Diffuse intraembryonic hemopoiesis in normal and chimeric avian development

Hemangioblast development and regulation

Hematopoietic and endothelial cell lineages are the first to mature from mesoderm in the developing embryo. However, little is known about the molecular and (or) cellular events leading to hematopoietic commitment. The recent applications of technology utilizing gene targeted mice and the employment of many available in vitro systems have facilitated our understanding of hematopoietic establishment in the developing embryo. It is becoming clear that embryonic hematopoiesis occurs both in the extra-embryonic yolk sac and within the embryo proper in the mouse. The existence of the long pursued hemangioblast, a common progenitor of hematopoietic and endothelial cells, is now formally demonstrated. Based on this new information, many studies are being conducted to understand hematopoietic commitment events from mesoderm. In this review, we will first discuss the establishment of the hematopoietic system with special emphasis on the most primitive hematopoietic committed cells, the hemangioblast. We will then discuss mesoderm-inducing factors and their possible role in hematopoietic lineage commitment.Key words: hematopoietic commitment, hemangioblast, in vitro embryonic stem cell differentiation.

Cooperative Effects of Growth Factors Involved in the Induction of Hematopoietic Mesoderm

Hematopoietic induction occurs on the ventral side ofXenopus gastrulae and is thought to be triggered by the growth factor bone morphogenetic protein 4 (BMP-4). To characterize this process, we developed a quantitative and sensitive assay for the induction of erythroid cells from totipotent ectoderm of the embryo. When high doses of BMP-4 were used in this explant assay, few erythroid cells were detected. In contrast, large numbers of differentiated erythroid cells were induced when ectoderm was treated with BMP-4 and the mesoderm inducers, activin, or fibroblast growth factor (FGF). Ectopic expression of GATA-1 also induced abundant erythroid cells in ectoderm treated with bFGF. This induction of erythroid cells by GATA-1 was blocked by coexpression with a dominant negative BMP-4 receptor, showing that GATA-1 requires the BMP signaling cascade to function. These results suggest that BMP-4 requires mesoderm induction to generate a program of gene expression, which regulates the specification of hematopoietic mesoderm by GATA factors.

Intraaortic hemopoietic cells are derived from endothelial cells during ontogeny

We have investigated the developmental relationship of the hemopoietic and endothelial lineages in the floor of the chicken aorta, a site of hemopoietic progenitor emergence in the embryo proper. We show that, prior to the onset of hemopoiesis, the aortic endothelium uniformly expresses the endothelium-specific membrane receptor VEGF-R2. The onset of hemopoiesis can be determined by detecting the common leukocyte antigen CD45. VEGF-R2 and CD45 are expressed in complementary fashion, namely the hemopoietic cluster-bearing floor of the aorta is CD45(+)/VEGF-R2(−), while the rest of the aortic endothelium is CD45(−)/VEGF-R2(+). To determine if the hemopoietic clusters are derived from endothelial cells, we tagged the E2 endothelial tree from the inside with low-density lipoproteins (LDL) coupled to DiI. 24 hours later, hemopoietic clusters were labelled by LDL. Since no CD45(+) cells were inserted among endothelial cells at the time of vascular labelling, hemopoietic clusters must be concluded to derive from precursors with an endothelial phenotype.

Cell-cell signalling in the regulation of blood cell formation and function

Blood cell formation is continuous in adult life and requires complex regulatory control to achieve the necessary cell proliferation, differentiation commitment and maturation. Much of this control is achieved by specific glycoprotein haemopoietic regulators, more than 20 of which have now been identified and produced in recombinant form. For the granulocyte-macrophage lineage, major regulators are the colony stimulating factors (CSF) that are produced by many cell types dispersed throughout the body. The production of CSF is rapidly increased in response to induction signals, most usually of microbial origin, representing a demand-generated signalling system designed to activate and generate cells to rapidly eliminate the micro-organisms. The CSF not only control cell division but certain aspects of differentiation commitment, maturation initiation and functional activation. These multiple responses are initiated by signals arising from different regions of the cytoplasmic domains of the CSF membrane receptors. Less well understood are the signalling systems controlling the special gene activation events required to initiate the formation of the first haemopoietic cells and the specialized microenvironmental cells needed to support haemopoietic cells as they respond to haemopoietic regulators.

Antigenic phenotype of early intra-embryonic lymphoid progenitors in the chicken

The stem cells for the definitive haematopoiesis are derived from intra-embryonic sources originally described in an avian model and later also in mammals. However, the molecular make-up of the early embryonic haematopoietic progenitors is not yet clearly defined. We have recently characterized the phenotype of prethymic intra-embryonic progenitors capable of thymic colonization. Here we studied the ontogeny of cell-surface antigens HEMCAM, alpha2beta1 integrin, thrombomucin, chL12 and c-kit and their co-expression on prethymic T-cell progenitors. The early intra-embryonic expression of avian B-cell antigen chB6 was also demonstrated on cells derived from the intra-embryonic areas. We suggest that in the chicken, embryonic B-cell progenitors segregate earlier than T-cell progenitors in the differentiation of multipotent haematopoietic stem cells to committed progenitors.

The cloche and spadetail genes differentially affect hematopoiesis and vasculogenesis

In vertebrates, hematopoietic and vascular progenitors develop from ventral mesoderm. The first primitive wave of hematopoiesis yields embryonic red blood cells, whereas progenitor cells of subsequent definitive waves form all hematopoietic cell lineages. In this report we examine the development of hematopoietic and vasculogenic cells in normal zebrafish and characterize defects in cloche and spadetail mutant embryos. The zebrafish homologs of lmo2, c-myb, fli1, flk1, and flt4 have been cloned and characterized in this study. Expression of these genes identifies embryonic regions that contain hematopoietic and vascular progenitor cells. The expression of c-myb also identifies definitive hematopoietic cells in the ventral wall of the dorsal aorta. Analysis of b316 mutant embryos that carry a deletion of the c-myb gene demonstrates that c-myb is not required for primitive erythropoiesis in zebrafish even though it is expressed in these cells. Both cloche and spadetail mutant embryos have defects in primitive hematopoiesis and definitive hematopoiesis. The cloche mutants also have significant decreases in vascular gene expression, whereas spadetail mutants expressed normal levels of these genes. These studies demonstrate that the molecular mechanisms that regulate hematopoiesis and vasculogenesis have been conserved throughout vertebrate evolution and the clo and spt genes are key regulators of these programs.

