L) occurs in the intraembryonic "intermediate cell mass" as well as the extraembryonic blood islands on the yolk sac. This phase of erythropoiesis is transient, continuing during the 7th to 12th day after fertilization and ceasing by the 15th day (5th day before hatching). There is a mixed population of Ery L, ImA, and A in the peripheral blood of posthatching alevins. Ery ImA and A showed immunofluorescence with FITC anti-Hb A antibody but Ery L did not. Erythroid cells, stainable with FITC anti-Hb A antibody, were observed in the kidney and the spleen 1 day before hatching and thereafter, but not in the liver throughout all stages examined. Therefore, we conclude that new erythropoiesis (erythropoiesis for Ery ImA and Ery A) began in the kidney and the spleen 1 day before hatching. These findings indicate that hemoglobin switching during the ontogeny of rainbow trout is based on erythrocyte replacement, correlated with a shift in the site of erythropoiesis from one organ to another.

Migration of erythropoietic and prebursal stem cells from the early chicken embryo to the yolk sac

Lymphocyte development and ontogenetic changes in erythroid cells have been studied in chick-chick yolk sac-embryo chimeras constructed of histoincompatible partners. The results obtained indicate that the early chick yolk sac produces transiently erythroid stem cells whereas definitive erythrocytes are derived from the intraembryonic stem cells. Such a change from the yolk sac-derived cells into embryo-derived cells is not observed in the lymphocytes which are exclusively derived from the embryo-borne stem cells. Experiments with cell transfers from the chimeric yolk sacs demonstrate that erythropoietic and prebursal stem cells migrate from the early embryo to the yolk sac during the second to the seventh day of incubation. The results obtained also exclude the de novo generation of prebursal stem cells in the yolk sac.

Hemoglobin biosynthesis in individual bursts from human adult peripheral and umbilical cord blood: analysis of the relative rates of synthesis of G gamma and A gamma globin chains

We cultured human adult peripheral and umbilical cord blood mononuclear cells in methylcellulose and measured the synthetic rates of globin chains in individual erythropoietic bursts. Globin chains were labeled with 14C-amino acids in culture, separated by isoelectric focusing in slab gels containing 8 M urea and 3% Nonidet P-40, and quantitated by fluorographic methods. All bursts exhibited G gamma, A gamma, and beta chains in varying ratios. Cumulative frequency distributions of individual bursts differing in the ratios of G gamma/(G gamma + A gamma) and gamma/(gamma + beta) approached normal distributions. These results indicated that perinatal hemoglobin switching and the hemoglobin switching of cultured adult erythropoietic precursors do not involve population changes in the precursors. Further, a positive correlation between gamma/(gamma + beta) and G gamma/(G gamma + A gamma) ratios of individual bursts existed in all of the samples. Switching between gamma and beta chains and switching of G gamma: A gamma ratios may share common regulatory mechanisms.

Developmental relationships between vitelline and intra-embryonic haemopoiesis studied in avian 'yolk sac chimaeras'

Using the quail-chick marker technique, cells have been traced in the haemopoietic organs of 149 developing chimaeras composed of a quail embryo with a chick yolk sac. The existence of intraembryonic stem cell relaying yolk sac stem cells, previously demonstrated with this system, is confirmed. Thymus, bursa of Fabricius and bone marrow are preferentially populated by intraembryonic stem cells. The spleen shows a transitory phase of colonization by chick yolk sac stem cells at 11--12 days of incubation. From 7 days onwards the yolk sac receives quail stem cell emigrating from the embryo.

Early origins of definitive erythroid cells in the chick embryo

The early maturation stages of definitive erythroid cells are observed in the embryonic circulation of the chick yolk sac at 4.5--5 days of incubation. Light and electron microscope observation of the mesoderm of the yold sac membrane indicate that individual presumptive precursors of the definitive-line are present as early as 2 days of incubation and give rise to sequestered populations of immature erythroblasts within sinusoids during the period of 2.5-6 days incubation. Such isolated populations of definitive-line erythroblasts eventually connect with the established capillary circulation of yolk sac membrane but a large proportion of the erythroblasts temporarily remain associated with the endothelium prior to free circulation.

Studies of the development of frog hemopoietic tissue in vitro. I. Spleen culture assay of an erythropoietic factor in anemic frog blood

A new in vitro technique has been described for demonstrating the presence of an erythropoietic factor in the circulating blood of frogs. The assay system consisted of MC33 medium, erythropoietically active spleen cells from Rana pipiens, and plasma or serum from frogs made anemic via phenylhydrazine or bleeding. The spleen cells, which remain erythropoietically active for up to nine days, were found to incorporate 59Fe, [3H]thymidine, [3H]uridine, and [3H]leucine at a greater rate in the presence of plasma or serum from anemic versus normal frogs. The hormones triiodothyronine, prolactin, and erythropoietin were not effective in eliciting an hemopoietic response. The data presented suggest that the spleen from that adult frog is a major site of erythroid differentiation and maturation.

In vitro activation of the in vivo colony-forming units of the mouse yolk sac

Experiments were performed to investigate the presence of colony-forming units (CFU) in the mouse embryonic yolk sac during the developmental period in which the yolk sac is the sole hemopoietic organ. Injection of yolk sac cell suspensions from normal embryos into syngeneic, lethally irradiated adult recipients evoked a very low number of spleen colonies. However, prior cultivation of yolk sacs in vitro caused a dramatic increase in the spleen colony-forming capacity--as high as 84-fold--following 48 hours in culture. The yolk sac origin of the spleen colonies was confirmed by: (a) Chromosomal marker analysis; (b) dose-response analysis; (c) demonstrating that the above colonies were not of endogenous origin induced by the mere injection of grafted cells. We conclude that the yolk sac contains many precursors of colony-forming cells which though undetectable by immediate grafting apparently become activated in culture by an as yet unknown induction process.

