Ectodermal Epithelium Research Articles

Morphology and ultrastructure of a specialized bacterial pouch in the digestive tract of Tetraponera ants (Formicidae, Pseudomyrmecinae)

The digestive tract in workers of some species of the pseudomyrmecine ant genus Tetraponera is characterized by a conspicuous pear-shaped diverticulum at the transition between the midgut and the intestine, that so far has not been found in any other ant species. As this organ is filled with a mass of bacteria, we propose to designate it as a bacterial pouch. Its distal wall is formed by a thin ectodermal epithelium, through which tens of tracheal branches penetrate into the bacterial mass that fills the pouch lumen. The proximal wall, in contrast, is formed by a cylindrical epithelium with a conspicuous microvillar differentiation of the apical cell membrane, but without a cuticular lining. The contact region between both epithelia occurs as a complex fold surrounding the pouch like a belt. The Malpighian tubules open into the pouch through the cylindrical epithelium adjacent to the belt fold. The functional significance of the bacterial pouch remains unknown, although the abundant presence of bacteria may indicate a symbiotic function. The conspicuous tracheolar supply illustrates the metabolic activity in the pouch, while the microvillar differentiation of the cylindrical epithelium may be interpreted in the uptake of metabolites from the pouch lumen.

Inversion of left–right asymmetry in the formation of the adult rudiment in sea urchin larvae: removal of a part of embryos at the gastrula stage

At the late pluteus stage, sea urchin larvae form an adult rudiment left–right (LR) asymmetrically, on only the left side. Little is known about how the LR asymmetry of the adult rudiment is established in earlier stages during which the larval body is basically LR symmetric. To investigate how the different regions of the embryo function to establish LR asymmetry, we removed different regions at the gastrula stage and assessed the effects of the operation on the establishment of LR asymmetry of the adult rudiment.Surgery was performed on mid- to late-gastrula embryos of two indirect developing species: Scaphechinus mirabilis and Hemicentrotus pulcherrimus. The embryos were placed under a dissecting microscope (SMZ8, Leica) and ectodermal epithelium together with underlying mesenchyme cells was dissected out with a glass needle. The region along the midline of the embryo at the width of the archenteron was designated the ‘midline region’, and the region lateral to the ‘midline region’ was designated the ‘lateral region’. When the left and/or the right side was excised, the whole lateral region was precisely removed on the animal side, but the plane of incision deviated more laterally on the vegetal side to avoid the ventro-lateral cluster of primary mesenchyme cells, so that a part of the defined ‘lateral region’ was left on the vegetal side. The vertical excision was made by cutting mid-gastrula embryos vertically to the archenteron at a level just superior to the tip of the archenteron. In the sham operation, embryos were pressed with the needle as in the vertical excision, but the incision was stopped slightly before the animal and vegetal halves of the embryos were completely divided from each other. The operated embryos were examined through an optical microscope (Optiphoto, Nikon) to confirm that the excision was correct, and they were then cultured with a food supply (diatoms, Chaetoceros gracilis). The handedness of the adult rudiment was examined at the six-armed or eight-armed pluteus stage through an optical microscope (Table 1).

Developmental characterization of the gene for laminin α-chain in sea urchin embryos

We describe the isolation and characterization of a cDNA clone encoding a region of the carboxy terminal globular domain (G domain) of the α-1 chain of laminin from the sea urchin, Strongylocentrotus purpuratus. Sequence analysis indicates that the 1.3 kb cDNA (spLAM- α) encodes the complete G2 and G3 subdomains of sea urchin a-laminin. The 11 kb spLAM- α mRNA is present in the egg and declines slightly in abundance during development to the pluteus larva. The spLAM- α gene is also expressed in a variety of adult tissues. Whole mount in situ hybridization of gastrula stage embryos indicates that ectodermal and endodermal epithelia and mesenchyme cells contain the spLAM- α mRNA. Immunoprecipitation experiments using an antibody made to a recombinant fusion protein indicates spLAM- α protein is synthesized continuously from fertilization as a 420 kDa protein which accumulates from low levels in the egg to elevated levels in the pluteus larva. Light and electron microscopy identify spLAM- α as a component of the basal lamina. Blastocoelic microinjection of an antibody to recombinant spLAM- α perturbs gastrulation and skeleton formation by primary mesenchyme cells suggesting an important role for laminin in endodermal and mesodermal morphogenesis.