Antigenic profiles of endothelial and hemopoietic lineages in murine intraembryonic hemogenic sites

Two hemogenic sites are present in mouse embryos before the onset of fetal liver hemopoiesis. While the yolk sac provides for immediate erythropoiesis, an intraembryonic region encompassing the dorsal aorta produces definitive hematopoietic stem cells, as shown experimentally. At early developmental stages this region, that we named paraaortic splanchnopleura, produces multipotent progenitors. At the time of fetal liver colonisation, the paraaortic splanchnopleura further evolves into aorta, gonads and mesonephros (AGM) and contains progenitors capable of long term multilineage reconstitution. Only then are cytologically identifiable collections of early hemopoietic cells present in various arteries and in the mesentery. The present report focuses on the antigenic characterisation of immature hemopoietic progenitors in order to trace back the intraembryonic precursors at earlier developmental stages. CD34, an antigen expressed by immature progenitors and endothelial cells, labels all potential hemopoietic sites. Markers, supposed to counterstain endothelial cells and spare CD34+ hemopoietic cells, also stain various hemopoietic cells. The meaning of these shared antigenic expressions between cells of the endothelial and hemopoietic lineages in the early embryo is discussed.

Induction and early development of the hematopoietic and immune systems in Xenopus

This paper reviews several aspects of the development of the hematopoietic and lymphoid systems in Xenopus. The developmental biology of hematopoietic stem cells and the early development of the thymus and B-cells are discussed. Recent advances in the development of molecular indices of the hematopoietic program are also considered. Previous studies as well as new data demonstrate that the hematopoietic program is initiated at the time of gastrulation. Recent advances suggest that both positive and negative regulation is necessary for the appropriate spatial and temporal expression of the hematopoietic program during development.

Development of the definitive hematopoietic hierarchy in the mouse

Recent research on the ontogeny of the hematopoietic system in mammals has shown that a simple textbook steady-state hematopoietic hierarchy can not be strictly applied to the hematopoietic cells found within the embryo. During embryonic development, hematopoietic cells originate, migrate and differentiate in a number of distinct anatomical sites such as the yolk sac AGM region and liver and thus represent various classes of cells within diverse microenvironments. In this manuscript we review both cellular and molecular aspects of developmental hematopoiesis and present our current views on the numerous complex mechanisms underlying the establishment of definitive hematopoiesis.

Blood-borne seeding by hematopoietic and endothelial precursors from the allantois

Until now the allantois has not been considered as a hematopoietic organ. Here we report experimental evidence demonstrating the in situ emergence of both hematopoietic and endothelial precursors in the avian allantoic bud. When the prevascularized allantoic bud from a quail embryo was grafted in the coelom of a chicken host, hematopoietic and endothelial cells later were found in the bone marrow of the host. Because the graft was located at a distance from the limb bud, these cells could reach the bone marrow only by the circulatory pathway. This blood-borne seeding may be accomplished by distinct hematopoietic and endothelial precursors, or by hemangioblasts, the postulated common precursors of these two lineages; we consider the latter interpretation more likely. We also show by reverse transcription-PCR that the allantois region expresses very early the GATA genes involved in hematopoiesis and some beta-globin chain genes.

In vitro expansion of murine multipotential hematopoietic progenitors from the embryonic aorta-gonad-mesonephros region

The origin of hematopoietic stem cells (HSCs) and their growth factor requirement are poorly understood. Here we describe a new in vitro culture system of the aorta-gonad-mesonephros (AGM) region, where long-term repopulating HSCs first arise. We demonstrate that oncostatin M (OSM) is expressed in the AGM and is absolutely required for the expansion of multipotential hematopoietic progenitors in vitro. In addition, OSM enhances the formation of endothelial cell clusters. Thus, OSM appears to be a key cytokine for the development of multipotential hematopoietic progenitors in the AGM, possibly acting on common precursor cells between HSCs and endothelial cells. By using the AGM culture derived from macrophage colony-stimulating factor (M-CSF)-deficient op/op mutant embryos, we also show a pivotal role for M-CSF in fetal myelopoiesis.

TAL1/SCL is expressed in endothelial progenitor cells/angioblasts and defines a dorsal-to-ventral gradient of vasculogenesis

In this study we establish that TAL1/SCL, a member of the helix-loop-helix family of transcription factors, and an important regulator of the hematopoietic lineage in mice, is expressed in the endothelial lineage of avians. The earliest events of vascular development were examined using antibodies to TAL1/SCL, and the QH1 antibody, an established marker of quail endothelial cells. Analyses using double immunofluorescence confocal microscopy show that: (i) TAL1/SCL is expressed by both quail and chicken endothelial cells; (ii) TAL1/SCL expression precedes that of the QH1 epitope; and (iii) TAL1/SCL, but not QH1, expression defines a subpopulation of primordial cells within the splanchnic mesoderm. Collectively these data suggest that TAL1/SCL-positive/QH1-negative cells are angioblasts. Further, using TAL1/SCL expression as a marker of the endothelial lineage, we demonstrate that in addition to the previously described cranial-to-caudal gradient, there is a dorsal-to-ventral progression of vasculogenesis.

Intraembryonic hematopoietic stem cells

Intraembryonic hematopoietic stem cells (HSC) were first detected in avian chimeras associating an embryo with a yolk sac (YS). Cell markers were used to construct chimeras. The results showed that YS blood precursors undergo primitive erythropoiesis and become extinct, whereas intraembryonic precursors colonize rudiments of blood-forming organs and settle in the bone marrow as self-renewable HSC. The model is valid in the mouse as shown by in vitro cultures of cells obtained from embryo structures or YS separated prior to circulation. This approach, as well as restoration of irradiated adults, demonstrates that YS precursors have a limited potential compared with embryo precursors. The emergence of hematopoietic precursors in both YS and embryos is closely linked to the emergence of the endothelial network and is restricted to the mesoderm layer associated with endoderm.