Erythropoiesis in the yolk sac of the bat Tadarida brasiliensis cynocephala

The process of erythropoiesis and vasculogenesis in the yolk sac of the bat (Tadarida brasiliensis cynocephala) has been studied through the use of both light and electron microscopy. Stem cells arise from the leading edge of the migrating splanchnic mesoderm and transform into primitive erythroblasts. Differentiation involves either contact or association with the endodermal cells, since all erythropoietic activity occurs on the endodermal side of the expanding vascular bed, and many of the cells are in close apposition to the lateral or basal plasma membranes of the endodermal cells. Endodermal cells also phagocytize developing primitive erythroblasts during the later stage of the process when erythropoiesis is subsiding in the yolk sac. Cells destined to become the endothelium of the expanding vascular bed also arise from the leading edge of the migrating splanchnic mesoderm. Their process of differentiation involves the development of cytoplasmic extensions that may surround a group of differentiating erythroblasts, enclosing them in the newly formed lumen of the blood vessel. The cytoplasmic extensions make contact and develop junctional complexes with similar processes from other cells to complete the lumen of the lengthening vascular bed. Cells of the granulocyte series or megakaryocytes are not observed in the yolk sac of the bat as has been described in certain other species.

The abundance of ferritin in yolk-sac derived red blood cells of the embryonic mouse

Circulating red blood cells formed early in development have several distinctive properties which include retention of the nucleus (mammals), large size and characteristic haemoglobin type (mammals, birds, amphibia). The primitive or embryonic red cells of early development are replaced by the definitive red cells which contain fetal or adult haemoglobin; a second developmental change occurs in the haemoglobin of some mammals (man, cattle, sheep) but does not involve a cell replacement. Circulating yolk-sac derived red cells from embryonic mice are siderocytes; elevated ferritin levels are associated with the circulating red cells of bullfrog tadpoles, but not with those of the adult frog, again indicating that red cell iron metabolism can change during development. In order to extend the observations made on an amphibian to a mammal, the ferritin content of circulating red cells from embryonic mice was determined and found to be 0.65 mg/100 mg of soluble protein; no ferritin (less than or equal to 0.007 mg/100 mg of soluble protein) was detected in adult mouse red cells. Elevated ferritin levels appeared to be specifically associated with the yolk-sac derived population of red cells since a decline in red-cell ferritin content coincided with the replacement of yolk-sac derived red cells by definitive red cells derived from the liver. Fractionation of mixtures of yolk-sac derived and liver derived red cells showed that fractions rich in the definitive red cells contained less ferritin than the mixture. The results suggest that elevated ferritin levels may be a general characteristic of the circulating, haemoglobinized red cells formed early in development.

Stimulation of fetal hemoglobin synthesis in bone marrow cultures from adult individuals

The regulation of fetal hemoglobin in adult erythroid cells was investigated with bone marrow cultures. Fetal hemoglobin (Hb F) was identified in individual erythroid colonies with fluorescent antibodies against Hb F and synthesis of gamma chains was determined with analyses of radioactive globins. The appearance of fetal hemoglobin in erythroid colonies was clonal. All the cells of the Hb F synthesizing colonies contained fetal hemoglobin. The frequency of erythroid colonies showing Hb F was higher than expected compared to the frequency of Hb F containing cells in the blood. Production of Hb F in culture, as shown by analysis of the radioactive globins, was 5 to 14 times higher than baseline Hb F synthesis. These results suggest that the ability for gamma chain synthesis in erythroid cells is determined at or above the level of the precursor cell from which the erythroid colonies, in vitro, derive (probably an erythropoietin responsive stem cell), and that stimulation of fetal hemoglobin synthesis in adult erythroid cells is possible.

Electron microscopic study of erythrocytes in developing rainbow trouts, Salmo gairdnerii irideus, with particular reference to changes in the cell line

The fine structure of the first erythroid cells in the embryonic circulation of the rainbow trout and new erythroid cells appearing after hatching was examined by scanning and transmission electron microscopy. The first erythroid cells in the embryos, where circulation has just begun, are very immature in ultrastructural characteristics; they are spherical, relatively large, and contain a large nucleus with diffuse chromatin and numerous free ribosomes mostly occurring as polyribosomes. The cells divide in the circulatory system. As development proceeds, the entire population undergoes a synchronous maturation, i.e., transformation into round and flat discs, progressive heterochromatization, reduction in number of cytoplasmic organelles, especially of ribosomes, and a concomitant increase of hemoglobin content. The fully mature first erythroid cells persist until after hatching and are gradually replaced by a new population of erythroid cells which begin to appear in the circulatory system within a few days after hatching. The new cells are at first small and spherical, and show immature ultrastructural features, but no dividing cells were encountered. The cytological changes accompanying maturation are almost parallel to those seen in the first class of erythroid cells. In the circulatory system of fry, where yolk absorption has just been completed, almost all the erythroid cells are mature. These cells are uniformly flat and elliptical discs. The differences in ribosome numbers in electron micrographs of erythroid cells from different stages of development are also consistent with the existence of two series of erythroid cell maturation in the peripheral blood of the developing rainbow trout.