From the hemangioblast to self-tolerance: a series of innovations gained from studies on the avian embryo

Chick embryo in experimental embryology and more

Chicken remains an undisputed, powerful, useful, and practical model in developmental research. Chick embryos have been used as model systems for studies in experimental embryology and teratology. As the chicken embryo develops outside the mother, effects of external stresses on cardiovascular development can be studied without interferences of maternal hormonal, metabolic, or hemodynamic alterations. In 2004, the first draft sequence of the complete chicken genome was released, allowing broad genetic analysis and comparison to humans, and enabling expansion of transgenic techniques within the chick model. The chick embryo is relatively simple, quick, and low-cost model. The main advantages of the chick as a model system for experimental embryology are the ease with cells and tissues can be labeled, transplanted, and cultured, along with its similarity to mammalian systems.

Dorsal aorta formation: separate origins, lateral-to-medial migration, and remodeling

Blood vessel formation is a highly dynamic tissue-remodeling event that can be observed from early development in vertebrate embryos. Dorsal aortae, the first functional intra-embryonic blood vessels, arise as two separate bilateral vessels in the trunk and undergo lateral-to-medial translocation, eventually fusing into a single large vessel at the midline. After this dramatic remodeling, the dorsal aorta generates hematopoietic stem cells. The dorsal aorta is a good model to use to increase our understanding of the mechanisms controlling the establishment and remodeling of larger blood vessels in vivo. Because of the easy accessibility to the developing circulatory system, quail and chick embryos have been widely used for studies on blood vessel formation. In particular, the mapping of endothelial cell origins has been performed using quail-chick chimera analysis, revealing endothelial, vascular smooth muscle, and hematopoietic cell progenitors of the dorsal aorta. The avian embryo model also allows conditional gene activation/inactivation and direct observation of cell behaviors during dorsal aorta formation. This allows a better understanding of the molecular mechanisms underlying specific morphogenetic events during dynamic dorsal aorta formation from a cell behavior perspective.

Contribution of cells derived from the area pellucida to extraembryonic mesodermal cell lineages in heterospecific quail chick blastodermal chimeras

The current study has two main objectives: first, to determine if cells derived from the area pellucida are able to populate extraembryonic membranes, and second, to determine if donor cells have the potential to differentiate to endothelial (EC) and hematopoietic cells (HC) in the yolk sac and allantois, the two extraembryonic membranes functioning as hematopoietic organs in the avian embryo. To this end, quail chick chimeras were constructed by transferring dissociated cells from the areae pellucidae of the stage X-XII (EG&K) quail embryo into the subgerminal cavity of the unincubated chick blastoderm. The distribution of quail cells in the allantois, yolk sac, amnion, and chorion of resulting putative chimeras was examined using quail cell-specific antibody against a perinuclear antigen (QCPN) after 6 days of incubation. The presence of EC, HC, and smooth muscle cells among the QCPN(+) donor cells was examined using QH-1, a quail-specific marker identifying HC and EC and an anti-α-smooth muscle actin antibody. Evidence gathered in the present study demonstrates that quail cells derived from the areae pellucidae are able to populate all of the extraembryonic membranes of resulting heterospecific quail chick chimeras and, most importantly, give rise to HC, EC, and smooth muscle cells, all of the three main mesodermal lineages derived from the posterior mesoderm both in the yolk sac and allantois.

Avian biomodels for use as pharmaceutical bioreactors and for studying human diseases

Animal-based biotechnologies involve the use of domestic animals for the production of pharmaceuticals and various proteins in milk and eggs, as disease models, as tools for stem cell research and animal cloning, and as sources of organs for xenotransplantation into humans. Avian species offer several advantages over mammalian models, and they have been used historically to advance the fields of embryology, immunology, oncology, virology, and vaccine development. In addition, avian species can be used for studying the etiology of human ovarian cancer and other human diseases such as disorders based on the abnormal metabolism of lipids and as unique mechanisms for the biosynthesis and transport of cholesterol. This review integrates recent progress and insight into the molecular and physiologic mechanisms associated with transgenic birds and gives an overview of the use of avian models as pharmaceutical bioreactors and as tools for studying human diseases.

