The control of embryonic hemoglobin synthesis

ERYTHROPOIESIS DURING AMPHIBIAN METAMORPHOSIS : II. Immunochemical Study of Larval and Adult Hemoglobins of Rana catesbeiana

Rabbit antibodies were prepared against the major hemoglobin components of the larval and adult stages of R. catesbeiana. The properties of the antisera were studied by double immunodiffusion, precipitation, and complement fixation. The antisera to tadpole and frog hemoglobins did not cross-react with either hemoglobin or apohemoglobin. The anti-serum against frog hemoglobin was used for the detection of frog hemoglobin in tadpoles undergoing either natural or thyroxine-induced metamorphosis. It was shown that frog hemoglobin is detectable first in the liver, indicating that the liver is the site of erythrocyte maturation during metamorphosis.

ERYTHROPOIESIS DURING AMPHIBIAN METAMORPHOSIS : III. Immunochemical Detection of Tadpole and Frog Hemoglobins (Rana catesbeiana) in Single Erythrocytes

Rabbit antibodies specific for the major tadpole and frog hemoglobin components of R. catesbeiana were used for the detection of the two hemoglobins inside single cells. The antisera, after fractionation by ammonium sulfate precipitation and diethylaminoethyl (DEAE)-cellulose chromatography, were conjugated with fluorescein isothiocyanate for the antifrog hemoglobin serum and tetramethylrhodamine isothiocyanate for the antitadpole hemoglobin serum. The conjugated fractions, refractionated by stepwise elution from a DEAE-cellulose column, were used for the fluorescent staining of blood smears, liver tissue imprints, and smears of liver cell suspensions. Both simultaneous and sequential staining with the two fluorescent preparations indicated that larval and adult hemoglobins were not present within the same erythrocyte during metamorphosis. In other experiments, erythroid cells from animals in metamorphosis were spread on agar containing specific antiserum. Precipitates were formed around the cells which contain the particular hemoglobin. The percentages of cells containing either tadpole or frog hemoglobin were estimated within the experimental error of the method. The data showed that the two hemoglobins are in different cells. It is concluded that the hemoglobin change observed during the metamorphosis of R. catesbeiana is due to the appearance of a new population of erythroid cells containing exclusively frog hemoglobin.

Definitive erythropoiesis in chicken yolk sac

The first wave of erythropoiesis in amniotic animals generates all primitive erythrocytes and takes place exclusively in yolk sac mesoderm. It is less clear, however, to what extent and for how long the yolk sac contributes to the second wave of erythropoiesis which gives rise to definitive erythrocytes for later embryonic and adult use. Here, we examine the initiation, duration, and site of definitive erythrocyte formation in chicken yolk sac. We show that the earliest definitive erythrocytes are generated in yolk sac venous vessels surrounding major arteries at embryonic day (E) 4-4.5, and that mature definitive erythrocytes enter circulating at E4.5-E5. This takes place at a time when yolk sac vasculature remodels extensively to generate paired arterial/venous vessels. The yolk sac remains the predominant site for definitive erythropoiesis from E5 to E10, and continues to generate definitive erythrocytes at least until E15. Similar to primitive erythropoiesis, definitive erythropoiesis in the yolk sac is accompanied by the expression of transcriptional regulators gata1, scl, and lmo2. Furthermore, our data suggest that one main source of definitive erythropoietic cells is the pre-existing vascular endothelial cells. It remains unclear whether yolk sac derived hematopoietic progenitors that do not undergo erythropoiesis in the yolk sac may take up intraembryonic niches and contribute to erythropoietic stem cell population after hatching.

Negative regulation of primitive hematopoiesis by the FGF signaling pathway

Hematopoiesis is controlled by multiple signaling molecules during embryonic and postnatal development. The function of the fibroblast growth factor (FGF) pathway in this process is unclear. Here we show that FGF plays a key role in the regulation of primitive hematopoiesis in chicks. Using hemoglobin mRNA expression as a sensitive marker, we demonstrate that timing of blood differentiation can be separated from that of initial mesoderm patterning and subsequent migration. High FGF activity inhibits primitive blood differentiation and promotes endothelial cell fate. Conversely, inhibition of FGFR activity leads to ectopic blood formation and down-regulation of endothelial markers. Expression and functional analyses indicate that FGFR2 is the key receptor mediating these effects. The FGF pathway regulates primitive hematopoiesis by modulating Gata1 expression level and activity. We propose that the FGF pathway mediates repression of globin gene expression and that its removal is essential before terminal differentiation can occur.

