Evidences that hemoglobin switch in the chick embryo depends on erythroid cell line substitution

In situ hybridization analysis of globin mRNAs in the primitive erythroid cells of the chick embryo

The possibility that the minor embryonic chick hemoglobins might be present in a particular subgroup of primitive erythroid cells has been investigated by in situ hybridization. Probe to detect the mRNA for the alpha A globin chain of the minor embryonic hemoglobin was used, and the results of the hybridization were compared with those obtained using as probes the cDNAs for total globin mRNAs. All erythroid cells circulating in a 4-day-old chick embryo gave positive signals with both probes at an approximately constant ratio. This shows that all cells contain a similar assortment of hemoglobin types, excluding the possibility that a subgroup might contain the minor primitive hemoglobins exclusively. However, the cells are not homogeneous, since about 10% of them show a distinctly higher concentration of mRNA of all globin types.

Differences in erythropoiesis in normal chicken and quail embryos

Using antibodies against the fetal and adult forms of alpha- and beta-globin, it has been shown that erythropoiesis in the para-aortic foci (PAF) constitutes a major species-specific difference between chicken and quail embryos. In quail embryos, para-aortic foci are rare, small and rather heterogeneous with regard to their erythropoietic and haemopoietic cell composition. In contrast, the PAFs in chicken embryos are abundant and consist of large numbers of erythropoietic cells. In both species a time difference (approximately 1 day) is observed between the first expression of the fetal alpha- and beta-globin and the adult alpha- and beta-globin in erythropoietic cells. Adult erythropoiesis in both species can be detected first in the stalk of the yolk sac; this is similar to the situation in mammalian and amphibian species. From this time onward the number of circulating adult erythrocytes increases steadily. Whereas in chicken, large intraembryonic foci that can serve as sources for these adult cells arise concomitantly, no such foci can be detected in quail embryos, suggesting that the quail yolk sac is a major source for these adult red blood cells.

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.

Chick embryo survival under acute carbon monoxide challenges

1. Early chick embryos (up to 7 days) survive longer in 1% CO than older ones. As shown by the partition constants of the oxygen-carbon monoxide equilibrium, such a behavior cannot be related to the different hemoglobin patrimonies. 2. In vivo, early embryo hemoglobin is completely turned into its CO derivative within about 15 min according to a step-wise progression which reflects the peculiar embryonic pathway of the blood circulation. 3. Diffusing oxygen is sufficient to keep early embryos alive for a long time. 4. Before the chorioallantois is formed, the vascular area acts as the embryonic respiratory organ.

Immunohistochemical analysis of the distribution of histone H5 and hemoglobin during chicken development

We used immunohistochemical procedures to investigate embryonic erythropoiesis in serial sections of chicken embryos after 2-13 days of incubation. Antibodies specific for the erythrocyte-specific histone H5, for embryonic hemoglobin, and for adult hemoglobin were used as markers for general, primitive, and definitive erythropoiesis, respectively. Histone H5 was present in erythrocytes at all of the stages studied, i.e., in both the primitive and definitive cells. Cell of the definitive lineage were first detected, at about 5-6 days of incubation, in erythroid foci in the mesenchyme around the vitelline stalk. At 7-9 days of incubation, a massive mesenchymal conglomeration of erythropoietic cells developed, extending from the cervical to the abdominal region and ventrally to the vertebral body, with its largest extensions being around the arteries in the mediastinum. Immunostaining revealed that these erythroid cells belonged to the definitive erythropoietic lineage. These cells had disappeared completely after 12 days of incubation, i.e., before erythropoiesis is visible in the bone marrow. These observations are consistent with the notion that the yolk sac is essential for the formation of the definitive erythroid lineage.