Surface marker for hemopoietic and endothelial cell lineages in quail that is defined by a monoclonal antibody

The avian embryo as a model in developmental studies: chimeras and in vitro clonal analysis

The avian embryo is a model in which techniques of experimental embryology and cellular and molecular biology can converge to address fundamental questions of development biology. The first part of the chapter describes two examples of transplantation and cell labeling experiments performed in ovo. Thanks to the distinctive histologic and immunocytochemical characteristics of quail and chick cells, the migration and development of definite cells are followed in suitably constructed chimeric quail-chick embryos. Isotopic transplantations of neural tube portions between quail and chick, combined with in situ hybridization with a nucleic probe specific for a quail oligodendrocyte marker, allowed study of the origin and migration of oligodendroblasts in the spinal cord. Heterotopic transplantations of rhombomeres were performed to establish the degree of plasticity of these segments of the hindbrain regarding Hox gene expression, which was revealed by labeling with chick-specific nucleic probes. The second part describes in vitro cell cloning experiments devised to investigate cell lineage segregation and diversification during development of the NC. An original cloning procedure and optimal culture conditions permitted analysis of the developmental potentials of individual NC cells taken at definite migration stages. The results revealed a striking heterogeneity of the crest cell population, which appeared to be composed of precursors at different states of determination. Clonal cultures also provide a means to identify subsets of cells that are the target of environmental factors and to understand how extrinsic signals influence the development of responsive cells.

The development of the coronary vessels and their differentiation into arteries and veins in the embryonic quail heart

Research concerning the embryologic development of the coronary plexus has enriched our understanding of anomalous coronary vessel patterning. However, the differentiation of the coronary vessel plexus into arteries, veins, and a capillary network is still incomplete. Immunohistochemical techniques have been used for whole mounts and serial sections of quail embryo hearts to demonstrate endothelium, vascular smooth muscle cells, and fibroblasts. From HH35 onward, the lumen of the coronary plexus was visualized by injecting India ink into the aorta. In HH17, branches from the sinus venosus plexus expand into the proepicardial organ to reach the dorsal side of the atrioventricular sulcus. From HH25 onward, vessel formation proceeds toward the ventral side and the apex of the heart. After lumenized connections of the coronary vessels with the aorta and right atrium are established, a media composed of smooth muscle cells and an adventitia composed of procollagen-producing fibroblasts are formed around the coronary arteries. In the early stage, bloodflow through the coronary plexus is possible, although connections with the aorta have yet to be established. After the coronary plexus and the aorta and right atrium are interconnected, coronary vessel differentiation proceeds by media and adventitia formation around the proximal coronary arteries. At the same time, the remodeling of the vascular plexus is manifested by disappearance of arteriovenous anastomoses, leaving only capillaries to connect the arterial and venous system.

Synergism of hemopoietic growth factors on endothelial cell proliferation

Endothelial cells, which are nonhemopoietic cells, express and/or produce most of the known hemopoietic receptors and cytokines. The biological role of these factors, and their respective receptors, on endothelial cells is still unknown. In this study, the authors assessed the effect of different hemopoietic growth factors, ie, interleukin-3 (IL-3), erythropoietin (EPO), macrophage-colony stimulating factor (M-CSF), granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), singly or in conjunction with others, on proliferation and chemotaxis of human umbilical vein endothelial cells (HUVECs). They found growth stimulatory activity with IL-3, EPO, and GM-CSF and potent synergism between EPO and IL-3, less with IL-3 and GM-CSF, and none with EPO and either GM-CSF or G-CSF. All the singly tested hemopoietic growth factors stimulated the migration of HUVECs, but in conjunction with other factors, they did not show any additive or synergistic effect.

Embryonic neural chimeras in the study of vertebrate brain and head development

Construction of neural chimeras between quail and chick embryos has been employed since 1969 when the unique nucleolar structure of the quail nucleus and its use to devise a cell marking technique by associating quail and chick cells in ovo were described in the "Bulletin Biologique de la France et de la Belgique." This method was first applied to the ontogeny of the neural crest, a structure whose development involves extensive cell migration, and, since 1984, to that of the central nervous system (CNS). This chapter highlights some of the most significant findings provided by this approach concerning the CNS, such as (i) demonstration of the common origin of the floor plate and notochord from a group of cells localized in the "organizer", i.e., Hensen's node, and the way in which these two structures become positioned respectively within and under the neural tube during gastrulation and neurulation in Amniotes; (ii) the neural crest origin of the skull vault and the facial and hypobranchial skeleton. This means that the mesodermal contribution to the skull is limited to the occipital and otic regions and extends only to the rostral limit of the notochord. A correlation can be drawn between the development of the telencephalon and the mesectodermally derived skull in the vertebrate phylum; (iii) demonstration that the midbrain-hindbrain junction, at the stage of the encephalic vesicles, acts as an organizing center for tectal and cerebellar structures. This function was correlated with the activity of several developmental genes, thus providing insight into their function during neurogenesis; (iv) the pattern of morphogenetic movements and cell migration taking place in defined brain-to-be areas, as well as the origin of various cell types of nervous tissues; and (v) a new avenue for studying brain localization of either behavioral traits or genetically encoded brain disorders.