A role for neural cell adhesion molecule in the formation of the avian inner ear

The inner ear forms by a series of folds within an ectodermal placode. Previous work has shown that changes in surrounding tissues play a more prominent role in invagination than changes in the cytoskeleton of the primordium. Interference with the integrity of the extracellular matrix causes abnormalities in the folding process, primarily related to abnormalities in the paraxial mesoderm which lies ventral to the placode. In this study, the role of the neural cell adhesion molecule (N-CAM) was investigated, based on the expression of this component of the plasmalemma at the time the otic placode begins to fold. Microinjection of blocking antibodies to N-CAM into the paraxial mesoderm adjacent to the otic placode resulted in two major classes of defects, detachment of the primordium from the neural tube and interference with formation of the folds. Microinjection of saline, control immunoglobulin, or antibody against cytoplasmic domain had no effect. These defects correlate with the pattern of N-CAM expression at the time of injection, along the neural ectoderm and otic epithelium and the mesenchyme cells ventral to the primordium. It seems likely that N-CAM is playing a role in heterophilic associations rather than through the homophilic binding domain during formation of the otic vesicle. Dev. Dyn.1998;213:359–369. © 1998 Wiley-Liss, Inc.

A role for neural cell adhesion molecule in the formation of the avian inner ear.

The inner ear forms by a series of folds within an ectodermal placode. Previous work has shown that changes in surrounding tissues play a more prominent role in invagination than changes in the cytoskeleton of the primordium. Interference with the integrity of the extracellular matrix causes abnormalities in the folding process, primarily related to abnormalities in the paraxial mesoderm which lies ventral to the placode. In this study, the role of the neural cell adhesion molecule (N-CAM) was investigated, based on the expression of this component of the plasmalemma at the time the otic placode begins to fold. Microinjection of blocking antibodies to N-CAM into the paraxial mesoderm adjacent to the otic placode resulted in two major classes of defects, detachment of the primordium from the neural tube and interference with formation of the folds. Microinjection of saline, control immunoglobulin, or antibody against cytoplasmic domain had no effect. These defects correlate with the pattern of N-CAM expression at the time of injection, along the neural ectoderm and otic epithelium and the mesenchyme cells ventral to the primordium. It seems likely that N-CAM is playing a role in heterophilic associations rather than through the homophilic binding domain during formation of the otic vesicle.

FREAC-1 contains a cell-type-specific transcriptional activation domain and is expressed in epithelial-mesenchymal interfaces.

The forkhead transcription factor FREAC-1 is a potent transcriptional activator. We have localized a transcriptional activation domain in the C-terminus of FREAC-1 and another one to a stretch of approximately 60 amino acids in the central part of the protein. While the C-terminal activation domain activates in all cell lines tested, the activation domain in the central part of the protein is functional only in cell lines derived from lung. This cell-type-specific activity is retained when the activation domain is fused to the heterologous DNA binding domain of Gal4. The human FREAC-1 gene was found to consist of two exons separated by an intron of 1.2 kb. Exon 1 encodes the forkhead DNA binding domain and the cell-type-specific activation domain. Exon 2 encodes the general activation domain. The distribution of FREAC-1 expression during embryogenesis was investigated by in situ hybridization. FREAC-1 mRNA was found in mesenchyme in immediate proximity to endodermal epithelia throughout the digestive, urinary, and respiratory tracts. Mesenchyme surrounding the notochord and adjacent to the ectodermal epithelia of the oral cavity and developing teeth also expresses FREAC-1. The pattern of FREAC-1 expression, with highest levels in the mesenchyme next to the epithelium and gradually diminishing as the distance from the epithelium increases, suggests that FREAC-1 expression is a response to epithelial paracrine signaling and that FREAC-1 may play a role in epitheliomesenchymal interactions.

Faint sausage encodes a novel extracellular protein of the immunoglobulin superfamily required for cell migration and the establishment of normal axonal pathways in the Drosophila nervous system.

