Single cell transcriptome dynamics from pluripotency to FLK1+ mesoderm.

Hemangiogenic progenitors generating blood and endothelial cells are specified from FLK1-expressing (FLK1+) mesoderm by the transcription factor ETV2. FLK1+ mesoderm also contributes to smooth muscle and cardiomyocytes. However, the developmental process of FLK1+ mesoderm generation and its allocation to various cell fates remain obscure. Recent single cell RNA-sequencing studies of early embryos or in vitro-differentiated human embryonic stem (ES) cells have provided unprecedented information on the spatiotemporal resolution of cells in embryogenesis. These snapshots, however, lack information on continuous dynamic developmental processes. Here, we performed single cell RNA sequencing of in vitro-differentiated mouse ES cells to capture the continuous developmental process leading to hemangiogenesis. We found that hemangiogenic progenitors from ES cells develop through intermediate gastrulation stages, which are gradually specified by 'relay'-like highly overlapping transcription factor modules. Moreover, the transcriptional program of the Flk1+ mesoderm was maintained in the smooth muscle lineage, suggesting that smooth muscle is the default fate of Flk1+ mesoderm. We also identified the SRC kinase contributing to ETV2-mediated activation of the hemangiogenic program. This continuous transcriptome map will facilitate both basic and applied studies of mesoderm development.

Tissue growth pressure drives early blood flow in the chicken yolk sac.

BACKGROUND: Understanding how molecular and physical cues orchestrate vascular morphogenesis is a challenge for developmental biology. Only little attention has been paid to the impact of mechanical stress caused by tissue growth on early blood distribution. Here we study the peripheral accumulation of blood in the chicken embryonic yolk sac, which precedes sinus vein formation. RESULTS: We report that blood accumulation starts before heart-induced blood circulation. We hypothesized that the driving force for the primitive blood flow is a growth-induced gradient of tissue pressure in the yolk sac mesoderm. Therefore, we studied embryos in which heart development was arrested after 2 days of incubation, and found that yolk sac growth and blood peripheral accumulation still occurred. This suggests that tissue growth is sufficient to initiate the flow and the formation of the sinus vein, whereas heart contractions are not required. We designed a simple mathematical model which makes explicit the growth-induced pressure gradient and the subsequent blood accumulation, and show that growth can indeed account for the observed blood accumulation. CONCLUSIONS: This study shows that tissue growth pressure can drive early blood flow, and suggests that the mechanical environment, beyond hemodynamics, can contribute to vascular morphogenesis. Developmental Dynamics 246:573-584, 2017. © 2017 Wiley Periodicals, Inc.

Refuting the hypothesis that semaphorin-3f/neuropilin-2 exclude blood vessels from the cap mesenchyme in the developing kidney.

BACKGROUND: During murine kidney development, new cortical blood vessels form and pattern in cycles that coincide with cycles of collecting duct branching and the accompanying splitting of the cap mesenchyme (nephron progenitor cell populations that "cap" collecting duct ends). At no point in the patterning cycle do blood vessels enter the cap mesenchyme. We hypothesized that the exclusion of blood vessels from the cap mesenchyme may be controlled, at least in part, by an anti-angiogenic signal expressed by the cap mesenchyme cells. RESULTS: We show that semaphorin-3f (Sema3f), a known anti-angiogenic factor, is expressed in cap mesenchymal cells and its receptor, neuropilin-2 (Nrp2), is expressed by newly forming blood vessels in the cortex of the developing kidney. We hypothesized that Sema3f/Nrp2 signaling excludes vessels from the cap mesenchyme. Genetic ablation of Sema3f and of Nrp2, however, failed to result in vessels invading the cap mesenchyme. CONCLUSIONS: Despite complementary expression patterns, our data suggest that Sema3f and Nrp2 are dispensable for the exclusion of vessels from the cap mesenchyme during kidney development. These results should provoke additional experiments to ascertain the biological significance of Sema3f/Nrp2 expression in the developing kidney. Developmental Dynamics 246:1047-1056, 2017. © 2017 Wiley Periodicals, Inc.

Cyp26 enzymes are required to balance the cardiac and vascular lineages within the anterior lateral plate mesoderm.