Differentiation commitment in normal hemopoiesis and leukemic transformation

Differentiation commitment events are essential for the initiation of hemopoiesis and, in one form or another, occur continuously during adult hemopoiesis. The most studied type of differentiation commitment decision a hemopoietic cell can make involves the alternative choice of self-renewal versus the formation of progeny destined for maturation. Aberration in this commitment choice is a key abnormality necessary for the formation of a leukemic population.

CD34 Expression Patterns During Early Mouse Development Are Related to Modes of Blood Vessel Formation and Reveal Additional Sites of Hematopoiesis

CD34 is a cell surface glycoprotein that is selectively expressed within the human hematopoietic system on stem and progenitor cells, and in early blood vessels. To elucidate its functions during early blood vessel formation and hematopoiesis, we analyzed the expression patterns, in day 8 to day 10 mouse embryos, of CD34 RNA by in situ hybridization and protein by immunohistochemistry using the monclonal antibody RAM 34. Levels of expression in embryonic blood vessels were correlated with the mode of vessel formation, being high in pre-endothelial cells and in vessels forming by vasculogenesis (particularly the dorsal aortae) or angiogenesis, but low in vessels forming by coalescence (the cardinal veins). CD34+ erythroid cells, presumably of yolk sac origin, were present in the liver of day 10 embryos; at the same stage, putative definitive hematopoietic cells, strongly CD34+, were present in the para-aortic mesenchyme. Possible sites of hemangioblastic differentiation were detected in the form of CD34+ endothelium-attached hematopoietic cells in the dorsal aorta and in two previously unreported locations, the proximal umbilical and vitelline arteries. These observations suggest functions for CD34 in relation to specific modes of blood vessel formation, and a hemangioblastic role in both embryonic and extraembryonic sites.

CD34 Expression Patterns During Early Mouse Development Are Related to Modes of Blood Vessel Formation and Reveal Additional Sites of Hematopoiesis

CD34 is a cell surface glycoprotein that is selectively expressed within the human hematopoietic system on stem and progenitor cells, and in early blood vessels. To elucidate its functions during early blood vessel formation and hematopoiesis, we analyzed the expression patterns, in day 8 to day 10 mouse embryos, of CD34 RNA by in situ hybridization and protein by immunohistochemistry using the monclonal antibody RAM 34. Levels of expression in embryonic blood vessels were correlated with the mode of vessel formation, being high in pre-endothelial cells and in vessels forming by vasculogenesis (particularly the dorsal aortae) or angiogenesis, but low in vessels forming by coalescence (the cardinal veins). CD34+ erythroid cells, presumably of yolk sac origin, were present in the liver of day 10 embryos; at the same stage, putative definitive hematopoietic cells, strongly CD34+, were present in the para-aortic mesenchyme. Possible sites of hemangioblastic differentiation were detected in the form of CD34+ endothelium-attached hematopoietic cells in the dorsal aorta and in two previously unreported locations, the proximal umbilical and vitelline arteries. These observations suggest functions for CD34 in relation to specific modes of blood vessel formation, and a hemangioblastic role in both embryonic and extraembryonic sites.

Bipotential primitive-definitive hematopoietic progenitors in the vertebrate embryo

Two regions of the vertebrate embryo, the blood islands and the dorsal lateral plate (DLP), participate in early hematopoietic development. In Xenopus, primitive erythrocytes are derived solely from the ventral blood islands (VBI), while definitive hematopoietic cells such as lymphocytes are derived from both VBI and DLP. We have utilized a transplantation technique to demonstrate in vivo that all hematopoietic cells (embryonic, fetal, or adult) originate from ventral mesoderm. Reciprocal grafts between VBI and DLP demonstrated that both regions are bipotential with respect to primitive and definitive hematopoiesis. Commitment of the VBI to primitive erythropoiesis and restriction of the DLP to definitive hematopoiesis occurs during neurula stages. Thus, hematopoietic development involves the induction of the blood program on the ventral axis of the embryo followed by environmentally regulated specification to the primitive or definitive lineages.

A requirement for Flk1 in primitive and definitive hematopoiesis and vasculogenesis

Mouse embryos lacking the receptor tyrosine kinase, Flk1, die without mature endothelial and hematopoietic cells. To investigate the role of Flk1 during vasculogenesis and hematopoiesis, we examined the developmental potential of Flk1-/- embryonic stem cells in chimeras. We show that Flk1 is required cell autonomously for endothelial development. Furthermore, Flk1-/- cells do not contribute to primitive hematopoiesis in chimeric yolk sacs or definitive hematopoiesis in adult chimeras and chimeric fetal livers. We also demonstrate that cells lacking Flk1 are unable to reach the correct location to form blood islands, suggesting that Flk1 is involved in the movement of cells from the posterior primitive streak to the yolk sac and, possibly, to the intraembryonic sites of early hematopoiesis.

Embryonic endothelial cells transdifferentiate into mesenchymal cells expressing smooth muscle actins in vivo and in vitro

All blood vessels are lined by endothelium and, except for the capillaries, surrounded by one or more layers of smooth muscle cells. The origin of the embryonic vascular smooth muscle cell has until now been described from neural crest and locally differentiating mesenchyme. In this study, we have substantial evidence that quail embryonic endothelial cells are competent in the dorsal aorta of the embryo to transdifferentiate into subendothelial mesenchymal cells expressing smooth muscle actins in vivo. At the onset of smooth muscle cell differentiation, QH1-positive endothelial cells were experimentally labeled with a wheat germ agglutinin-colloidal gold marker (WGA-Au). No labeled subendothelial cells were observed at this time. However, 19 hours after the endothelial cells had endocytosed, the WGA-Au-labeled subendothelial mesenchymal cells were observed in the aortic wall. Similarly, during the same time period, subendothelial cells that coexpressed the QH1 endothelial marker and a mesenchymal marker, alpha-smooth muscle actin, were present. In such cells, QH1 expression was reduced to a cell membrane localization. A similar antigen switch was also observed during endocardial-mesenchymal transformation in vitro. Our results are the first direct in vivo evidence that embryonic endothelial cells may transdifferentiate into candidate vascular smooth muscle cells. These data arouse new interpretations of the origin and differentiation of the cells of the vascular wall in normal and diseased vessels.