We examined the structure of the nervous system in Drosophila embryos homozygous for a null mutation in the faint sausage (fas) gene. In the peripheral nervous system (PNS) of fas mutants, neurons fail to delaminate from the ectodermal epithelium; in the central nervous system (CNS), the positions of neuronal cell bodies and glial cells are abnormal and normal axonal pathways do not form. Sequence analysis of fas cDNAs revealed that the fas protein product has characteristics of an extracellular protein and that it is a novel member of the immunoglobulin (Ig) superfamily. In situ hybridization demonstrated that fas transcripts are expressed throughout the embryo but they are in relatively high concentrations in the lateral ectoderm, from which the peripheral nervous system delaminates and in the CNS. Antiserum directed against Fas protein was found to stain neurons but not glia in the CNS. We conclude that fas encodes a protein that, in the developing nervous system, is present on the surface of neurons and is essential for nerve cell migration and the establishment of axonal pathways.

The role of F-cadherin in localizing cells during neural tube formation in Xenopus embryos.

The cell adhesion molecule F-cadherin is expressed in Xenopus embryos at boundaries that subdivide the neural tube into different regions, including one, the sulcus limitans, which partitions the caudal neural tube into a dorsal and ventral half (alar and basal plate, respectively). Here we examine the role of F-cadherin in positioning cells along the caudal neuraxis during neurulation. First, we show that ectopic expression of F-cadherin restricts passive cell mixing within the ectodermal epithelium. Second, we show that F-cadherin is first expressed at the sulcus limitans prior to the extensive cell movements that accompany neural tube formation, suggesting that it might serve to position cells at the sulcus limitans by counteracting their tendency to disperse during neurulation. We test this idea using an assay that measures changes in cell movements during neurulation in response to differential cell adhesion. Using this assay, we show that cells expressing F-cadherin localize preferentially to the sulcus limitans, but still disperse when located away from the sulcus limitans. In addition, inhibiting cadherin function prevents cells from localizing precisely at the sulcus limitans. These results indicate that positioning of cells at the sulcus limitans is mediated in part by the differential expression of F-cadherin.

Type XII collagen up-regulation is associated with regeneration of fully developed but not embryonic limbs of the salamander and newt

Monoclonal antibodies (mAbs) MT1 and MT2 provided the opportunity to compare developing and regenerating limbs with regard to the temporal and spatial appearance of the developmentally regulated, extracellular matrix (ECM) proteins tenascin (TN) and type XII collagen (col XII), respectively. In limb buds and in regenerates of both limb buds and mature limbs, mAb MT1 reactivity was seen within the ECM between mesenchyme cells and as a layer subjacent to the ectoderm and wound epithelium. In contrast, reactivity of mAb MT2 was absent from limb buds or regenerating limb buds, but was strong in the distal limb stump and blastema of regenerating mature limbs. At differentiation stages of limb development, TN and col XII were seen in association with perichondrium, myotendinous junctions, and tendons, typical of their distributions in mature limbs. In situ hybridization with a probe that recognizes transcripts of both the long form and the alternatively spliced short form of the alpha chain of type XII collagen (alpha col XII) confirmed that mAb MT2 reactivity represented alpha col XII expression and further showed that neither form of alpha col XII mRNA is expressed in early limb buds of chicken or urodele embryos. Thus, when the expression patterns of antigens are compared during limb development and regeneration, both similarities and differences are revealed. The results are consistent with the view that col XII has a role in the regeneration of mature limbs but not in the development or regeneration of embryonic limbs. J. Exp. Zool. 279:71-80, 1997. © 1997 Wiley-Liss, Inc.

Cadherin-mediated cell adhesion and cell motility in Drosophila trachea regulated by the transcription factor Escargot.

Coordination of cell motility and adhesion is essential for concerted movement of tissues during animal morphogenesis. The Drosophila tracheal network is formed by branching, migration and fusion of tubular ectodermal epithelia. Tracheal tip cells, located at the end of each branch that is going to fuse, extend filopodia to search for targets and later change their cell shape to a seamless ring to allow passage of lumen. The cell adhesion molecule DE-cadherin accumulates at the site of contact to form a ring that marks the site of lumen entry and is essential for the fusion. DE-cadherin expression in tip cells of a subset of branches is dependent on escargot, a zinc finger gene expressed in all tip cells. Such escargot mutant tip cells failed to adhere to each other and continued to search for alternative targets by extending long filopodia. We present evidence indicating escargot positively regulates transcription of the DE-cadherin gene, shotgun. Overexpression of DE-cadherin rescued the defect in one of the fusion points in escargot mutants, demonstrating an essential role of DE-cadherin in target recognition and identifying escargot as a key regulator of cell adhesion and motility in tracheal morphogenesis.