The regeneration of the cephalic neural crest, a problem revisited: the regenerating cells originate from the contralateral or from the anterior and posterior neural fold

The mesencephalic and rhombencephalic levels of origin of the hypobranchial skeleton (lower jaw and hyoid bone) within the neural fold have been determined at the 5-somite stage with a resolution corresponding to each single rhombomere, by means of the quail-chick chimera technique. Expression of certain Hox genes (Hoxa-2, Hoxa-3 and Hoxb-4) was recorded in the branchial arches of chick and quail embryos at embryonic days 3 (E3) and E4. This was a prerequisite for studying the regeneration capacities of the neural crest, after the dorsal neural tube was resected at the mesencephalic and rhombencephalic level. We found first that excisions at the 5-somite stage extending from the midmesencephalon down to r8 are followed by the regeneration of neural crest cells able to compensate for the deficiencies so produced. This confirmed the results of previous authors who made similar excisions at comparable (or older) developmental stages. When a bilateral excision was followed by the unilateral homotopic graft of the dorsal neural tube from a quail embryo, thus mimicking the situation created by a unilateral excision, we found that the migration of the grafted unilateral neural crest (quail-labelled) is bilateral and compensates massively for the missing crest derivatives. The capacity of the intermediate and ventral neural tube to yield neural crest cells was tested by removing the chick rhombencephalic neural tube and replacing it either uni- or bilaterally with a ventral tube coming from a stage-matched quail. No neural crest cells exited from the ventral neural tube but no deficiency in neural crest derivatives was recorded. Crest cells were found to regenerate from the ends of the operated region. This was demonstrated by grafting fragments of quail neural fold at the extremities of the excised territory. Quail neural crest cells were seen migrating longitudinally from both the rostral and caudal ends of the operated region and filling the branchial arches located inbetween. Comparison of the behaviour of neural crest cells in this experimental situation with that showed by their normal fate map revealed that crest cells increase their proliferation rate and change their migratory behaviour without modifying their Hox code.

Hematopoietic stem cell emergence in embryonic life: developmental hematology revisited

In utero, hematopoiesis takes place initially in the extraembryonic yolk sac, then switches to the liver, thymus, and, finally, bone marrow. This chronologic sequence and the fact that all blood-forming tissues but the yolk sac sustain hematopoiesis after colonization by stem cells of external origin have led to the hypothesis that the whole prenatal and postnatal blood system is founded by yolk sac-derived stem cells. Experimental data recently obtained from bird and mouse embryo models strongly suggest, however, that definitive hematopoiesis is established from an intraembryonic source of stem cells arising in the vicinity of the developing aorta. In agreement, an abundant population of CD34+ primitive hematopoietic cells has been identified in the equivalent area of the human embryo. These novel findings will contribute to our understanding of blood cell homeostasis and may help to further develop therapeutic protocols making use of fetal hematopoietic cells transplanted in utero or in postnatal life.

Morphological aspects of the vascularization in intraventricular neural transplants from embryo to embryo

Intraventricular transplants of neural tissues were performed in ovo from embryo to embryo. Fragments of the nervous wall of the optic lobe (tectum) from 14-day chick or 12-day quail embryos (donor) were inserted into the ventricle of the right optic lobe of 6-day chick or 5-day quail embryos (host). Chick-to-chick, chick-to-quail and quail-to-chick grafts were carried out. The vascularization changes occurring in the host tectum and in the grafted neural tissues were analysed under light, transmission, and scanning electron microscopes and by morphometric methods. In the host embryo tectum, the neural graft stimulates a statistically significant increment in vessel density and a vessel sprouting into the ventricle of the optic lobe. The vascular sprouts reach the transplanted tissue and establish connections with its native microvasculature. The chick-to-quail and quail-to-chick grafts, submitted to immunoreaction with a quail-specific antibody which recognizes an antigen (MB1) present on endothelial cells, indicate that re-establishment of the circulation in the graft depends upon anastomoses between host and donor vasculatures and the rapid new growth of host-derived and donor-native vessels. The presence of macrophage-like cells escorting the new-growing vessels suggests that these cells are involved in the host and donor tissue angiogenesis.

Vascular endothelial cell lineage-specific promoter in transgenic mice

Vascular endothelial cells play essential roles in the function and development of the cardiovascular system. However, due to the lack of lineage-specific markers suitable for molecular and biochemical analyses, very little is known about the molecular mechanisms that regulate endothelial cell differentiation. We report the first vascular endothelial cell lineage-specific (including angioblastic precursor cells) 1.2 kb promoter in transgenic mice. Moreover, deletion analysis of this promoter region in transgenic embryos revealed multiple elements that are required for the maximum endothelial cell lineage-specific expression. This is a powerful molecular tool that will enable us to identify factors and cellular signals essential for the establishment of vascular endothelial cell lineage. It will also allow us to deliver genes specifically into this cell type in vivo to test specifically molecules that have been implicated in cardiovascular development. Furthermore, we have established embryonic stem (ES) cells from the blastocysts of the transgenic mouse that carry the 1.2 kb promoter-LacZ reporter transgene. These ES cells were able to differentiate in vitro to form cystic embryoid bodies (CEB) that contain endothelial cells determined by PECAM immunohistochemistry. However, these in vitro differentiated endothelial cells did not express the LacZ reporter gene. This indicates the lack of factors and/or cellular interactions which are required to induce the expression of the reporter gene mediated by this 1.2 kb promoter in this in vitro differentiation system. Thus this system will allow us to screen for the putative inducers that exist in vivo but not in vitro. These putative inducers are presumably important for in vivo differentiation of vascular endothelial cells.

Blood vessel formation in the avian limb bud involves angioblastic and angiotrophic growth

The vasculature of the avian limb bud takes its origin from the intersomitic vessels as can be shown by ink perfusion of the embryo. While the primitive vessels form a central network in the early limb bud, an area of about 100 microns in width from the ectoderm inward remains free from lumenized vessels. However, this subectodermal avascular zone contains isolated angioblasts, which can be demonstrated by confocal laser scanning microscopy in connection with QH-1-staining. QH-1-positive cells from the avascular zone are capable of giving rise to endothelial cells when grafted ectopically into a "permissive" environment such as the dorso-lateral paraxial mesoderm. Several grafting sites are compared regarding their permissiveness for capillary formation. In order to investigate the origin of the QH-1-positive angioblasts we carried out injections of DiI-Ac-LDL, which is specifically taken up by endothelial cells and macrophages, and found the lumenized vessels and a few isolated cells in the peripheral limb mesoderm stained. In double-labelling studies combining DiI-Ac-LDL and QH-1, it can be shown that there exists a pool of isolated angioblasts that are only QH-1-positive, but have not incorporated DiI-Ac-LDL. In contrast to the lumenized vessels in the core of the limb bud, we found that angioblasts in the avascular zone do not proliferate, as shown by proliferation studies applying the BrdU-method to semithin sections in connection with QH-1-labelled parallel sections. We conclude that the vascularization of the avian limb bud is achieved by a combination of angiotrophic growth (sprouting of vessels) and angioblastic growth (recruitment of angioblasts from the limb mesoderm.