Vessel patterning in the embryo of the zebrafish: guidance by notochord

We have cloned the zebrafish homolog of the receptor tyrosine kinase flk-1 to provide us with a tool to study normal vascular pattern formation in the developing zebrafish embryo and to compare it to mutants in which vascular pattern is perturbed. We find that during normal development the first angioblasts arise laterally in the mesoderm and then migrate medially to form the primordia of the large axial vessels, the dorsal aorta (axial artery) and the axial vein. Lumen formation occurs shortly before onset of circulation at 24 hr postfertilization. We examined the specification of vascular progenitors in the mutant cloche, which fails to form both vessels and blood. cloche lacks all flk-expressing cells and therefore appears to lack angioblasts. The axial vessels of the trunk form in close proximity to notochord and endoderm, which may provide cues for their formation. The dorsal aorta is normally just ventral to the notochord; the axial vein is just below the dorsal aorta and above the endoderm. floating head (flh) and no tail (ntl) mutants both have defects in the formation of notochord. Both are cell-autonomous lesions, flh abolishing notochord and ntl preventing its differentiation. In both mutants the dorsal aorta fails to form, while formation of the axial vein is less affected. Mosaic analysis of mutant embryos shows that transplanted wild-type cells can become notochord in mutant flh embryos. In these mosaic embryos flh cells expressing flk assemble at the midline, beneath the wild-type notochord, and form an aortic primordium. This suggests that signals from the notochord may guide angioblasts in the fashioning of the dorsal aorta. The notochord seems to be less important for the formation of the vein.

Characterization of Adult α- and β-Globin Genes in the Zebrafish

Developmental switching of hemoglobins (Hbs) occurs in most vertebrates, yet the cellular and molecular basis for this process remains elusive. The zebrafish is a new genetic and developmental system that can be used to study embryogenesis, and mutants with a variety of defects in hematopoiesis have recently been derived. To initiate our studies on Hb switching in this organism, we have characterized the globins expressed in the adult. Reversed-phase high performance liquid chromatography and mass spectrometric analyses of adult peripheral blood hemolysates showed that there are three major α globins and two β globins in circulating erythroid cells. In addition, we have isolated and characterized zebrafish adult α- and β-globin cDNA clones that encode some of these globins. High levels of α- and β-globin gene expression were detected in adult erythroid cells, whereas embryonic erythroid cells expressed little, if any, of these RNAs. We have also shown that the α- and β-globin genes are tightly linked on the same chromosome and are arrayed in a 3′-5′ to 5′-3′ configuration, respectively. The characterization of these genes and regulatory elements in this globin locus will provide insight into the process of globin gene transcription. With these reagents, future studies of Hb switching in zebrafish mutants with defective hematopoiesis will be possible.

Avian models in developmental biology

The avian embryo is uniquely amenable to experimental manipulation. The most widely used models are chimeras resulting from heterotopic or orthotopic exchanges of rudiments between chick and quail embryos, according to Le Douarin's technique (1969). Cell migrations and fates are traced in these chimeras either through the identification of quail cell nuclei stained for DNA or by means of monoclonal antibodies that recognize a particular lineage in only one of the two species. The ontogeny of the hemopoietic and endothelial lineages, as enlightened through appropriately designed chimeras, is reviewed in the present article. Homologies recently disclosed in mouse and human embryo are emphasized. Finally, the possibilities afforded by retroviral somatic transgenesis in the avian embryo will be envisaged.

Tenascin-C lines the migratory pathways of avian primordial germ cells and hematopoietic progenitor cells

Tenascin-C is a large hexameric extracellular matrix glycoprotein associated with epithelial-mesenchymal interactions, connective tissue development, and the formation of the central nervous system. Tenascin-C also lines the pathways followed by migrating avian neural crest cells, although its role in neural crest morphogenesis remains unclear. In vitro, tenascin-C interferes with cell-fibronectin interactions, and promotes the motility of many cell types including the neural crest. To determine if tenascin-C is a consistent component of matrices through which invasive embryonic cells migrate, we have investigated if tenascin-C is associated with 2 additional populations of motile, embryonic cells: primordial germ cells and hematopoietic progenitor cells. We have found that HNK-1, a monoclonal antibody used as a marker of neural crest, also stains avian primordial germ cells. Double-label immunohistochemistry reveals that tenascin-C is found in the mesenchyme adjacent to the ventral half of the dorsal aorta where the primordial germ cells penetrate the vessel wall, and both tenascin-C and fibronectin are present in the extracellular matrix through which the primordial germ cells migrate to reach the genital ridges. Unlike fibronectin, which is found throughout the splanchnic mesoderm, tenascin-C is concentrated in the proximal part of the splanchnic region where the primordial germ cells are concentrated. In embryos where the gonadal anlagen are surgically removed before the primordial germ cells leave the bloodstream, ectopic primordial germ cells were found exclusively in head and trunk mesenchyme containing tenascin-C. Like primordial germ cells, a subset of hematopoietic progenitor cells migrate through the mesenchyme ventral to the dorsal aorta where they form hematopoietic clusters. Others bud directly into the lumen of the aorta. Anti-tenascin-C stains the mesenchyme surrounding the migrating cells as well as the basal surfaces of the cells that appear to be budding into the lumen. In situ hybridization with a tenascin-C-specific cDNA probe shows that the major sources of the tenascin-C mRNA in this region are the hematopoietic progenitor cells themselves as well as the cells in the wall of the ventral aorta. mRNAs encoding 3 major splice variants of tenascin-C were identified by reverse transcriptase polymerase chain reaction (PCR) in the embryonic aorta and adjacent mesenchyme dissected from both the region of primordial germ cell and hematopoietic precursor cell migration. These experiments indicate that tenascin-C is a component of the migratory environment for many motile cells in the early embryo, where it has the potential to mediate cell-fibronectin interactions.