'Membranous Aplasia Cutis' With Hair Collars

The skin and the nervous system are both derived from ectoderm. Separation of neural ectoderm from epithelial ectoderm occurs concurrently with the closure of the neural tube. This chronologic association may explain the cutaneous abnormalities often found overlying neural tube defects. A ring of dark long hair encircling a congenital scalp lesion (the hair collar sign) is one such marker and is often associated with encephaloceles, meningoceles, and heterotopic brain tissue. We describe six children with aplasia cutis who displayed the hair collar sign. Aplasia cutis is a relatively heterogeneous disorder; however, these lesions had a unique and strikingly similar appearance. This subtype of aplasia cutis, which we have termed membranous aplasia cutis, shares several clinical and histologic features with cranial neural tube defects. We propose that membranous aplasia cutis is a form fruste of a neural tube defect and may be derived from a similar embryological defect. Recent advances in the understanding of cranial neural tube closure may provide support for this hypothesis.

Expression pattern of osteogenic protein-1 (bone morphogenetic protein-7) in human and mouse development.

Osteogenic protein-1 (OP-1; BMP-7) is a member of the bone morphogenetic protein subfamily. Because members of the TGF-beta superfamily have a role in tissue development, the distribution of OP-1 expression in developing human embryos (5-8 gestational weeks) and fetuses (8-14 gestational weeks) and mouse (9.5-17.5 gestational days) fetuses was examined. Northern hybridization with specific OP-1 probes revealed two mRNA species of 4 and 2.2 KB. Highest levels of OP-1 mRNA were found in human fetal kidney and heart between 12-14 weeks of gestation. By in situ hybridization, the OP-1 transcripts were found in various tissues, i.e., the ectodermal epithelium of the mouse fore- and hindlimbs, heart, teeth, intestinal epithelium, perichondrium, hypertrophic chondrocytes, and periosteum/osteoblast layer of developing human bones. In kidneys, transcripts were first detected in the epithelium of the branching uretheric buds, whereas at later stages glomeruli were the major site of OP-1 mRNA accumulation. These data suggest that, although OP-1 has been isolated from bone matrix, it may have additional regulatory roles in the morphogenesis and/or function of the kidney, limb bud, tooth, heart, and intestine.

Expression of crumbs confers apical character on plasma membrane domains of ectodermal epithelia of drosophila

The crumbs protein of Drosophila is an integral membrane protein, with 30 EGF-like and 4 laminin A G domain-like repeats in its extracellular segment, which is expressed on the apical plasma membrane of all ectodermally derived epithelia. Here, we present evidence to show that the insertion of crumbs into the plasma membrane is necessary and sufficient to confer apical character on a membrane domain. Over-expression of crumbs results in an enormous expansion of the apical plasma membrane and the concomitant reduction of the basolateral domain. This is followed by the redistribution of β Heavy-spectrin, a component of the membrane cytoskeleton, and by the ectopic deposition of cuticle and other apical components into these areas. Strikingly, overexpression of the membrane-bound cytoplasmic portion of crumbs alone is sufficient to produce this dominant phenotype. Our results suggest that crumbs plays a key role in specifying the apical plasma membrane domain of ectodermal epithelial cells of Drosophila.

Scanning electron microscopy of the muscle system of Hydra magnipapillata

The fine structure of the ectodermal and endodermal muscle layers of Hydra magnipapillata has been analyzed by scanning electron microscopy after hydrolytic removal of the mesoglea with NaOH and subsequent exposure of the basal and lateral aspects of the layers by mechanical dissection. The ectodermal muscle layer consists of fibrous processes of epithelial cells extending longitudinally to the body axis, whereas the endodermal muscle layer comprises cells with hexagonal bases and several strands of myonemes oriented circularly. In each layer, the muscular elements tightly interdigitate, extending a continuous muscle sheet along the mesoglea. The ectodermal and endodermal muscle sheets communicate with each other via foliate microprojections penetrating the mesoglea. On the lateral aspect of the ectodermal epithelium, spiny nerve fibers run along the upper surface of the muscle processes. The spines are often attached to muscle processes, suggesting that the former monitor muscle contraction. Nerve fibers occasionally come into contact with the mesoglea through narrow gaps between the muscle processes. In the hypostomal ectoderm, a small spindle-shaped cell, probably sensory in nature, extends an apical cilium and a long basal process.