Discovery and characterization of novel vascular and hematopoietic genes downstream of etsrp in zebrafish

The transcription factor Etsrp is required for vasculogenesis and primitive myelopoiesis in zebrafish. When ectopically expressed, etsrp is sufficient to induce the expression of many vascular and myeloid genes in zebrafish. The mammalian homolog of etsrp, ER71/Etv2, is also essential for vascular and hematopoietic development. To identify genes downstream of etsrp, gain-of-function experiments were performed for etsrp in zebrafish embryos followed by transcription profile analysis by microarray. Subsequent in vivo expression studies resulted in the identification of fourteen genes with blood and/or vascular expression, six of these being completely novel. Regulation of these genes by etsrp was confirmed by ectopic induction in etsrp overexpressing embryos and decreased expression in etsrp deficient embryos. Additional functional analysis of two newly discovered genes, hapln1b and sh3gl3, demonstrates their importance in embryonic vascular development. The results described here identify a group of genes downstream of etsrp likely to be critical for vascular and/or myeloid development.

Human embryonic stem cell-derived hematoendothelial progenitors engraft chicken embryos

OBJECTIVE

To investigate whether human embryonic stem cells (hESC) committed in culture into hematopoietic/endothelial cell progenitors can be further developed into mature blood and vascular cells following transplantation into chicken embryos.

MATERIALS AND METHODS

The yolk sac of 42- to 44-hour chicken embryos received yolk sac injections of unfractionated human embryoid body (hEB) cells, CD34-positive hEB cells, or CD34+CD45+ granulocyte colony-stimulating factor-mobilized human peripheral blood hematopoietic stem-progenitor cells. Human cells in the host were detected by flow cytometry and immunohistochemistry.

RESULTS

All injected cell populations engrafted chicken hematopoietic organs, as assessed by detection of CD45+ cells in the spleen, bursa of Fabricius, and thymus. CD34+ day -10 hEB cells showed the highest efficiency for producing human CD45+ cells in the hosts and yielded human glycophorin A+ erythroid, CD13+ myeloid, and CD19+ lymphoid cells in the spleen and bursa of Fabricius. Spleen cells from chimeric embryos also contained human colony-forming units-granulocyte macrophage, as assessed in methylcellulose colony-forming assays. Human endothelial cells expressing vascular endothelial-cadherin, von Willebrand factor, CD31, and the receptor for the Ulex europaeus lectin were also observed in the yolk sac vasculature following injection of either unfractionated or CD34+ day -10 hEB cells.

CONCLUSION

Primitive angiohematopoietic stem cells (total and CD34+ day -10 hEB cells) as well as adult hematopoietic stem cells could home to intraembryonic blood-forming organs following injection into the yolk sac. These observations demonstrate the utility of the avian embryo as a convenient and reliable host to model the angiohematopoietic development of human embryonic, or other early stem cells.

Molecular and developmental biology of the hemangioblast

The hemangioblast hypothesis was proposed a century ago. The existence of hemangioblasts is now demonstrated in mouse and human embryonic stem cell (ESC)-derived embryoid bodies (EBs), in the mouse and zebrafish gastrula, and in adults. The hemangioblast is believed to derive from mesodermal cells, and is enriched in the Bry+Flk1+ and Flk1+Scl+ cell populations in EBs and in the posterior primitive streak of the mouse gastrula and in the ventral mesoderm of the zebrafish gastrula. However, recent studies suggest that the hemangioblast does not give rise to all endothelial and hematopoietic lineages in mouse and zebrafish embryos. Although several signaling pathways are known to involve the generation of hemangioblasts, it remains largely unknown how the hemangioblast is formed and what are the master genes controlling hemangioblast development. This review will summarize our current knowledge, challenges, and future directions on molecular and developmental aspects of the hemangioblast.