GATA factors and the origins of adult and embryonic blood in Xenopus: responses to retinoic acid

The transcription factors, GATA-1, -2 and -3 play essential roles in the differentiation of haematopoietic cells. To study the process of blood formation during vertebrate development we have used the expression of these GATA factors to locate haematopoietic cells in Xenopus embryos and to act as sensors for the effects of all-trans retinoic acid (RA), a signalling molecule which influences both anteroposterior patterning and haematopoietic differentiation. GATA factor expression was detected in the leading edge of the gastrulating mesoderm, in the ventral blood island (VBI) and dorsolateral plate (DLP) mesoderms and in a population of cells between the VBI and DLP. The VBI contributes to both embryonic and adult blood, whereas the DLP contains precursors of adult blood only, which have not been identified previously with molecular markers. The possibility that the GATA-2-expressing cells between the VBI and DLP were haematopoietic progenitors migrating from the VBI to the DLP was ruled out by transplantation analysis. Differential effects of RA on the expression of GATA-1 and GATA-2 suggest that RA has a direct action on haematopoietic differentiation, rather than on the formation of haematopoietic mesoderm.

Two distinct endothelial lineages in ontogeny, one of them related to hemopoiesis

We have shown previously by means of quail/chick transplantations that external and visceral organs, i.e., somatopleural and splanchnopleural derivatives, acquire their endothelial network through different mechanisms, namely immigration (termed angiogenesis) versus in situ emergence of precursors (or vasculogenesis). We have traced the distribution of QH1-positive cells in chick hosts after replacement of the last somites by quail somites (orthotopic grafts) or lateral plate mesoderm (heterotopic grafts). The results lead to the conclusion that the embryo becomes vascularized by endothelial precursors from two distinct regions, splanchnopleural mesoderm and paraxial mesoderm. The territories respectively vascularized are complementary, precursors from the paraxial mesoderm occupy the body wall and kidney, i.e., they settle along with the other paraxial mesoderm derivatives and colonize the somatopleure. The precursors from the two origins have distinct recognition and potentialities properties: endothelial precursors of paraxial origin are barred from vascularizing visceral organs and from integrating into the floor of the aorta, and are never associated with hemopoiesis; splanchnopleural mesoderm grafted in the place of somites, gives off endothelial cells to body wall and kidney but also visceral organs. It gives rise to hemopoietic precursors in addition to endothelial cells.

In vitro characterization of a novel avian haemopoietic growth factor derived from stromal cells

To better define the role of the chicken haemopoietic microenvironment in supporting haemopoiesis, a continuous cell line was generated by RSV transformation of avian spleen stromal cells (SSL-1). Supernatants from this line were found to contain haemopoietic growth factor activity as measured by the ability to induce proliferation and differentiation of precursor cells present in embryonic and post-hatched haemopoietic tissues. Comparison of cultures grown in the presence of cMGF and SSL-1 conditioned media (CM) revealed that both cytokine sources induced similar types of cell populations. Both sources supported the proliferation of predominantly macrophage-like cells based on colony morphology, differential staining, non-specific esterase staining, and phagocytosis activity. Interestingly, SSL-1 does not express any message for cMGF, nor does it secrete any IL-2 or interferon activities suggesting that the growth factor activities seen in SSL-1 are novel.

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.

Potential intraembryonic hemogenic sites at pre-liver stages in the mouse

In the course of a previous experimental study on the early development of the mouse embryo hemopoietic system, we found that, at the 10-25 pairs of somite stages, the para-aortic splanchnopleure contains hemopoietic progenitors. Trying to discover a structural basis for this potentiality, we have looked for cytological signs of hemopoiesis in the embryo proper between 8.5 and 12 days post-coitum, i.e. prior to full-blown fetal liver hemopoiesis. Two suggestive findings are reported: (1) intra-arterial hemopoietic cells aggregates are present in the omphalomesenteric and umbilical arteries and to a lesser degree in the dorsal aorta; (2) cells groups resembling yolk sac blood islands are observed in the mesentery. The intra-arterial aggregates are strikingly similar to the intra-aortic clusters of avian embryos. These cytological aspects provide the anatomical basis underlying recent functional data that revealed the hemogenic capacity of the para-aortic splanchnopleure.

Does the paraxial mesoderm of the avian embryo have hemangioblastic capacity?

In a previous study of the hemangioblastic capacity of lateral plate mesoderm, we showed that the endoderm-associated splanchnopleural layer is capable of giving rise to both endothelial and hemopoietic cells while the ectoderm-associated somatopleural layer is not (Pardanaud and Dieterlen-Lièvre 1993a). In order to complete the inventory of territories able to produce these two cell lineages, we assayed the paraxial mesoderm, and report the results here. Quail somites or segmental plates were treated with mab QH1+complement in order to eliminate attached aortic endothelial cells, which cling to the ventral aspects of these structures. They were grafted in the limb bud or the coelom of chick host, since these sites promote the differentiation of endothelial and hemopoietic cells, respectively. Vascular development and hemopoietic cell emergence were analyzed using QH1 immunocytology. Segmental plate and somites both produced abundant endothelial cells. In addition, the segmental plate gave rise to small groups of hemopoietic cells when grafted in the coelom.

B-lymphoid potential in pre-liver mouse embryo

Increasing interest in the initial steps of hemopoiesis in the embryo is prompted by the lack of information about the primordial origin and the nature of hemopoietic stem cells. It appears critical to understand the emergence, diversification and differentiation potential of hemopoietic cells in the preliver embryo. In-vitro studies of B-potential in the mouse embryo prior to liver colonization show that, contrary to interpretations prevalent 10 years ago, there are two sources of B-cell progenitors (and most probably stem cells) at these stages: the yolk sac and the paraaortic splanchnopleura within the embryo proper. The analysis of the phenotype and differentiation potential of precursors from both sources shows them to be very similar, while different from the fetal liver progenitors. Indications for an independent generation of precursors in both sites are reviewed.