Domains of differential cell proliferation and formation of amnion folds in chick embryo ectoderm.

Patterns of cell proliferation in ectoderm epithelium that will form avian amnion correlate with morphogenesis, but not in an obvious pattern with respect to large-scale folding. At sites where the pre-axial amnion folds will first appear in 4- to 8-somite embryos, patterns of proliferation do not separate into domains that presage location of the single pre-axial fold that is commonly described in embryology texts. Instead, increased cell proliferation occurs in a significant, bilateral pattern. In stages with 13 to 27 somites, when lateral amnionic folds are prominent, five paraxial domains of cell proliferation correlate with morphology and show decreasing levels of cell proliferation with distance from the neural axis. Slowly growing areas surround rapidly growing areas and could assist buckling of epithelium by providing constraints on expansion of faster growing areas. Proliferation domains in ectoderm correlate with morphology and morphological events when localized changes in cell shape are lacking and suggest a role for differential cell proliferation in formation of large-scale epithelial folds in early chick embryos.

The Development of Cellular Junctions in the Drosophila Embryo

The pattern and development of cellular junctions in the different tissues of the Drosophila embryo from the blastoderm stage until hatching were analyzed. The cellular junctions found include: gap junctions, two types of septate junctions, and several types of cell-cell and cell-substrate adherens junctions. During early and mid embryogenesis (stages 4 to 13) only spot adherens junctions, gap junctions, and zonulae adherentes prevail. Scattered spot adherens junctions are already formed at the blastoderm stage. During and shortly after gastrulation, spot adherens junctions become concentrated at the apical pole and fuse into continuous zonulae adherentes in the posterior endoderm and the ectoderm. In addition to the zonulae adherentes, ectodermally derived epithelia possess scattered gap junctions and form pleated septate junctions and hemiadherens junctions during late embryogenesis (stages 14 to 17). Mesenchymal tissues (i.e., all nonepithelial tissues of the embryo, including the neural primordium and, transiently, the mesoderm and endoderm) possess both spot adherens junctions and gap junctions at a low frequency. Initially, the midgut epithelium does not establish a junctional complex and possess only gap junctions and spot adherens junctions. Only late in development does a circumferential smooth septate junction develop; zonulae adherentes are missing. The various derivatives of the mesoderm express spot adherens junctions, hemiadherens junctions, and gap junctions, but never zonulae adherentes or septate junctions. After organogenesis, several different types of tissue-specific adherens junctions are formed, among them connecting hemiadherens junctions (between gut epithelium and visceral muscle and early during the formation of the muscle tendon junction), muscle tendon junctions (between somatic muscle and tendon cells), fasciae adherentes (between the cells of both the visceral muscle and the dorsal vessel), and autocellular nephrocyte junctions (in nephrocytes). Interesting exceptions to the general pattern of junctional development are provided by the outer epithelial layer of the proventriculus and the Malpighian tubules. Both tissues develop as typical ectodermal epithelia and possess zonulae adherentes. During late embryogenesis, both epithelia lose the zonulae adherentes and form smooth rather then pleated septate junctions, thereby expressing a junctional complex similar to that of the endodermally derived midgut epithelium.

A Novel Substance Localizing on the Apical Surface of the Ectodermal and the Esophageal Epithelia of Sea Urchin Embryos: (egg-surface substance/apical location/larval epithelium/metamorphosis/sea urchin).

A new substance (ES-1) which localizes on the ectodermal and espophageal epithelia of sea urchin embryos was identified by a monoclonal antibody, McA ES-1. McA ES-1 recognized a 175 KDa protein of fertilized and 200 KDa in proteins of unfertilized egg-cortices. By indirect fluorescent antibody staining, ES-1 was found on the plasma membrane of fertilized eggs and in the cortical region of unfertilized eggs. ES-1 was not contained in the cortical granules and remained fixed in the cortex after centrifugation of unfertilized eggs for 30 min at 20,000 g. The polarized localization of ES-1 on the apical surface of ectodermal epithelial cells continued to the metamorphosis. It disappeared from mesenchyme cells and other migrating cells of the gastrula, while ES-1 was reexpressed in the presumptive esophagus to be connected with ectodermal epithelium. This may suggest a functional significance of ES-1 in establishment of cell polarity in the epithelium of larvae. In metamorphosing larvae and adults, the apical localization of ES-1 could no longer be found, and it was found in coelomocytes. From these findings, it is concluded that ES-1 was a novel surface substance of embryos and is probably phagocytosed at metamorphosis.