spib is required for primitive myeloid development in Xenopus

Vertebrate blood formation occurs in 2 spatially and temporally distinct waves, so-called primitive and definitive hematopoiesis. Although definitive hematopoiesis has been extensively studied, the development of primitive myeloid blood has received far less attention. In Xenopus, primitive myeloid cells originate in the anterior ventral blood islands, the equivalent of the mammalian yolk sac, and migrate out to colonize the embryo. Using fluorescence time-lapse video microscopy, we recorded the migratory behavior of primitive myeloid cells from their birth. We show that these cells are the first blood cells to differentiate in the embryo and that they are efficiently recruited to embryonic wounds, well before the establishment of a functional vasculature. Furthermore, we isolated spib, an ETS transcription factor, specifically expressed in primitive myeloid precursors. Using spib antisense morpholino knockdown experiments, we show that spib is required for myeloid specification, and, in its absence, primitive myeloid cells retain hemangioblast-like characteristics and fail to migrate. Thus, we conclude that spib sits at the top of the known genetic hierarchy that leads to the specification of primitive myeloid cells in amphibians.

Expression profiling of circulating non-red blood cells in embryonic blood

Background

In addition to erythrocytes, embryonic blood contains other differentiated cell lineages and potential progenitor or stem cells homed to changing niches as the embryo develops. Using chicken as a model system, we have isolated an enriched pool of circulating non red blood cells (nRBCs) from E4 and E6 embryos; a transition period when definitive hematopoietic lineages are being specified in the peri-aortic region.

Results

Transcriptome analysis of both nRBC and RBC enriched populations was performed using chicken Affymetrix gene expression arrays. Comparison of transcript profiles of these two populations, with verification by RT-PCR, reveals in nRBCs an expression signature indicative of hematopoietic stem cells (HSCs) and progenitor cells of myeloid and lymphoid lineages, as well as a number of previously undescribed genes possibly involved in progenitor and stem cell maintenance.

Conclusion

This data indicates that early circulating embryonic blood contains a full array of hematopoietic progenitors and stem cells. Future studies on their heterogeneity and differentiation potentials may provide a useful alternative to ES cells and perinatal blood.

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.

Development of Early PCLP1-Expressing Haematopoietic Cells within the Avian Dorsal Aorta

The first haematopoietic stem cells (HSC) develop in the dorsal aorta as haematopoietic intra-aortic clusters (HIAC). To evaluate the initial steps of definitive haematopoiesis, we have studied the emergence and the expression profile of podocalyxin-like protein 1 (PCLP1)-expressing cells in early chick embryos. Here we demonstrate that at embryonic day 2 (E2), the PCLP1+ cells are present in the splanchnic mesoderm and in the ventral lining of the paired dorsal aorta. Following aortic fusion at E3, the PCLP1-expressing cells are exclusively found in the aortic floor and as the development proceeds, both the haematopoietic clusters and the aortic endothelial cells express PCLP1. In parallel with the early PCLP1 expression, bone morphogenetic protein 4 (BMP4) expression was detected in the splanchnopleura and thereafter in the densely packed mesenchymal cells beneath the HIAC. The microarray analyses of early E3 PCLP1+ cells revealed elevated expression of genes known to be involved in the stem cell function. These data suggest that splanchnopleura-derived PCLP1-expressing cells give rise to the earliest definitive haematopoietic progenitors.

The chick; a great model system becomes even greater

The chick embryo has a long and distinguished history as a major model system in developmental biology and has also contributed major concepts to immunology, genetics, virology, cancer, and cell biology. Now, it has become even more powerful thanks to several new technologies: in vivo electroporation (allowing gain- and loss-of-function in vivo in a time- and space-controlled way), embryonic stem (ES) cells, novel methods for transgenesis, and the completion of the first draft of the sequence of its genome along with many new resources to access this information. In combination with classical techniques such as grafting and lineage tracing, the chicken is now one of the most versatile experimental systems available.