Steel and c-kit in the development of avian melanocytes: a study of normally pigmented birds and of the hyperpigmented mutant silky fowl

We describe here the expression of c-kit and Steel (Sl) genes during the development of melanocytes in normally pigmented strains of chick and quail compared to unpigmented (White Leghorn) and hyperpigmented (Silky Fowl) strains of chickens. By using the quail/chick chimera system, we found that the neural crest cells, which migrate dorso-laterally in the subectodermal mesenchyme to give rise to the melanocytes, express c-kit as early as E4, that is about 2 days after they have left the neural primordium. The Sl gene is expressed from E4 onward in the epidermis but not at all in the dermis at any developmental stage. As feather buds develop, Sl mRNA becomes restricted to the apical region of the feather filaments. During formation of the barbs and barbules of the down feather, production of the Steel factor is restricted to the external epidermal cells of the barbules. The cell bodies of the c-kit-positive melanocytes are then located in the internal border of the epidermal ridges and extend their processes toward the source of the Steel factor. We propose that the spatial restriction of Sl gene activity at that stage accounts for the morphology of the melanocytes and their vectorial secretion of melanin to the external barbule cells. As a whole, these results show that during skin development c-kit positive cells are present in the Steel factor-producing areas at the time when melanoblasts proliferate and differentiate. Interestingly, in the mouse, previous studies showed that the Sl gene is activated in the dermis where melanoblasts undergo most of their expansion (Nishikawa et al. [1991] EMBO J. 10:2111-2118). In the unpigmented and hyperpigmented mutants that we studied, expression of the Sl message, as judged quantitatively in Northern blots (for the SF embryos) or spatially by in situ hybridization, is similar to that observed in normal birds. In SF embryos the c-kit expressing melanoblasts migrate initially in the dorso-lateral migration pathway as in normal birds. However their number increases considerably in the dermis from E5 onward. From E7, they invade mesodermally derived organs that do not express the Sl gene. This suggests that another, still unknown, factor(s) is responsible for the survival, the proliferation, and the extensive spreading of melanocytic cells within the mesoderm of this mutant.

Embryogenesis of the bursa of Fabricius: stem cell, microenvironment, and receptor-paracrine pathways

The bursa of Fabricius is an ideal model system to answer the plethora of questions related to the origin of B cells and microenvironmental issues leading to the education of the stem cell. Prior to the 1960s, lymphocytes were thought to be derived from epithelial or mesenchymal cells. Later work demonstrated the bloodborne nature of the stem cell contributing to B cells. Stem cells entered the bursa of quail and chicken between 7 and 11 and 7.5 and 14 d of embryogenesis, respectively. Interspecific chimeric studies, quail and chick, emphasized the intraembryonic origin and sites of the stem cell. The bloodborne and stromal cells that contribute to the microenvironment of the bursa orchestrate the events leading to B cell differentiation. The separation of the endodermal and mesodermal components of the bursa revealed a singular role for the endoderm in the genesis of the bursa but did not exclude a role for the mesoderm. A dark mesenchymal cell was shown to play a role in bud formation. This cell gave rise to the bursal secretory dendritic cell (BSDC), unique in its membrane association with IgG. A receptor-paracrine thesis has been proposed to explain the interaction between in-frame B cells and Ig-positive BSDC in the expansion of in-frame B cells and the subsequent development of the B cell repertoire. Cell adhesion molecules have been integrated into this thesis.

Intraembryonic haemopoietic cells and early T cell development

T cell precursors in the chick embryo have been localized into the intraembryonic mesenchyme (IEM) and into the para-aortic region before the first wave of the thymic colonization on embryonic day (ED) 6,5-8. The cell surface markers of avian prethymic stem cells are not known. It is also not known whether these precursor cells are already committed to the T cell lineage before their thymic colonization. In 7-day-old chick embryos Ov+ cells were found in the para-aortic region. Also the endothelial cells of the embryonic dorsal aorta were positively stained. Ov antigen might represent a very primitive marker for precursor cells having the potentiality to differentiate both to haemopoietic and endothelial cells. Scattered CD45+ cells were observed in the same para-aortic area as in many haemopoietic areas in the loose embryonic mesenchymal tissues. CD8 alpha (MoAb 3-298) expressing haemopoietic cells were detected before thymic colonization on ED6. In flow cytometric analysis of IEM precursors Ov, CD45 and CD8 alpha expressing cells seemed to form distinct subsets suggesting heterogeneity of these haemopoietic cells.

Development of hematopoietic stem cell activity in the mouse embryo

The precise time of appearance of the first hematopoietic stem cell activity in the developing mouse embryo is unknown. Recently the aorta-gonad-mesonephros region of the developing mouse embryo has been shown to possess hematopoietic colony-forming activity (CFU-S) in irradiated recipient mice. To determine whether the mouse embryo possesses definitive hematopoietic stem cell activity in the analogous AGM region and to determine the order of appearance of stem cells in the yolk sac, AGM region, and liver, we transferred these embryonic tissues into adult irradiated recipients. We report here the long-term, complete, and functional hematopoietic repopulation of primary and serial recipients with AGM-derived cells. We observe potent hematopoietic stem cell activity in the AGM region before the appearance of yolk sac and liver stem cell activity and discuss a model for the maturation of stem cell activity in mouse embryogenesis.

Early development of macrophages in intact and organ cultured hearts of rat embryos

BACKGROUND

Macrophage precursors are present early in embryonic life, being demonstrable in placental and embryonic connective tissues of rats at the neurula stage and as potential macrophages in the brain, liver, and lungs near the onset of organogenesis. We examined the development of macrophages in the heart and the possibility that they initially appear at sites of programmed cell death (apoptosis).

METHODS

Precursors were recognized by the binding of peroxidase-coupled Griffonia simplicifolia isolectin B4 (GSA) on the cell membrane. Their capacity for conversion into macrophages was assayed in organ cultures; confirmation of the progeny as bona fide macrophages was obtained from their responses to particle exposure and macrophage colony-stimulating factor (M-CSF).

RESULTS

GSA+cells were first seen on gestational day 12 (4 mm embryos) as 2-3 cycling, nonvacuolated cells located in cardiac tissue outside the blood vessels. This population increased to approximately 12 cells by day 14 (9 mm embryos). Two-thirds were distributed along the bulbus cordis in the jellylike endocardium and a more densely cellular connective tissue closer to the aortic arches where apoptotic sites are expected to develop. Such sites were not found in serial glycol methacrylate sections through our 14-day specimens, although in whole heart explants of this age an area of necrosis developed along the prospective line of bulbar endocardial fusion on the second day of organ culturing, and by then macrophages were fairly abundant. Organ culturing of 13-day embryonic hearts also yielded large, highly vacuolated, GSA+mononuclear phagocytes. After a few days in culture most of the macrophages migrated onto the medium where they formed a tight corona of cells about the explants. They readily ingested iron oxide particles and concentrated supravitally administered neutral red in their vacuoles. Macrophages from 14-day cultures exposed to M-CSF developed significantly larger coronas than macrophages from explants grown in serum-rich control medium (p < 0.001). In the presence of cytokines, moreover, these cardiac macrophages survived as many as 100 (92 "postnatal") days.