A Chicken Homeobox Gene Related to Drosophila paired Is Predominantly Expressed in the Developing Limb

We identified a homeobox-containing gone, Prx-1, isolated from the chick limb bud cDNA library. The homeodomain sequence is related to Drosophila paired and gooseberry and mouse Pax-3, Pax-6, and Pax-7. The deduced amino acid sequence of the Prx-1 gene product reveals the absence of a paired-box sequence and extensive similarity to mouse S8 and MHox homeodomain proteins, thus constituting a new class of homeobox gene. Using an in situ hybridization method, the Prx-1 gene is shown to be expressed predominantly in the limb bud and visceral arches. At early stages of limb development, distal mesodermal cells express the homeobox gene with an apparent gradient along the proximal-distal axis. The signal is absent in the apical and nonridge ectoderm. Removal of the apical ectodermal ridge had no apparent effect on the subsequent expression of Prx-1 in the limb mesenchyme. The Prx-1-expressing cells are later confined to the interdigital and perichondrial regions. The Prx-1 transcripts are also detectable in the mesenchyme of the visceral arches and facial primordia subjacent to the ectoderm. The Prx-1 gene is weakly expressed in somites and condensing vertebrae. No signal is detectable in neural tube and ectodermal epithelium. These results suggest that the Prx-1 homeodomain protein is involved in the differentiation of bone, muscle, and other tissues of mesodermal origin during limb development.

Congenital Nonpigmented Epithelial Iris Cyst after Amniocentesis: Clinicopathologic Report on Two Children

Congenital nonpigmented epithelial iris cysts are not common. They may arise spontaneously from developmental entrapment of surface ectodermal epithelium or from occult ocular trauma prenatally or at birth. Between 1989 and 1991, an 8-month-old child and a 6-year-old child presented with large, progressive congenital epithelial iris cysts. Both children had a maternal history of diagnostic amniocentesis after an ultrasound scan, and there was no history of postnatal ocular trauma. The cysts were successfully removed by a modified block excision and tectonic corneoscleral grafting. A dense adherence of the cyst wall to Descemet's membrane resembled old anterior synechiae after occult perforation of the globe in both patients. On histopathologic examination, the epithelial lining of the cysts consisted of non-keratinizing stratified squamous epithelium with goblet cells resembling conjunctival epithelium. A perforating limbal scar with a corresponding break in Descemet's membrane could be detected in one eye. The long-term visual acuity of both children was encouraging, and there was no evidence of recurrence of the iris cyst during the follow-up period (average, 23 months). The authors conclude that the clinical and histopathologic features of these congenital iris cysts may be consistent with an occult intrauterine limbal perforation of the anterior chamber with a needle during amniocentesis. Amniocentesis, when not guided by a real-time ultrasound scan, may be a risk factor for prenatal ocular trauma, which should be considered in the differential diagnosis of congenital ocular disorders.

The effects of growth factors on the day 13 chorioallantoic membrane (CAM): a study of VEGF165 and PDGF-BB.

The in vivo effects of two growth factors, VEGF165 and PDGF-BB, were studied in the chick chorioallantoic membrane (CAM). The factors were air-dried on Thermanox discs and the inverted discs were placed on the day-13 CAM for a period of 3 days. The specimens were then fixed, examined under a stereomicroscope and processed for semi- and ultrathin sectioning. VEGF165 induces marked vascular growth. Many new blood vessels emerge from the precapillary arterioles, and a brush-like formation of vessels can be seen in this region. In the venous part of the vascular system, the formation of sinusoidal or lacunar vessels can be seen. Edema does not develop. PDGF-BB induces thickening of the CAM due to extracellular-matrix production and the proliferation or immigration of fibrocytes. These lie just beneath the ectodermal epithelium and are oriented parallel to it. Out of the four factors we have already studied (PDGF-BB, VEGF165, Angiogenin, bFGF), only VEGF165 specifically induces the growth of blood vessels.