Normal heart development requires appropriate levels of retinoic acid (RA) signaling. RA levels in embryos are dampened by Cyp26 enzymes, which metabolize RA into easily degraded derivatives. Loss of Cyp26 function in humans is associated with numerous developmental syndromes that include cardiovascular defects. Although previous studies have shown that Cyp26-deficient vertebrate models also have cardiovascular defects, the mechanisms underlying these defects are not understood. Here, we found that in zebrafish, two Cyp26 enzymes, Cyp26a1 and Cyp26c1, are expressed in the anterior lateral plate mesoderm (ALPM) and predominantly overlap with vascular progenitors (VPs). Although singular knockdown of Cyp26a1 or Cyp26c1 does not overtly affect cardiovascular development, double Cyp26a1 and Cyp26c1 (referred to here as Cyp26)-deficient embryos have increased atrial cells and reduced cranial vasculature cells. Examining the ALPM using lineage tracing indicated that in Cyp26-deficient embryos the myocardial progenitor field contains excess atrial progenitors and is shifted anteriorly into a region that normally solely gives rise to VPs. Although Cyp26 expression partially overlaps with VPs in the ALPM, we found that Cyp26 enzymes largely act cell non-autonomously to promote appropriate cardiovascular development. Our results suggest that localized expression of Cyp26 enzymes cell non-autonomously defines the boundaries between the cardiac and VP fields within the ALPM through regulating RA levels, which ensures a proper balance of myocardial and endothelial lineages. Our study provides novel insight into the earliest consequences of Cyp26 deficiency that underlie cardiovascular malformations in vertebrate embryos.

UBR box N-recognin-4 (UBR4), an N-recognin of the N-end rule pathway, and its role in yolk sac vascular development and autophagy.

The N-end rule pathway is a proteolytic system in which destabilizing N-terminal residues of short-lived proteins act as degradation determinants (N-degrons). Substrates carrying N-degrons are recognized by N-recognins that mediate ubiquitylation-dependent selective proteolysis through the proteasome. Our previous studies identified the mammalian N-recognin family consisting of UBR1/E3α, UBR2, UBR4/p600, and UBR5, which recognize destabilizing N-terminal residues through the UBR box. In the current study, we addressed the physiological function of a poorly characterized N-recognin, 570-kDa UBR4, in mammalian development. UBR4-deficient mice die during embryogenesis and exhibit pleiotropic abnormalities, including impaired vascular development in the yolk sac (YS). Vascular development in UBR4-deficient YS normally advances through vasculogenesis but is arrested during angiogenic remodeling of primary capillary plexus associated with accumulation of autophagic vacuoles. In the YS, UBR4 marks endoderm-derived, autophagy-enriched cells that coordinate differentiation of mesoderm-derived vascular cells and supply autophagy-generated amino acids during early embryogenesis. UBR4 of the YS endoderm is associated with a tissue-specific autophagic pathway that mediates bulk lysosomal proteolysis of endocytosed maternal proteins into amino acids. In cultured cells, UBR4 subpopulation is degraded by autophagy through its starvation-induced association with cellular cargoes destined to autophagic double membrane structures. UBR4 loss results in multiple misregulations in autophagic induction and flux, including synthesis and lipidation/activation of the ubiquitin-like protein LC3 and formation of autophagic double membrane structures. Our results suggest that UBR4 plays an important role in mammalian development, such as angiogenesis in the YS, in part through regulation of bulk degradation by lysosomal hydrolases.

The distribution and ultrastructure of the forming blood capillaries and the effect of apoptosis on vascularization in mouse embryonic molar mesenchyme.

Vascularization is essential for organ and tissue development. Teeth develop through interactions between epithelium and mesenchyme. The developing capillaries in the enamel organ, the dental epithelial structure, occur simultaneously by mechanisms of vasculogenesis and angiogenesis at the onset of dentinogenesis. The vascular neoformation in the dental mesenchyme has been reported to start from the cap stage. However, the mechanisms of vascularization in the dental mesenchyme remain unknown. In the hope of understanding the mechanisms of the formation of dental mesenchymal vasculature, mouse lower molar germs from embryonic day (E) 13.5 to E16.5 were processed for immunostaining of CD31 and CD34, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and transmission electron microscopy (TEM). In addition, the role of apoptosis for the vascularization in dental mesenchyme was examined by in vitro culture of E14.0 lower molars in the presence of the apoptosis inhibitor (z-VAD-fmk) and a subsequent subrenal culture. Our results showed that CD31- and CD34-positive cells progressively entered the central part of the dental papilla from the peridental mesenchyme. For TEM, angioblasts, young capillaries with thick endothelium and endothelial cells containing vacuoles were observed in peripheral dental mesenchyme, suggesting vasculogenesis was taking place. The presence of lateral sprouting, cytoplasmic filopodia and transluminal bridges in the dental papilla suggested angiogenesis was also occurring. Inhibition of apoptosis delayed the angiogenic vascularization of the dental papilla. Therefore, these data demonstrated that molar mesenchyme is progressively vascularized by mechanisms of both vasculogenesis and angiogenesis and apoptosis partially contributes to the vascularization of the dental papilla.