CONCLUSIONS

Macrophage precursors first appear in embryonic rat hearts well before they are needed to clear debris generated by programmed cell death and are capable of rapid conversion into outright phagocytic cells as early as the 13th prenatal day.

Intra-embryonic haemopoietic cells and early MHC expression

In the avian embryo the haemopoietic stem cells originate from the intra-embryonic area near dorsal aorta. The surface-marker expression of haemopoietic stem cells and their potential to produce different haemopoietic cells are still largely unknown. The surface antigen expression and particularly the MHC antigen expression on intra-embryonic haemopoietic cells was studied. Expression of B-F antigens, homologous to mammalian MHC class-I antigens, was found already on embryonic day (ED) 5. The first B-L antigens, analogous to mammalian MHC class-II antigens, were detected also from ED5 onwards. The appearance of surface antigens defined by MoAbs T10A6 and 3-298 during embryogenesis also was studied. The antigen defined with T10A6 was detected from ED4 onwards on endothelial cells but not on haemopoietic cells in the para-aortic region. The first 3-298+ haemopoietic cells were found on ED6, whereas endothelial cells were negative. These findings imply that some surface markers are shared with haemopoietic and endothelial cells indicating either a common embryonic origin or the importance of these molecules in embryonic stem-cell homing.

Ontogeny of IgM and IgM-bearing cells in rainbow trout

We have studied the ontogenic development of immunoglobulin M (IgM) and of IgM-bearing cells in the rainbow trout, Oncorhynchus mykiss. Lymphocytes showing cytoplasmic IgM were first observed in embryos at 12 days before hatch (14 degrees C). At this stage, no cells positive for surface IgM were present. Lymphocytes bearing surface IgM were observed at 8 days before hatch (14 degrees C). Unfertilized trout eggs contained detectable amounts of IgM (11.2 +/- 2.6 micrograms/g of egg weight), indicating that transfer of IgM from mother to embryo can occur in salmonids. The levels of IgM from whole fish increase slowly after the appearance of intraembryonic cells that express surface IgM. The amount of IgM/g of tissue peaks around hatch, but this parameter shows lower values up to 2 months after hatch.

Differentiation and characterization of B-cell precursors detected in the yolk sac and embryo body of embryos beginning at the 10- to 12-somite stage

The embryonic sites in which progenitors of the hematopoietic lineages first emerge are ideal regions to characterize both the cells and environment needed to initiate blood cell development. For a number of years both the murine yolk sac and embryo have been recognized to contain progenitors of B lymphocytes. However, clonal, quantitative in vitro assays, which allow precise observation of precursors and their progeny, have been lacking. Moreover, the site of origin of the initial events remains controversial. In this report we document the presence of B-cell progenitors in yolk sac and embryonic tissue obtained from mouse fetuses beginning at the 10-somite stage, day 8.5. We determine the frequency, cell-surface phenotype, and growth properties of these progenitors. We show that these cells can differentiate into immunoglobulin-secreting cells and that the progeny derived from single progenitors are diverse with respect to immunoglobulin heavy-chain allotype expression, diversity-joining region use, and heavy-chain variable-region utilization.

An early pre-liver intraembryonic source of CFU-S in the developing mouse

It is widely accepted that during murine embryogenesis, totipotent haematopoietic stem cells first originate in the yolk sac, then migrate to the fetal liver and finally colonize the bone marrow shortly before birth. This view is based on in vitro studies showing that yolk sac cells can differentiate into various haematopoietic lineages and in vivo studies showing that yolk sac contains spleen colony-forming units (CFU-S) beginning at day 8 of gestation. However, some investigators have failed to find statistically significant numbers of CFU-S arising from day 9 yolk sac and, although one group reported that yolk sac could repopulate the haematopoietic system of W mutant mice, others have failed to confirm yolk sac-derived repopulation of adults. In the avian and amphibian systems, the yolk sac gives rise only to early, transitory haematopoiesis whereas the definite adult haematopoietic stem cells in these vertebrates are derived from the mesodermal region containing the dorsal aorta. Because this analogous area of the mouse embryo has not been previously examined for haematopoietic activity, we directly compared the CFU-S activity of the aorta, gonad, mesonephros (AGM) region with the yolk sac and fetal liver during embryogenesis. Here we report that this intra-embryonic AGM region contains CFU-S activity at a higher frequency than that in embryonic yolk sac and that such activity appears in the AGM region before the fetal liver.

Avian pluripotent haemopoietic progenitor cells: detection and enrichment from the para-aortic region of the early embryo

In the avian embryo, the wall of the aorta is a site where haemopoiesis occurs in large diffuse foci from day 3 to day 10. In contrast to haemopoiesis in other organs of the embryo, para-aortic haemopoiesis is sustained by stem cells, which emerge in situ. Previous studies have demonstrated that the para-aortic region from the day-4 chick embryo harbours committed myeloid progenitors and committed erythroid progenitors. The present paper reports the in vitro development of para-aortic progenitors with both myelomonocytic and erythroid potentialities. Three types of myelo-erythroid progenitors were observed, giving rise to erythroblasts and monocytes, to erythroblasts and granulocytes, or to erythroblasts, monocytes and granulocytes. Their frequency in the para-aortic cell suspension was 1 per 10,000 cells. In cell sorting experiments, they co-sorted with committed progenitors in the cell population that immunolabeled with the VI-A2 monoclonal antibody, which is specific for chicken haemopoietic cells. Cell sorting also demonstrated that these multipotential progenitors did not express the BEN cell surface molecule, in contrast to late myeloid progenitors. The BEN molecule belongs to the immunoglobulin superfamily and is expressed by haemopoietic progenitors from bone marrow, selective sets of neurons and epithelial cells from the bursa of Fabricius. The myelo-erythroid progenitors were enriched 4 times in the VI-A2-positive cell population, and 2 to 5 times in the BEN-negative population. These results represent the first in vitro demonstration of avian normal myelo-erythroid progneitors.