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.

Cloning and characterization of a basic helix-loop-helix protein expressed in early mesoderm and the developing somites

Basic helix-loop-helix (bHLH) heterodimer protein complexes regulate transcription of genes during the processes of differentiation and development. To study the molecular basis of early mesodermal differentiation, we sought to identify bHLH proteins from cells of mesodermal origin. By using an interaction cloning strategy with a radiolabled recombinant bHLH protein, E12, a clone encoding a potential heterodimer partner was isolated from an endothelial cell library. This gene (bHLH-EC2) is most homologous to Twist but shares similarity within the bHLH domain with TAL1 and other members of this family. bHLH-EC2 is expressed in cultured endothelial cells and in embryonic stem cell, erythroleukemia, and muscle cell lines in a differentiation-dependent manner. In situ hybridization studies of mouse embryos reveal that bHLH-EC2 is expressed throughout the primitive mesoderm as early as 7.5 days postcoitum. Expression then becomes restricted to the paraxial mesoderm and to the dermamyotome of the developing somite. Expression of bHLH-EC2 in cells destined to become myoblasts thus predates expression of myogenic bHLH factors. bHLH-EC2 is expressed in early endothelial and hematopoietic cells in vivo, as shown by RNA studies of embryonic yolk sac and cultured cells derived from yolk sac explants. These findings suggest that bHLH-EC2 plays a role in the development of multiple cell types derived from the primitive mesoderm.

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.

A new model of vasculogenesis and angiogenesis in vitro as compared with vascular growth in the avian area vasculosa

In cultures of dissociated quail epiblast the basic constituents of the vascular system, blood cells and endothelial cells can be induced by basic fibroblast growth factor (Flamme and Risau, Development, 116: 435-439, 1992). As we show here, in those cultures three types of vascular plexus differentiate spontaneously under different culture conditions: At the 3rd day a vascular plexus appears in situ closely resembling the vascular plexus of the quail area opaca vasculosa (vasculogenesis). Vascular sprouts are formed, extending long filopodia at their tips. Such filopodia are shown to build the first intervascular bridges in the growing vascular plexus of the area vasculosa at embryonic day 3. Connections of filopodia turn out to be precursors of new capillaries interconnecting pre-existing blood vessels (angiogenesis). Two further types of in vitro capillary plexus differentiate in long term endothelial cell cultures derived from induced angioblasts. Whereas one closely resembles so-called angiogenesis in vitro, the third type comprises mainly multinucleated giant endothelial cells lining loop like capillaries and represents a differentiation of aging endothelial cell culture. Thus, the present in vitro model is an approach to the sequence of angioblast induction, vasculogenesis, and angiogenesis.

Development of the cardiac coronary vascular endothelium, studied with antiendothelial antibodies, in chicken-quail chimeras

The endothelium of the coronary vascular system has been described in the literature as originating from different sources, varying from aortic endothelium for the main coronary stems, endocardium for the intramyocardial network, and sinus venosus lining for the venous part of the coronary system. Using an antibody against quail endothelial cells (alpha-MB1), we investigated the development of the coronary vascular system in the quail (Hamburger and Hamilton stages 15 to 35) and in a series of 36 quail-chicken chimeras. In the chimeras, pieces of quail epicardial primordium and/or liver tissue were transplanted into the pericardial cavity of a chicken host. The results showed that the coronary vascular endothelial distribution closely followed the formation of the epicardial covering of the heart. However, pure epicardial primordium transplants did not lead to endothelial cell formation, whereas a liver graft with or without an epicardial contribution did have this capacity. The first endothelial cells were seen to reach the heart at the sinus venosus region, subsequently spreading through the inner curvature to the atrioventricular sulcus and the outflow tract and, last of all, over the ventricular surfaces. At these sites, the precursor cells and small vessels were seen to invade the sinus venosus wall, the ventricular and atrial myocardium, and the mesenchymal border of the aortic orifice. Connections with the endocardium of the heart tube were only observed in the right ventricular outflow region. Initially, the connections with the aortic endothelium were multiple, but later in development only two of these connections persisted to form the proximal part of the two main coronary arteries. Connections to the pulmonary orifice were never observed. Our transplantation data showed that the entire coronary endothelial vasculature originated from an extracardiac source. Moreover, using the developing subepicardial layer as a matrix, we showed that the endothelial cells reached the heart from the liver region. Ingrowth into the various cardiac segments was also observed. Implications for the relation to specific congenital cardiac malformations are discussed.

Infantile (juvenile) capillary hemangioma: a tumor of heterogeneous cellular elements

Infantile (juvenile) capillary hemangiomas are vascular neoplasms which can appear quite infiltrative histologically and are characterized by cords of cells with areas of marked cellularity. While vessels can be distinguished in most cases, there are many cells which do not appear to be endothelial in origin. We labeled 5 such cases with antibodies directed against factor VIII-related antigen, CD34, alpha actin, factor XIIIa and PC-10. Anti-factor VIII-related antigen labeled all endothelial cells and did not label cells away from vessels. Anti-CD34 recognized similar cells and also stained a subset of interstitial cells. Anti-alpha muscle defined pericytes and stained few interstitial cells and none of the endothelial cells. Many of the interstitial spindled and cuboidal cells stained strongly with anti-factor XIIIa. The majority of the mitotic activity was concentrated in the interstitial cells. These observations lend support to the concept that infantile (juvenile) capillary hemangioma is a tumor of primitive cells with capabilities for differentiating toward endothelial cells and pericytes. It is not clear whether a similar stem cell population gives rise to dermal dendrocytes, or whether these represent an immune response to the neoplasm.

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.