Dusp-5 and Snrk-1 coordinately function during vascular development and disease.

Mitogen-activated protein kinases play an integral role in several cellular processes. To regulate mitogen-activated protein kinases, cells express members of a counteracting group of proteins called phosphatases. In this study, we have identified a specific role that one member of this family of phosphatases, dual-specific phosphatase-5 (Dusp-5) plays in vascular development in vivo. We have determined that dusp-5 is expressed in angioblasts and in established vasculature and that it counteracts the function of a serine threonine kinase, Snrk-1, which also plays a functional role in angioblast development. Together, Dusp-5 and Snrk-1 control angioblast populations in the lateral plate mesoderm with Dusp-5 functioning downstream of Snrk-1. Importantly, mutations in dusp-5 and snrk-1 have been identified in affected tissues of patients with vascular anomalies, implicating the Snrk-1-Dusp-5 signaling pathway in human disease.

Developmental biology of the pancreas: a comprehensive review.

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.

RBP-J, the transcription factor downstream of Notch receptors, is essential for the maintenance of vascular homeostasis in adult mice.

In adults, angiogenic abnormalities are involved in not only tumor growth but several human inherited diseases as well. It is unclear, however, concerning how the normal vascular structure is maintained and how angiogenesis is initiated in normal adults. Using the Cre-LoxP-mediated conditional gene deletion, we show in the present study that in adult mice disruption of the transcription factor recombination signal-binding protein Jkappa (RBP-J) in endothelial cells strikingly induced spontaneous angiogenesis in multiple tissues, including retina and cornea, as well as in internal organs, such as liver and lung. In a choroidal neovascularization model, which mimics the angiogenic process in tumor growth and age-related macular degeneration, RBP-J deficiency induced a more intensive angiogenic response to injury. This could be transmitted by bone marrow, indicating that RBP-J could modulate bone marrow-derived endothelial progenitor cells in adult angiogenesis. In addition, in the absence of RBP-J, proliferation of endothelial cells increased significantly, leading to accumulative vessel outgrowth. These findings suggest that in adults RBP-J-mediated Notch signaling may play an essential role in the maintenance of vascular homeostasis by repressing endothelial cell proliferation.

Requirement of vasculogenesis and blood circulation in late stages of liver growth in zebrafish.

BACKGROUND: Early events in vertebrate liver development have been the major focus in previous studies, however, late events of liver organogenesis remain poorly understood. Liver vasculogenesis in vertebrates occurs through the interaction of endoderm-derived liver epithelium and mesoderm-derived endothelial cells (ECs). In zebrafish, although it has been found that ECs are not required for liver budding, how and when the spatio-temporal pattern of liver growth is coordinated with ECs remains to be elucidated. RESULTS: To study the process of liver development and vasculogenesis in vivo, a two-color transgenic zebrafish line Tg(lfabf:dsRed; elaA:EGFP) was generated and named LiPan for liver-specific expression of DsRed RFP and exocrine pancreas-specific expression of GFP. Using the LiPan line, we first followed the dynamic development of liver from live embryos to adult and showed the formation of three distinct yet connected liver lobes during development. The LiPan line was then crossed with Tg(fli1:EGFP)y1 and vascular development in the liver was traced in vivo. Liver vasculogenesis started at 55-58 hpf when ECs first surrounded hepatocytes from the liver bud surface and then invaded the liver to form sinusoids and later the vascular network. Using a novel non-invasive and label-free fluorescence correction spectroscopy, we detected blood circulation in the liver starting at approximately 72 hpf. To analyze the roles of ECs and blood circulation in liver development, both cloche mutants (lacking ECs) and Tnnt2 morphants (no blood circulation) were employed. We found that until 70 hpf liver growth and morphogenesis depended on ECs and nascent sinusoids. After 72 hpf, a functional sinusoidal network was essential for continued liver growth. An absence of blood circulation in Tnnt2 morphants caused defects in liver vasculature and small liver. CONCLUSION: There are two phases of liver development in zebrafish, budding and growth. In the growth phase, there are three distinct stages: avascular growth between 50-55 hpf, where ECs are not required; endothelium-dependent growth, where ECs or sinusoids are required for liver growth between 55-72 hpf before blood circulation in liver sinusoids; and circulation-dependent growth, where the circulation is essential to maintain vascular network and to support continued liver growth after 72 hpf.