Mixed cultures of avian blastoderm cells and the quail mesoderm cell line QCE-6 provide evidence for the pluripotentiality of early mesoderm

During the early stages of embryogenesis, the mesoderm gives rise to cells of the cardiovascular system which include cardiac myocytes and vascular endothelial and red blood cells. We have investigated the development of these cell phenotypes using aggregate cultures of avian blastoderm cells, which replicated mesodermal cell diversification. The cell phenotypes expressed by the blastoderm cells were dependent upon the age of the blastoderm cells, with Hamburger-Hamilton stage 3 or 4 cells giving rise to endothelial and red blood cells and stage 5 cells producing endothelial and myocardial cells. To begin to understand the stage dependency of the cellular diversification of these aggregate cultures, we treated the cultures with various signaling factors that have been shown to be present in the early avian embryo. These experiments showed that stem cell factor and TGF alpha altered cell phenotypes by stimulating red blood cell and myocardial differentiation, respectively. The ability of these growth factors to shift the differentiation profile of aggregate cultures demonstrated the plasticity of early embryonic cells. To explore the diversification of individual mesodermal cells, labeled QCE-6 cells were incorporated within these blastoderm aggregate cultures. Previous studies have shown that this quail mesodermal cell line possesses characteristics of early nondifferentiated mesodermal cells and can be induced to express either myocardial or endothelial cell phenotypes (C. A. Eisenberg and D. M. Bader, 1996, Circ. Res. 78, 205-216). In the present study, we show that when these cells were cultured as a component of blastoderm cell aggregates, they differentiated into fully contractile cardiomyocytes or endothelial or red blood cells. Moreover, QCE-6 cell differentiation was in accordance with that displayed by the blastoderm cells. Specifically, QCE-6 cells differentiated into red blood cells when cultured within stage 3 or stage 4, but not stage 5, blastoderm cell aggregates. Accordingly, the differentiation of QCE-6 cells into beating cardiomyocytes only occurred when these cells were incorporated into stage 5 blastoderm cell aggregates. The identical sorting and differentiation patterns that were exhibited by QCE-6 and blastoderm cells suggest that expression of differentiated cell types within the early mesoderm is directed by the surrounding environment without immediate cellular commitment. In addition, these results provide further evidence that QCE-6 cells are representative of a multipotential mesodermal stem cell and that they possess the potential to exhibit fully differentiated cell phenotypes.

Cardiac differentiation induced by dopamine in undifferentiated cells of early chick embryo

Evidence suggests that neurotransmitters can act as possible chemical signals involved in cell division and morphogenetic movements long before neurons appear in the embryo. However, whether they are playing a role in differentiation is now unknown. It was recently observed (M. Sarasa and S. Climent, 1987, J. Exp. Zool. 241, 181-190) that the neurotransmitter dopamine exerted a stimulating effect on cardiac differentiation in the chick in ovo. We show here that dopamine acts as a specific inducer of heart muscle differentiation in vitro. When cells of the gastrula of embryos treated with dopamine were dissociated and reaggregated, the aggregates obtained almost entirely underwent cardiac muscle differentiation. Also, when small postnodal pieces obtained from the most posterior region of the gastrula were cultivated in the presence of dopamine, they differentiated into myocardic tissue instead of following their fate map. Therefore, dopamine can trigger a process that both causes undifferentiated cells to differentiate into heart muscle and compels cells already determined to another way of differentiation to become myocardic tissue.

The nucleosome repeat length increases during erythropoiesis in the chick

During erythropoiesis in the chick, the nucleosome repeat length increases from 190 base pairs to 212 base pairs. This increase is correlated with a dramatic increase in the concentration of the red cell specific histone H5 (from 0.2 molecules per nucleosome to 1 molecule per nucleosome) and with no change in the concentration of H1 (1 molecule per nucleosome).

The regulation of haemoglobin synthesis in cultured chick blastoderms by steroids related to 5beta-androstane

1. After 24h of incubation, the blastoderm may be dissected from the early developing chick embryo and successfully maintained under conditions of organ culture in vitro. 2. Low concentrations of steroids related to 5beta-androstane stimulate the synthesis of foetal haemoglobins, types E and P, in a highly steroid- and tissue-specific manner.

The hemoglobins of the developing chicken embryos. Fractionation and globin composition of the individual component of total erythrocytes and of a single erythrocyte type

The hemoglobins of the chicken embryo at several stages of development have been isolated in pure form by column chromatography and their relative amounts and globin compositions determined. The analyses on separated primitive and definitive erythrocytes show that the first contain four hemoglobins different from the adult ones. The two major ones at four days, decrease gradually and are no longer detectable from 15 days on. The two minor ones increase up to 6-7 days, then decrease but are still present at hatching. The definitive embryonic erythrocytes contain two hemoglobins identical to the adult ones but their ratios change gradually during development and approach that of the adult hemoglobins at hatching.