Development of the pharyngeal arch system related to the pulmonary and bronchial vessels in the avian embryo. With a concept on systemic-pulmonary collateral artery formation

BACKGROUND

The literature is ambiguous as to the question of the developmental background of systemic-pulmonary collateral arteries. These are found in combination with various congenital heart malformations such as pulmonary atresia. From a clinical point of view, it is of interest to know whether we are dealing with the persistence of transient embryological vessels such as ventral segmental arteries or parts of pharyngeal arch arteries or with the prenatal or postnatal recruitment of the bronchial vasculature that normally supplies the lung. This study of the embryology of the extrapulmonary and intrapulmonary vasculature aims at a better understanding of the variations in origin, course, branching pattern, and histology of collateral arteries.

METHODS AND RESULTS

Serial sections of quail embryos ranging between stage HH11 and stage HH28 were incubated with a monoclonal antibody (alpha MB1) against endothelial cells and their precursors. Additional series of chick embryos were injected with india ink to study the lumenized vascular patterns. A splanchnic plexus consisting of endothelial cells and precursors is present around the foregut before the lung buds develop. This plexus expands and gives rise to the pharyngeal arch arteries, the ventral pharyngeal veins, the pulmonary vessels, and the bronchial vessels, including the intrapulmonary vessel network. During two subsequent periods, the splanchnic plexus is transiently connected to the systemic arteries and veins. The bronchial arteries and veins develop in the second period from these transient vessels. The expansion and extension of the splanchnic plexus to many organs during the formation of the bronchial vessels explains the varying course and branching pattern of the bronchial vasculature.

CONCLUSIONS

These results show that we are not dealing with two or more individual vascular systems that contribute to the developing vessels of the lungs but with one vascular plexus that normally gives rise to the pulmonary and bronchial vasculature but has the potential to give rise to other systemic-pulmonary connections.

First appearance, distribution, and origin of macrophages in the early development of the avian central nervous system

A phagocytic cell system of hemopoietic origin exists in the early avian embryo (Cuadros, Coltey, Nieto, and Martin: Development 115:157-168, '92). In this study we investigated the presence of cells belonging to this system in the central nervous system (CNS) of chick and quail embryos by using both histochemical staining for acid phosphatase and immunolabelling with antibodies recognizing cells of quail hemangioblastic lineage. The origin of these cells was traced in interspecific chick-quail yolk sac chimeras. Hemopoietic cells were detected within the CNS from developmental stage HH15 on, and steadily increased in number at subsequent stages. Analysis of yolk sac chimeras revealed that most of these cells were of yolk sac origin, although some hemopoietic cells of intramebryonic origin were also found in the CNS. Immunocytochemical, histochemical, and ultrastructural characterization allowed us to identify hemopoietic cells in the CNS as macrophages. These cells were consistently found in the brain vesicles and spinal cord, appearing (1) between undifferentiated neuroepithelial cells at dorsal levels of the CNS; (2) in areas of cell death; (3) in the marginal layer in close relationship with developing axons; (4) in large extracellular spaces in the subventricular layer; (5) on vascular buds growing through the marginal and subventricular layers; and (6) in the ventricular lumen. Macrophages in different locations varied in morphology and ultrastructure, suggesting that in addition to their involvement in phagocytosis, they play a role in other processes in the developing CNS, such as axonal growth and vascular development. The first macrophages migrate to the CNS independently of its vascularization, apparently traversing the pial basal lamina to reach the nervous parenchyma. Other macrophages may enter the CNS together with vascular buds at subsequent stages during CNS vascularization.

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.

Expression of the human hematopoietic progenitor cell antigen CD34 in vascular and spindle cell tumors

The human hematopoietic progenitor cell antigen is known as CD34. This antigen is present on normal bone marrow progenitor cells and vascular endothelial cells. We used the monoclonal antibody anti-CD34 and immunoperoxidase staining techniques to evaluate the expression of CD34 in benign and malignant vascular and spindle cell tumors. All of the 42 vascular lesions, except two of three lesions of intravascular papillary endothelial hyperplasia, demonstrated diffuse membraneous staining of moderate to strong intensity of their endothelial cells. Also, normal placentas (five) showed similar staining. All neurofibromas (12), three of five neuromas, and one of four neurilemmomas revealed moderate to strong, diffuse, membraneous staining. Five of eight piloleiomyomas, two of seven angioleiomyomas, and one of five uterine leiomyomas showed focal to diffuse, and weak to moderate, membraneous staining in the smooth muscle component. Six dermatofibrosarcoma protuberans were studied: generalized, strongly positive membraneous staining was present in four. All specimens showed staining of the normal endothelial cells and the cells surrounding the hair follicles (bulge area), sebaceous glands, and eccrine glands. No staining was demonstrated in any of the following fibrohistiocytic tumors: atypical fibroxanthomas (two), fibrous dermatofibromas (23), giant cell tumor of tendon sheath (one), and hemosiderotic dermatofibromas (18). Melanocytic tumors [Spitz nevi (three) and spindle cell superficial spreading malignant melanoma (one)], Merkel cell carcinomas (six), and spindle cell squamous cell carcinomas (two) did not stain with anti-CD34. Glomus tumors (two) and a hemangiopericytoma were also negative except for their vascular channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Early formation of the vascular system in quail embryos

The relation between vascular development and translocation of the splanchnic mesodermal layers was studied in presomite to 20-somite quail embryos by scanning electron microscopy. In addition, serially sectioned embryos were stained immunohistochemically with monoclonal antibodies (alpha QH1 or alpha MB1) specific for endothelial and hemopoietic cells. By the formation of the foregut the anterior borders of the two splanchnic mesodermal layers of a presomite embryo are translocated to the lateral and ventral sides of the foregut and fuse in the ventral midline of a 4-somite embryo. Meanwhile the splanchnic mesoderm differentiates into a splanchnic mesothelial layer and a plexus of endothelial cells, facing the endoderm. From 4 somites onward the foregut is covered by a single endothelial plexus. At first the endothelial precursors bordering the anterior intestinal portal and those in the area of the ventral mesocardium lumenize, subsequently giving rise to the endocardium of the heart tube. Hereafter, the pharyngeal arch arteries and the dorsal aortae develop from the remaining precursors. During formation of the pharyngeal arches, the pharyngeal arch arteries maintain their connections with the splanchnic plexus through the developing ventral pharyngeal veins. After disappearance of the dorsal mesocardium, the midpharyngeal endothelial strand, which is a longitudinal strand of proendocardial cells, remains connected to the foregut. This strand will contribute to the formation of the pulmonary venous drainage into the left atrium. A bilateral accumulation of cardiac jelly developing between the promyocardium and proendocardial plexus only suggests that the heart develops from two tubes. The proendocardial layer, however, is not divided by the ventral mesocardium but initially forms just one endocardial heart tube.