The vascular pattern in the papillary region of rat incisor and molar tooth enamel organ.

Scanning electron microscopy of the enamel organ of rat incisor and molar teeth in the maturation stage of amelogenesis revealed two vascularization patterns of the papillary layer. In one pattern, the anastomosing capillaries formed loops of varying sizes around spherical or somehwat oblong papillae. In the second pattern, the capillaries were parallel to each other embedded in furrows between long ridges of papillary cells. It is postulated that each of these two patterns may be associated with a specific stage in the process of enamel maturation.

Visceral endoderm expression of Yin-Yang1 (YY1) is required for VEGFA maintenance and yolk sac development.

Mouse embryos lacking the polycomb group gene member Yin-Yang1 (YY1) die during the peri-implantation stage. To assess the post-gastrulation role of YY1, a conditional knock-out (cKO) strategy was used to delete YY1 from the visceral endoderm of the yolk sac and the definitive endoderm of the embryo. cKO embryos display profound yolk sac defects at 9.5 days post coitum (dpc), including disrupted angiogenesis in mesoderm derivatives and altered epithelial characteristics in the visceral endoderm. Significant changes in both cell death and proliferation were confined to the YY1-expressing yolk sac mesoderm indicating that loss of YY1 in the visceral endoderm causes defects in the adjacent yolk sac mesoderm. Production of Vascular Endothelial Growth Factor A (VEGFA) by the visceral endoderm is essential for normal growth and development of the yolk sac vasculature. Reduced levels of VEGFA are observed in the cKO yolk sac, suggesting a cause for the angiogenesis defects. Ex vivo culture with exogenous VEGF not only rescued angiogenesis and apoptosis in the cKO yolk sac mesoderm, but also restored the epithelial defects observed in the cKO visceral endoderm. Intriguingly, blocking the activity of the mesoderm-localized VEGF receptor, FLK1, recapitulates both the mesoderm and visceral endoderm defects observed in the cKO yolk sac. Taken together, these results demonstrate that YY1 is responsible for maintaining VEGF in the developing visceral endoderm and that a VEGF-responsive paracrine signal, originating in the yolk sac mesoderm, is required to promote normal visceral endoderm development.

Early embryonic vascular patterning by matrix-mediated paracrine signalling: a mathematical model study.

During embryonic vasculogenesis, endothelial precursor cells of mesodermal origin known as angioblasts assemble into a characteristic network pattern. Although a considerable amount of markers and signals involved in this process have been identified, the mechanisms underlying the coalescence of angioblasts into this reticular pattern remain unclear. Various recent studies hypothesize that autocrine regulation of the chemoattractant vascular endothelial growth factor (VEGF) is responsible for the formation of vascular networks in vitro. However, the autocrine regulation hypothesis does not fit well with reported data on in vivo early vascular development. In this study, we propose a mathematical model based on the alternative assumption that endodermal VEGF signalling activity, having a paracrine effect on adjacent angioblasts, is mediated by its binding to the extracellular matrix (ECM). Detailed morphometric analysis of simulated networks and images obtained from in vivo quail embryos reveals the model mimics the vascular patterns with high accuracy. These results show that paracrine signalling can result in the formation of fine-grained cellular networks when mediated by angioblast-produced ECM. This lends additional support to the theory that patterning during early vascular development in the vertebrate embryo is regulated by paracrine signalling.

Placental mesenchymal dysplasia presenting as a twin gestation with complete molar pregnancy.

BACKGROUND: Placental mesenchymal dysplasia is a rare abnormality characterized by placentomegaly, grapelike cystic vesicles, and villous hyperplasia. The clinical and ultrasonographic presentation may mimic molar pregnancy, provoking incorrect diagnoses and unnecessary therapeutic interventions. CASE: A 36-year-old nulliparous woman presented for prenatal ultrasonography that indicated the presence of one gestational sac containing both fetus and cystic mass, concerning for partial molar pregnancy. Amniocentesis returned a 46,XX karyotype, suggesting a twin gestation with complete mole. The patient was monitored closely and, because of fetal growth restriction, was induced successfully at term and delivered a healthy newborn. Histopathologic findings of the placenta were consistent with placental mesenchymal dysplasia. CONCLUSION: Although placental mesenchymal dysplasia is often confused with molar pregnancy, it is important to consider both in a differential to avoid inappropriate treatments.