Collagen heterogeneity within different growth regions of long bones of rachitic and non-rachitic chicks

The different anatomical regions involved in osteogenesis in the chick long bone have been examined for heterogeneities in collagen structure that might relate to the mechanism of ossification. Experimentally induced lathyrism was employed to enhance collagen solubility, and vitamin D deficiency to allow accumulation of osteoid, the precursor of bone matrix. The extractable lathyritic collagens of the cartilaginous and osseous regions of growing long bones from rachitic and non-rachitic chicks were examined for alpha-chain type and amino acid composition. In both groups of animals the growth plate and cartilaginous regions of the epiphysis gave collagen molecules of the constitution [alpha1(II)](3), whereas the ossifying regions contained [alpha1(I)](2) alpha2. The degree of hydroxylation of the lysine moieties was increased by approximately 50% in the alpha1(I)-chain and alpha2-chain of rachitic bone collagen. Since uncalcified osteoid is greatly enriched in rachitic bone, it is concluded that the collagen of osteoid has the configuration [alpha1(I)](2) alpha2, similar to that of bone matrix, but has an elevated hydroxylysine content. The possible relationship of this difference to the mechanism of calcification is discussed.

Protein synthesis: its control in erythropoiesis

Erythropoiesis in the fetal mouse provides a model to study several important aspects of the regulation of cell differentiation and differentiated protein synthesis. Changes in the patterns of hemoglobins formed during fetal and postfetal development are shown to be associated with the substitution of the liver erythroid cell line. In the course of differentiation of yolk sac erythroid cells there are at least two classes of proteins distinguishable with respect to dependence on continued RNA formatoin. The bulk of nuclear proteins, "nondifferentiated" proteins, appear to be dependent on relatively short-lived messenger RNA while synthesis of differentiated proteins, the hemoglobins, proceeds on relatively stable molecules of messenger RNA. Hemoglobin formation occurs in those cells which are actively synthesizing DNA and dividing. On the average, two to three cell divisions may occur after the formation and stabilization of the messenger RNA for globin. Yolk sac erythropoiesis, at least from day 10 of gestation, is unresponsive to erythropoietin. By comparison, in fetal liver erythropoiesis, the hormone, erythropoietin, acts selectively on the most immature erythroid cell precursor to induce differentiation, cell replication, and hemoglobin formation. The erythropoietin responsive cell in the liver is apparently differentiated from the progenitor, pluripotential stem cell and committed to erythroblast formation and hemoglobin synthesis on exposure to the hormone. The initial effects of erythropoietin on macromolecular synthesis are to stimulate RNA synthesis, which temporally is followed by cell replication and the increase in hemoglobin formation. During liver erythropoiesis, there appears to be a transition from hemoglobin synthesis dependent on RNA formation to hemoglobin synthesis directed by relatively stable messenger RNA.

Developmental changes of erythropoiesis in cultured chick blastoderms

The erythropoietic area of very early chick embryos was cultured as a tissue for up to nine days to study the changes in red cell type and hemoglobin type, the cell cycle time, the cell population kinetics, and the DNA synthetase activity of these cells. It was found that the area vasculosa without the participation of the embryo proper contained the information and the timing mechanism required to produce not only the early primitive erythroid cell population, but also in due course, the later definitive cell type, each with its appropriate hemoglobin types. Also the precursors of the definitive cell type are active in DNA synthesis and therefore are probably in cycle very early in the culture period.

The effects of 5-bromodeoxyuridine on yolk sac erythropoiesis in the chick embryo

The effects of the thymidine analog, 5-bromodeoxyuridine (BUdR), on the formation of red cells in the yolk sac of the chick embryo were examined. The prospective area opaca vasculosa from a definitive primitive streak embryo was excised, disaggregated, and deposited into a cell clump, and the cell clump was placed in organ culture. Hemoglobin synthesis is detectable after about 16 hr in culture. The formation of erythropoietic foci and incorporation of (55)Fe into heme were used to measure the extent of erythropoiesis. Exposure to 40 microg/ml of BUdR within 6 hr after explantation almost completely eliminated red cell formation; subsequent transfer to thymidine medium showed that the inhibition was reversible, and there was no histological evidence of analog toxicity. Between 6 and 12 hr after initiation of organ culture, the tissue became completely refractory to BUdR. DNA synthesis, as monitored by thymidine-(3)H and BUdR-(3)H pulses, was extensive both during and after the period of BUdR sensitivity. Hence, during both BUdR sensitive and insensitive periods the analog was incorporated into DNA of cells which had not yet synthesized hemoglobin. It is proposed that between 6 and 12 hr a crucial regulatory event for terminal differentiation is perturbed by the presence of BUdR in the chromosomes.

Cytodifferentiation during insect metamorphosis: the galea of silkmoths

In the galea of silkmoths undergoing metamorphosis, generalized epidermal cells, which had previousty secreted pupal cuticle, transform into highly specialized cells producing a new protein, the enzyme cocoonase. These cells first segregate by mitosis and displacement, then grow rapidly through endomitosis and accumulation of RNA-rich cytoplasm, and finally begin rapid synthesis of cocoonase. Replication of DNA continues in fully differentiated cells synthesizing cocoonase.