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.

Expression of alpha 1 integrin, a laminin-collagen receptor, during myogenesis and neurogenesis in the avian embryo

In this study, we have examined the spatiotemporal distribution of the alpha 1 integrin subunit, a putative laminin and collagen receptor, in avian embryos, using immunofluorescence microscopy and immunoblotting techniques. We used an antibody raised against a gizzard 175 × 10(3) M(r) membrane protein which was described previously and which we found to be immunologically identical to the chicken alpha 1 integrin subunit. In adult avian tissues, alpha 1 integrin exhibited a very restricted pattern of expression; it was detected only in smooth muscle and in capillary endothelial cells. In the developing embryo, alpha 1 integrin subunit expression was discovered in addition to smooth muscle and capillary endothelial cells, transiently, in both central and peripheral nervous systems and in striated muscles, in association with laminin and collagen IV. alpha 1 integrin was practically absent from most epithelial tissues, including the liver, pancreas and kidney tubules, and was weakly expressed by tissues that were not associated with laminin and collagen IV. In the nervous system, alpha 1 integrin subunit expression occurred predominantly at the time of early neuronal differentiation. During skeletal muscle development, alpha 1 integrin was expressed on myogenic precursors, during myoblast migration, and in differentiating myotubes. alpha 1 integrin disappeared from skeletal muscle cells as they became contractile. In visceral and vascular smooth muscles, alpha 1 integrin appeared specifically during early smooth muscle cell differentiation and, later, was permanently expressed after cell maturation. These results indicate that (i) the expression pattern of alpha 1 integrin is consistent with a function as a laminin/collagen IV receptor; (ii) during avian development, expression of the alpha 1 integrin subunit is spatially and temporally regulated; (iii) during myogenesis and neurogenesis, expression of alpha 1 integrin is transient and correlates with cell migration and differentiation.

Induction of vasculogenesis and hematopoiesis in vitro

Despite a large number of investigations of embryonic vascular development, in particular in avian embryos, the conditions under which the endothelial and hematopoietic cell lineages emerge remain unknown. As we demonstrate here, both endothelial and hematopoietic cells can be induced by treatment of dissociated quail epiblast with fibroblast growth factors in vitro. These cells aggregate in characteristic blood islands. In long-term culture, the induced endothelial cells gave rise to vascular structures in vitro, i.e. vasculogenesis. No induction was observed in the absence of fibroblast growth factors, and other growth factors like TGF-beta, TGF-alpha and EGF were not capable of inducing blood island formation. Thus, the dissociated quail epiblast provides a remarkably simple test system to investigate cell lineage diversification in higher vertebrates.

Endothelial cells and hematopoiesis: a light microscopic study of fetal, normal, and pathologic human bone marrow in plastic-embedded sections

The origin and morphological identity of hematopoietic progenitor cells, as well as their precursor, the pleuripotential hematopoietic stem cell (HSC), has not been established. Our studies of 2 microns sectioned undecalcified plastic-embedded bone marrow (BM) from healthy human fetuses; normal adults; patients with acute myeloblastic leukemia (AML), acute lymphoblastic leukemia (ALL), and chronic granulocytic leukemia (CGL) in various stages (chronic, accelerated, acute blastic phase, and after autografting); and patients recovering from therapy-induced marrow hypoplasia suggest that proliferative hematopoietic zones exist near the endosteum (endosteal marrow) and the vascular endothelium (capillary and sinus-lining endothelium) and a maturational zone distal to these regions. In some of these areas, morphologically recognizable hematopoietic cells were seen and interpreted as emerging and maturing in a sequential progression, suggesting an origin from the endosteal or endothelial progenitors. In other loci, early hematopoietic cells were seen in close contact with the endosteal or vascular endothelial (VE) cells. This latter relationship suggested that these areas of cellular contact were important and represented sites of cell to cell interaction that may be associated with the liberation of growth factors by endosteal and endothelial cells and their action on hematopoietic progenitor cells. Following treatment-induced hypoplasia, the endosteal and VE cells were seen to modulate, transform, and migrate into the surrounding empty and edematous marrow space as fibroblasts. Later, as hemopoietic regeneration began, clusters of regenerating hematopoietic cells were seen adjacent to bone trabecule (BT) and near the vascular endothelium. We postulate that endosteal and VE cells are the equivalent of embryonal-stage, undifferentiated mesenchyme and, under the appropriate regulatory influence, are capable of modulation and transformation (differentiation) into stromal (fibroblast-like) cells and precursors of hematopoietic cells in normal (physiologic) and stressed (pathologic) conditions. Recently, human endothelial cells have been shown to express a large number of cell surface antigens in common with hematopoietic (myeloid and lymphoid) cells. It is also possible that, in some situations, the VE cells act to establish a microenvironment and liberate growth factor(s), enabling pleuripotential and progenitor cell differentiation into mature hematopoietic cells adjacent to the vascular endothelium. Indeed, vascular endothelium has been shown to elaborate growth factors that participate in normal hematopoiesis.