HoxA3 is an apical regulator of haemogenic endothelium.

During development, haemogenesis occurs invariably at sites of vasculogenesis. Between embryonic day (E) 9.5 and E10.5 in mice, endothelial cells in the caudal part of the dorsal aorta generate haematopoietic stem cells and are referred to as haemogenic endothelium. The mechanisms by which haematopoiesis is restricted to this domain, and how the morphological transformation from endothelial to haematopoietic is controlled are unknown. We show here that HoxA3, a gene uniquely expressed in the embryonic but not yolk sac vasculature, restrains haematopoietic differentiation of the earliest endothelial progenitors, and induces reversion of the earliest haematopoietic progenitors into CD41-negative endothelial cells. This reversible modulation of endothelial-haematopoietic state is accomplished by targeting key haematopoietic transcription factors for downregulation, including Runx1, Gata1, Gfi1B, Ikaros, and PU.1. Through loss-of-function, and gain-of-function epistasis experiments, and the identification of antipodally regulated targets, we show that among these factors, Runx1 is uniquely able to erase the endothelial program set up by HoxA3. These results suggest both why a frank endothelium does not precede haematopoiesis in the yolk sac, and why haematopoietic stem cell generation requires Runx1 expression only in endothelial cells.

Epidermal growth factor receptor signaling modulates chemokine (CXC) ligand 5 expression and is associated with villus angiogenesis after small bowel resection.

BACKGROUND: Adaptive villus growth after a massive small bowel resection (SBR) is an important response to the loss of intestinal surface area and is regulated via epidermal growth factor receptor (EGFR) signaling. Increased levels of the proangiogenic chemokine ligand 5 (CXCL5) have been found within the adapting bowel in which angiogenesis is increased. We sought to determine whether CXCL5 was expressed specifically in the villus mesenchymal zone (area of increased blood vessel growth) and whether this expression was affected by EGF. METHODS: C57BL/6J mice were subjected to sham operation (bowel transaction with reanastomosis) or 50% proximal SBR. The remnant intestine was harvested, and the villus lamina propria was isolated by laser capture microdissection. The expression of CXCL5 messenger RNA (mRNA) was analyzed using real-time polymerase chain reaction (RT-PCR). Furthermore, CXCL5 mRNA levels were determined in EGF-stimulated human umbilical vein endothelial cells (HUVECs). RESULTS: A 2.39-fold increase (P < .05) in CXCL5 mRNA occurred in the lamina propria after SBR. In addition, villus height was found to be related directly to the degree of CXCL5 mRNA (R(2) = 0.97) expression. HUVECs treated with EGF demonstrated a 9-fold increase in CXCL5 mRNA expression. CONCLUSION: The villus growth observed in resection-induced adaptation is associated with increased expression of the chemokine CXCL5 within the lamina propria. Because EGF enhances CXCL5 expression directly in endothelial cells, EGFR-directed proangiogenic gene expression may be a critical mechanism for adaptive ileal villus growth.

Multipotential mesoangioblast stem cell therapy in the mdx/utrn-/- mouse model for Duchenne muscular dystrophy.

BACKGROUND: Duchenne muscular dystrophy is a progressive, lethal muscle-wasting disease for which there is no treatment. MATERIALS & METHODS: We have isolated wild-type mesoangioblasts from aorta and tested their effectiveness in alleviating severe muscle disease in the dystrophin/utrophin knockout (mdx/utrn-/-) mouse model for Duchenne muscular dystrophy. RESULTS: Mesoangioblast clones express Sca-1 and Flk-1 and differentiate into smooth and skeletal muscle, glial cells and adipocytes in vitro. Mesoangioblasts proliferate in vivo, incorporate into muscle fibers, form new fibers, and promote synthesis of dystrophin and utrophin. Muscle fibers that have incorporated mesoangioblasts, as well as surrounding fibers, are protected from damage, with approximately 50-fold less damage than fibers in muscle injected with saline. Some mesoangioblasts localize beneath the basal lamina and express c-met, whereas others differentiate into smooth muscle cells at the periphery of vessels and express alpha-smooth muscle actin. In mdx/utrn-/- muscle, some mesoangioblasts also form Schwann cells. DISCUSSION & CONCLUSION: Mesoangioblasts differentiate into multiple cell types damaged during the progression of severe muscle disease and protect fibers from damage. As such, they are good candidates for therapy of Duchenne muscular dystrophy and perhaps other neuromuscular diseases.