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.

Emergence of endothelial and hemopoietic cells in the avian embryo

During organogenesis, endothelial cells develop through two different mechanisms: differentiation of intrinsic precursors in organ rudiments constituted of mesoderm associated with endoderm, and colonization by extrinsic precursors in organs constituted of mesoderm associated with ectoderm (Pardanaud et al. 1989). On the other hand, both types of rudiment are colonized by extrinsic hemopoietic stem cells. In the present work we extend our former study by investigating the hemangioblastic (i.e. hemopoietic and angioblastic) potentialities of primordial germ layers in the area pellucida during the morphogenetic period. By means of interspecific grafts between quail and chick embryos, we show that splanchnopleural mesoderm gives rise to abundant endothelial cells, and to numerous hemopoietic cells in a permissive microenvironment, while somatopleural mesoderm produces very few cells belonging to these lineages, or none. Thus we confirm that the angioblastic capacities of the mesoderm differ radically, depending on its association with ectoderm or endoderm. Furthermore, at this embryonic period, both endothelial and hemopoietic potentialities are displayed by splanchnopleural mesoderm. However the site of emergence of intraembryonic hemopoietic stem cells appears spatially restricted by comparison to more widespread angioblastic capacities.

Cytokinetic studies on the aortic endothelium and limb bud vascularization in avian embryos

Cytokinetic studies on the aortic endothelium using the BrdU/anti-BrdU-method were carried out on 2.5- to 6-day chick and quail embryos. The mitotic activity of the aortic endothelium is related temporally to the age of the avian embryo and spatially to the embryonic region where the aorta originates. The mitotic activity of the aortic endothelium decreases with increasing age of the embryos. In the limb buds, however, the mitotic rate of the aortic endothelial cells increases independently of the age of the embryo. This increase in the mitotic activity of the aortic endothelium at the appropriate levels coincides with the vascularization of the outgrowing limb buds. We concluded therefore that the aortic endothelium probably supplies endothelial cells for the formation of limb vessels at this stage. Thus our results suggest that angiogenesis (sprouting of capillaries from pre-existing vessels) takes place during limb vascularization in avian embryos. On the other hand, immunohistochemical studies with QH-1 or MB-1 antibody show, beside a capillary network in the central core of the wing bud, individual immunolabelled cells of mesenchymal character within the primarily avascular subectodermal region from the onset of vascularization onwards. We suggest that these cells have partly to be regarded as endothelial precursor cells, which have differentiated in situ from the local limb mesenchyme, and which will contribute to the developing vascular plexus. This means that not only angiogenesis, but also vasculogenesis (in situ from mesenchymal precursors differentiated endothelial cells) appears to be involved in limb vessel formation.

BEN, a novel surface molecule of the immunoglobulin superfamily on avian hemopoietic progenitor cells shared with neural cells

BEN is a novel molecule of the immunoglobulin superfamily that we previously identified by means of a monoclonal antibody on neural cell populations during avian development and epithelial cells of the bursa of Fabricius. In this paper, we describe the expression of BEN by hemopoietic cells during ontogeny. In the thymus, BEN is expressed as early as E9, and from E12 until just after hatching 30-60% of thymocytes are BEN positive. Thus the cells expressing BEN are immature thymocytes and not yet differentiated T cells. In the spleen, BEN expression parallels the myelopoietic activity. It is present on 75% of splenocytes during embryonic development and falls rapidly to 20% of cells during the first week after hatching when the spleen is becoming a secondary lymphoid organ. BEN is also found on a large proportion (about 80% positive cells) of bone marrow cells during ontogeny. Post hatching, BEN is present on 40-50% of bone marrow cells. The population of BEN-positive cells in the bone marrow includes myeloid and erythroid progenitor cells, identified by their ability to form colonies in vitro. BEN expression is lost as progenitor cells proliferate and differentiate to develop mature colonies in the clonal assay. Mature myeloid cells, such as macrophages, granulocytes, thrombocytes, and erythrocytes do not express the BEN antigen. Taken together, these data demonstrated that BEN is a stage-specific rather than a lineage-specific differentiation antigen expressed by immature hemopoietic cells.

Macrophage development: II. Early ontogeny of macrophage populations in brain, liver, and lungs of rat embryos as revealed by a lectin marker

Earliest origins of macrophage populations in the central nervous system, the liver, and the lungs were studied in rat embryos aged between 10.5-11 days and 14 days of gestation, based on light and electron microscopic identification of macrophages using peroxidase-coupled isolectin B4 of Griffonia simplicifolia (GSA I-B4), which recognizes alpha-D-galactose groups on the cell membrane. During embryonic life macrophages and their precursors are GSA I-B4-positive and generally bereft of peroxidase-positive granules. At 10.5 days the yolk sac and embryonic circulations have just become joined, the brain has five vesicles but nerve cells are little differentiated, the liver exists as a diverticulum of the gut with fingerlike extensions of hepatocytes, and the lungs as a laryngotracheal groove. Macrophages and/or their precursors occurred in small numbers in embryonic mesenchyme and blood vessels but showed no special affinity for either liver or lung rudiments. The developing brain was the first organ to be colonized, beginning on prenatal day 12. The liver followed between days 12 and 13 and was succeeded by the lungs, beginning between days 13 and 14. Dividing macrophages were present in these organs at the outset of colonization and throughout the duration of the embryo series, indicating that from the beginning, replication of resident cells contributes to growth of the local population. Granulocyte precursors were first apparent in the liver around day 13; they are also GSA-positive but are distinguished from macrophages by their content of peroxidase-positive granules. Organ cultures of 13-day liver and lungs, and 14-day brain tissue, indicate that whereas isolated liver fragments support the formation of both granulocytes and macrophages, only the latter develop in brain or lung cultures. A resident population of macrophages evidently is set up very early in these organs, well before white cells colonize the spleen, bone marrow, and other future blood forming regions. The events outlined are seen as stages in an embryo-wide process that leads to establishment of macrophage populations in various organs.