Macrophage-like cells invading the suboptic necrotic centres of the avian embryo diencephalon originate from haemopoietic precursors

Macrophage-like cells have been previously shown within the suboptic necrotic centres of chick embryos during the period just previous to, and coinciding with, growth of the earliest optic axons through suboptic necrotic centres. In this paper, light and electron microscopy observations of chick embryos suggest that these macrophage-like cells originate from blood cells. Immunocytochemical techniques in chick-quail yolk sac chemeras, constituted of a chick embryo and a quail yolk sac, revealed that the macrophage-like cells within the suboptic necrotic centres are labelled with anti-MB1 antibody, which is specific for quail haemopoietic and endothelial cell lineage. These findings demonstrate that these phagocytic cells are of blood cell lineage, and originate in the extraembryonic tissues of the yolk sac. Diffuse staining around some suboptic necrotic centre macrophage-like cells suggests that they release MB1 antigens which may play a role in the growth of the optic axons through the suboptic necrotic centres.

Experimental analysis of blood vessel development in the avian wing bud

Shortly after its appearance, the avian limb bud becomes populated by a rich plexus of vascular channels. Formation of this plexus occurs by angiogenesis, specifically the ingrowth of branches from the dorsal aorta or cardinal veins, and by differentiation of endogenous angioblasts within limb mesoderm. However, mesenchyme located immediately beneath the surface ectoderm of the limb is devoid of patent blood vessels. The objective of this research is to ascertain whether peripheral limb mesoderm lacks angioblasts at all stages or becomes avascular secondarily during limb development. Grafts of core or peripheral wing mesoderm, identified by the presence or absence of patent channels following systemic infusion with ink, were grafted from quail embryos at stages 16-26 into the head region of chick embryos at stages 9-10. Hosts were fixed 3-5 days later and sections treated with antibodies that recognize quail endothelial cells and their precursors. Labeled endothelial cells were found intercalated into normal craniofacial blood vessels both nearby and distant from the site of implantation following grafting of limb core mesoderm from any stage. Identical results were obtained following grafting of limb peripheral mesoderm at stages 16-21. However, peripheral mesoderm from donors older than stage 22 did not contain endothelial precursors. Thus at the onset of appendicular development angioblasts are present throughout the mesoderm of the limb bud. During the fourth day of incubation, these cells are lost from peripheral mesoderm, either through emigration or degeneration.

Haemopoietic phagocytes in the early differentiating avian retina

The existence of specialised phagocytic cells is described in regions of the retinal neuroepithelium undergoing intense cell death during early differentiation of the avian embryo retina (2.5-5 days of incubation). These results were obtained using routine techniques for light microscopy, acid phosphatase histochemistry and immunocytochemical staining with antibodies MB-1 and QH-1, both specific for quail endothelial cells and all blood cells except mature erythrocytes. Specialised phagocytes were distinguishable from neuroepithelial cells on the basis of morphological criteria: in the former, the nucleus was not oval in shape and was not oriented perpendicular to basement membrane neuroepithelium. The cytoplasm of the specialised phagocytes was often filled with dead cell fragments. In contrast to neuroepithelial cells, the specialised phagocytes showed acid phosphatase activity and were labelled with both MB-1 and QH-1 antibodies in normal quail embryos and chick----quail yolk sac chimeras. Moreover, some acid phosphatase positive and MB-1/QH-1 positive cells also appeared in the presumptive vitreous body, at the edges of the optic cup and in the surrounding mesenchyme. As the vitreal cells and the specialised phagocytes of the neural retina were immunolabelled in chick----quail yolk sac chimeras, we conclude that they are derived from haemopoietic cells in the yolk sac. Some images suggest that these cells enter the vitreous body from the surrounding mesenchyme and traverse the basement membrane of the neuroepithelium in the optic disc region to give rise to the specialised phagocytes of the retinal neuroepithelium.

Distribution and migration of angiogenic cells from grafted avascular intraembryonic mesoderm

The hemangiogenic potencies of initially avascular intra-embryonic mesoderm were studied in chick and quail embryos and in chick-quail chimeras. The prechordal mesoderm, primitive streak and primitive node of quail embryos were heterospecifically grafted into limb buds of chick embryos. Hemangiopoietic quail cells in the host limb were detected by immunohistological staining with the monoclonal anti-MB-1 antibody after 3-6 days of re-incubation. The antibody is specifically directed against quail hemangiopoietic cells and their derivatives. Quail endothelial cells were found in pure quail and in chimeric vessels, inside as well as outside the graft. The main artery of the limb and the vessels inside the graft were connected by chimeric arteries. Proximal to the graft, quail endothelial cells were located predominantly within the lining of the main artery, while distally they were found mainly in the veins and the marginal sinus. The results show that, as early as stage 3 (according to Hamburger and Hamilton 1951, HH) all parts of the avascular intraembryonic mesoderm tested, give rise to endothelial cells. Both mechanisms, angiogenesis and vasculogenesis, contribute to the vascularization of the limb. Immunocytological and scanning electron microscopic studies indicate that centrifugal and centripetal migration of angiogenic cells occurs outside the vessels as well as on the inner surface of the endothelium.

Vascular endothelial cells migrate centripetally within embryonic arteries

Migration of vascular endothelial cells was traced in quail-chick chimeras. After heterospecific transplantations of quail limb bud pieces, or other tissues containing blood vessels, into the limbs or the coelomic cavity, the immunohistochemically stained endothelial cells of the quail were found to invade the chick host vessels, favouring the arteries. Within these vessels the endothelial cells regularly reach the host aorta, where they contribute to the endothelium on the ipsilateral side. It is concluded that the endothelial cells activity migrate, because microinjections of a synthetic peptide which contains the RGD-sequence and mimics fibronectin, stop the invasion of endothelial cells.