Isolation and characterization of mesoangioblasts from mouse, dog, and human tissues.

Mesoangioblasts are recently identified stem/progenitor cells, associated with small vessels of the mesoderm in mammals. Originally described in the mouse embryonic dorsal aorta, similar though not identical cells have been later identified and characterized from postnatal small vessels of skeletal muscle and heart (not described in this unit). They have in common the anatomical location, the expression of endothelial and/or pericyte markers, the ability to proliferate in culture, and the ability to undergo differentiation into various types of mesoderm cells upon proper culture conditions. Currently, the developmental origin of mesoangioblasts, their phenotypic heterogeneity, and the relationship with other mesoderm stem cells are not understood in detail and are the subject of active research. However, from a practical point of view, these cells have been successfully used in cell transplantation protocols that have yielded a significant rescue of structure and function in skeletal muscle of dystrophic mice and dogs. Since the corresponding human cells have been recently isolated and characterized, a clinical trial with these cells is planned in the near future. This unit provides detailed methods for isolation, culture, and characterization of mesoangioblasts.

Development of the chick embryo mesoblast: pronephros, lateral plate, and early vasculature.

The early development of the mesoblast in the intermediate and lateral regions of the chick embryo was examined with the scanning and transmission electron microscope. It was found that primary mesenchyme here becomes condenses into epithelial structures that emerge in a metameric pattern. Viewed in developmental sequence, the intermediate mesoblast condenses into a narrowing cord of axially oriented cells which divert medially at regular intervals into the intersegmental interfaces of somitomeres and somites. These cells give rise to the vascular channels of the posterior cardinal vein as well as to tubular elements of the pronephros. Intermediate mesenchyme cells become epithelial, forming zonular junctional complexes apically and depositing patchy basal lamina over their basal surfaces. The lateral plate mesenchyme organizes similarly into somatic and splanchnic epithelial sheets that utilize the body coelom as their lumenal surface. Cells of the lateral plate extend filopodia basally that interweave with adjacent cells, fibrillar extracellular matrix, as well as with interstitial bodies. The pattern in the lateral plate is subtly ribbed as bands of mesoblast undulate along the axis. The central region of each band is raised while ther are grooves created along lines of band abutment, corresponding to intersegmental clefts in the paraxial region and reflecting an underlying metameric pattern. These grooves are usually demarked medially by the protrusion of short segments of adjacent intermediate mesoblast. Most of the remaining primary mesenchyme develops into a non-metameric vascular epithelium, which forms a prominent anastamosing plexus between splanchnic mesoderm and endoderm. It is proposed that the emergence of primary mesenchyme into patterened epithelial anlage facilitates the distribution of neural crest cells introduced subsequently.

Ectodermal control of the avascular zone of the peripheral mesoderm in the chick embryo.

Prospective skin ectoderm is underlaid by a relatively thick (100 +/- 20 micrometer) avascular zone of mesoderm in most regions of the early embryo. To determine whether or not the ectoderm exercises a role in the establishment and maintenance of the avascular zone, trypsin-isolated pieces of backskin ectoderm from chick or quail embryos were implanted as a sheet into a slit cut deep into the capillary bed of the wing bud of host chick embryos of stages 19-23. In sham operations, slits were cut at various anteroposterior levels, and the wing was allowed to heal. At intervals of 3-48 hr after these operations, embryos were injected with India ink, fixed, and cleared. Implants formed flattened vesicles, usually in continuity with host ectoderm, but sometimes completely internalized. Periderm cells from each side of the vesicle faced each other, and the cells of the cuboidal layer faced an avascular mesodermal layer at least 100 micrometer thick at all points. The implantation of prospective skin ectoderm resulted in the formation of an avascular zone in normally vascularized mesoderm of the wing bud. In contrast, the vascular bed of the limb bud abutted directly on implants of Millipore filters or of Silastic silicone (Dow Corning). Likewise, the capillary bed came in direct contact with implants of retinal pigment epithelium, an ectodermal derivative normally in close contact with the vascular choroid coat of the eye. These results, taken in conjunction with earlier experiments that show the necessity of the apical ectodermal ridge for the formation of the marginal veins of the limb bud, suggest that epithelial-mesenchymal interactions are involved in important aspects of vasculogenesis in early embryos.