Origins and assembly of avian embryonic blood vessels

Two processes by which embryonic blood vessels develop are well-known: angiogenesis (growth by budding and branching of existing vessels) and local formation of endothelial vesicles that coalesce with elongating vessels. The former process appears to be more prevalent, with the latter restricted to vessels that form near the endoderm-mesoderm interface. The contributions of endothelial cells formed by each of these processes to specific blood vessels has not been defined, however, nor have the origins of precursors (angioblasts) of intraembryonic endothelial populations been established. To identify the origins of endothelial cells, precursor populations from quail embryos were transplanted into chick embryos. Antibodies that recognize quail endothelial cells were applied to sections from chimeric embryos fixed 2-5 days after surgery. These experiments reveal that all intraembryonic mesodermal tissues, except the notochord and prechordal plate, contain angiogenic precursors. Many angioblasts emigrate from the grafted tissue, invading surrounding mesenchyme and contributing to the formation of arteries, veins, and capillaries in a wide area. The invasive behavior of these angioblasts is unlike that of any other embryonic mesenchymal cell type and represents a third process operating during embryonic blood vessel formation. Transplanted angioblasts, even those excised from quail trunk regions, form normal craniofacial vascular channels, including the cardiac outflow tract. These results demonstrate that the control over blood vessel assembly resides within the connective tissue-forming mesenchyme of the embryo, not within endothelial precursors.

Development of the origin of the coronary arteries, a matter of ingrowth or outgrowth?

Inconsistencies still exist with regard to the exact mode of development of proximal coronary arteries and coronary orifices. In this regard 15 quail embryos were investigated using a monoclonal anti-endothelium antibody, enabling a detailed study of the development of endothelium-lined vasculature. Coronary orifices emerged at 7-9 days of incubation (Zacchei stages 24-26) and were invariably present at 10 days of incubation (Zacchei stage 27). We never observed more than 2 coronary orifices; these were always single in either of the facing sinuses of the aorta. A coronary orifice was always observed being connected to an already developed proximal coronary artery, which belonged to a peritruncal ring of coronary arterial vasculature. We did not find any coronary orifice without a connection to a proximal coronary artery. Moreover, at 7-9 days of incubation (Zacchei stages 24-26) we observed coronary arteries from the peritruncal ring penetrating the aortic media. In 2 specimen this coronary artery, with a lumen, was in contact with the still intact endothelial lining of the aorta. We conclude that coronary arteries do not grow out of the aorta, but grow into the aorta from the peritruncal ring of coronary arterial vasculature. This throws new light on normal and abnormal development of proximal coronary arteries and coronary orifices.

SEM characterization of a cellular layer separating blood vessels from endoderm in the quail embryo

The associations between the developing blood vessels and both endoderm and splanchnic mesoderm in quail embryos at stages 9-11 were examined by using scanning electron microscopy. Embryos were pinned ventral-side up on agar plates and the endoderm was surgically removed prior to fixation and dehydration. This procedure exposes a netlike layer of cells closely apposed to the ventral surface of paraxial mesoderm and all visible blood vessels; we are calling this the subvascular layer. Development of this layer proceeds rostral-to-caudal, and lateral-to-medial, with the earliest stages of formation being visible over the unsegmented paraxial mesoderm of the segmental plate. The subvascular layer increases markedly in density slightly medial to the innermost boundary of the intraembryonic vascular plexus. Cells of this layer eventually establish a continuous sheet beneath the lateral plate and paraxial mesoderm and the notochord. With maturation, the cells of the subvascular layer approach confluence. The spatial and temporal patterns of development of the embryonic vascular tissues and the subvascular layer are closely correlated, suggesting a possible role for the subvascular layer in normal embryonic vascular development.

Early events in human T cell ontogeny. Phenotypic characterization and immunohistologic localization of T cell precursors in early human fetal tissues

During early fetal development, T cell precursors home from fetal yolk sac and liver to the epithelial thymic rudiment. From cells that initially colonize the thymus arise mature T cells that populate T cell zones of the peripheral lymphoid system. Whereas colonization of the thymus occurs late in the final third of gestation in the mouse, in birds and humans the thymus is colonized by hematopoietic stem cell precursors during the first third of gestation. Using a large series of early human fetal tissues and a panel of monoclonal antibodies that includes markers of early T cells (CD7, CD45), we have studied the immunohistologic location and differentiation capacity of CD45+, CD7+ cells in human fetal tissues. We found that before T cell precursor colonization of the thymus (7-8 wk of gestation), CD7+ cells were present in yolk sac, neck, upper thorax, and fetal liver, and were concentrated in mesenchyme throughout the upper thorax and neck areas. By 9.5 wk of gestation, CD7+ cells were no longer present in upper thorax mesenchyme but rather were localized in the lymphoid thymus and scattered throughout fetal liver. CD7+, CD2-, CD3-, CD8-, CD4-, WT31- cells in thorax and fetal liver, when stimulated for 10-15 d with T cell-conditioned media and rIL-2, expressed CD2, CD3, CD4, CD8, and WT31 markers of the T cell lineage. Moreover, CD7+ cells isolated from fetal liver contained all cells in this tissue capable of forming CFU-T colonies in vitro. These data demonstrate that T cell precursors in early human fetal tissues can be identified using a mAb against the CD7 antigen. Moreover, the localization of CD7+ T cell precursors to fetal upper thorax and neck areas at 7-8.5 wk of fetal gestation provides strong evidence for a developmentally regulated period in man in which T cell precursors migrate to the epithelial thymic rudiment.

[Chimeras in biologic embryology]

Chimeras produced from amphibian, mammalian, and especially avian embryos have provided important insights into vertebrate development. Important contributions have led to new concepts in understanding the development of, for example, the nervous system, the vascular system, and the skeletal muscles. The migration of cells is particularly accessible in chimeras. More important results are to be expected from chimeras in the future, especially by combining this approach with other state-of-the-art techniques.

Surface markers of axolotl lymphocytes as defined by monoclonal antibodies

In an attempt to identify urodele amphibian lymphocyte subpopulations by their surface markers, we prepared hybridomas from BALB/c mice spleen immunized with axolotl (Ambystoma mexicanum) blood and splenic leucocytes and purified immunoglobulins. Sixty-five hybridomas were selected and subsequently subcloned. Among numerous monoclonal antibodies (mAbs) thus obtained, four mAbs were extensively characterized by immunoblotting, single and double fluorescence and immunohistology. MAb 34.38.6 recognizes polypeptides between 65,000 and 72,000 MW and labels in immunofluorescence nearly all thymocytes, 60-63% splenic lymphocytes of normal animals but only 9% splenic lymphocytes in thymectomized animals. MAb 19.14.2 reacts with a 98,000 MW protein and labels a restricted lymphocyte population in thymus (52-77%) and spleen (20-25%). The immunohistological study demonstrates that 34.38.6 and 19.14.2 label most thymocytes and a large proportion of spleen leucocytes including lymphocytes, granulocytes and macrophages. In addition, 19.14.2 labels some large interdigitating cells in thymic epithelial areas and splenic cords. MAbs 33.45.1 and 33.101.2, respectively, recognize heavy (72,000-88,000 MW) and light (20,000-27,000 MW) axolotl immunoglobulin chains. They do not react with thymocytes but label a splenic lymphocyte population not labelled by mAb 34.38.6. The proportion of surface immunoglobulin-positive (sIg+) lymphocytes in spleen is not altered by thymectomy. MAb 33.101.2 labels 40-48% of splenic lymphocytes, 33.45.1 stains only 14% of these same cells. This suggests some interesting heavy-chain isotypic differences in axolotl. For the first time in urodele amphibians, mAbs differentiate T-like and B-like lymphocyte populations by their membrane markers. This will allow further analysis of the axolotl immune system.

A chicken leukocyte common antigen: biochemical characterization and ontogenetic study

A monoclonal antibody, designated CL-1, was produced by immunizing mice with chicken peripheral blood lymphocytes (PBL) and selected in an indirect immunofluorescence assay. CL-1 characterizes a cell surface antigen expressed on all chicken hemopoietic cells except mature erythrocytes. Cells stained by CL-1 were detected at early stages of embryonic development in the various hemopoietic compartments. CL-1 reacts with two chicken PBL proteins of 200 kDa and 215 kDa which are probably the homologues of the T-200 and B-220 antigens previously described in the mouse. Moreover, CL-1 does not react with quail hemopoietic tissues and can be useful in experiments using quail-chick chimeras to study the ontogeny of the hemopoietic system.

Phylogenetically conserved antigen on nerve cells and lymphocytes resembles myelin-associated glycoprotein

The HNK-1 (Leu 7) and NC-1 monoclonal antibodies, raised against a human T-cell line and against nerve cells of quail embryos, respectively, have been shown to bind to a shared epitope present on the surface of human large granular lymphocytes and on nerve cells in species ranging from amphibians to humans. We demonstrate that a related antigen is also expressed on the lymphocyte surface in the avian central lymphoid organs, thymus and bursa, and in the spleen during embryonic and adult life. The expression of the HNK-1/NC-1-reactive determinant differs remarkably in the bursal and thymic compartments, antigen expression being stabilized at a high level early in development of the bursa, whereas its expression fluctuates in the thymus. The material immunoprecipitated from bursal and thymic lymphocytes by the HNK-1/NC-1 antibodies exhibits the same relative molecular mass as myelin-associated glycoprotein, which is one of the molecules recognized by these antibodies in the nervous system. Together with the observation that an antiserum reactive with the protein part of chicken myelin-associated glycoprotein detects similar material in membrane extracts of HNK-1/NC-1-positive thymocytes, this suggests that a molecule sharing structural analogies with this nerve cell component is expressed on cells of the immune system.

Homing of hemopoietic precursor cells to the embryonic thymus: characterization of an invasive mechanism induced by chemotactic peptides

During embryonic development, T cell precursors migrate to the thymus, where immunocompetency is acquired. Our previous studies have shown that avian hemopoietic precursor cells are recruited to the thymus by chemotactic peptides secreted by thymic epithelial cells (Champion, S., B. A. Imhof, P. Savagner, and J. P. Thiery, 1986, Cell, 44:781-790). In this study, we have characterized the homing of these precursor cells to the thymus in vivo by electron and light microscopy. Hemopoietic precursors could be seen to extravasate from blood or lymphatic vessels, migrate in the mesenchyme, traverse the perithymic basement membrane, and finally intercalate into the thymic epithelium. Labeled hemopoietic precursors injected into the blood circulation also followed the same pathway. Migrating hemopoietic precursor cells were found to express the fibronectin receptor complex. In the presence of thymic chemotactic peptides, hemopoietic precursors traverse a human amniotic basement membrane. This invasive process was inhibited by antibodies to laminin or to fibronectin, two major glycoproteins of the amniotic membrane, by monovalent Fab' fragments of antibodies to the fibronectin receptor, and, finally by synthetic peptides that contain the cell-binding sequence Arg-Gly-Asp-Ser of fibronectin. These results indicate that hemopoietic precursors respond to thymic chemotactic peptides by invasive behavior. Direct interactions between basement membrane components and fibronectin receptors appear to be required for this developmentally regulated invasion process.

MB1, a quail leukocyte-endothelium antigen: partial characterization of the cell surface and secreted forms in cultured endothelial cells

We describe here conditions allowing the selective growth in culture of embryonic capillary endothelial cells from quail yolk sac. Such cultures were set up to characterize an antigen present on the endothelial cell surface and to study whether it was secreted in the culture medium. This antigen, MB1, was previously evidenced by a monoclonal antibody raised to quail IgM heavy chain. It is present at the surface of all endothelial and hemopoietic cells (except mature erythrocytes) starting from the hemangioblast, the early mesodermal precursor of blood and vascular endothelial cells. The MB1 epitope is also found on quail plasma molecules of 80 and 125-200 kDa. By immunoprecipitation of either surface or metabolically labeled endothelial cellular material, we have chemically characterized MB1-bearing components as glycoproteins of apparent molecular mass ranging from 80 to 200 kDa and provided evidence for their release into the culture medium. This is consistent with the hypothesis that, in the quail, vascular endothelium participates in the secretion of the alpha-MB1-positive plasmatic components.