Apical ectodermal ridge

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Apical ectodermal ridge morphogenesis in limb development is controlled by Arid3b mediated regulation of cell movements

Apical ectodermal ridge morphogenesis in limb development is controlled by Arid3b mediated regulation of cell movements

The apical ectodermal ridge (AER) is a specialized epithelium located at the distal edge of the limb bud that directs outgrowth along the proximodistal axis. Although the molecular basis for its function is well known, the cellular mechanisms that lead to its maturation are not fully understood. Here, we show that Arid3b , a member of the ARID family of transcriptional regulators, is expressed in the AER in mouse and chick embryos, and that interference with its activity leads to aberrant AER development, in which normal structure is not achieved. This happens without alterations in cell numbers or gene expression in main signalling pathways. Cells that are defective in Arid3b show an abnormal distribution of the actin cytoskeleton and decreased motility in vitro. Moreover, movements of pre-AER cells and their contribution to the AER were defective in vivo in embryos with reduced Arid3b function. Our results show that Arid3b is involved in the regulation of cell motility and rearrangements that lead to AER maturation.

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The apical ectodermal ridge is a timer for generating distal limb progenitors

The apical ectodermal ridge is a timer for generating distal limb progenitors

The apical ectodermal ridge (AER) is an essential signaling center governing vertebrate limb development (Capdevila and Izpisua Belmonte, 2001; Martin, 1998; Niswander, 2003). The importance of the AER was demonstrated by classic experiments in chicken embryos showing that AER removal at progressively later stages of limb development causes a progressive loss of distal elements of the limb (Saunders, 1948; Summerbell, 1974). Although different models have been proposed to explain limb skeletal patterning along the proximal-distal (PD) axis, AER function in this process has remained largely unclear. For example, the Progress Zone (PZ) model postulates that the AER provides permissive signals to keep PZ cells labile until they exit the zone, at which time these cells are autonomously specified by ceasing to acquire ‘positional information’ (Summerbell et al., 1973). By contrast, the Early Specification (ES) model proposes that PD elements are specified, rather than progressive, at the earliest stages of limb bud development and that the AER regulates subsequent expansion of progenitor pools by promoting cell proliferation and survival (Dudley et al., 2002). Furthermore, recent models suggest that PD elements are specified via dynamic interactions between the flank of the lateral plate mesoderm and the AER (Mercader et al., 2000; Tabin and Wolpert, 2007).

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The Apical Ectodermal Ridge: morphological aspects and signaling pathways

The Apical Ectodermal Ridge: morphological aspects and signaling pathways

In the chick, the use of cell-fate tracers has permitted the identification of two distinct ectodermal compartments, dorsal and ventral, in the presumptive limb ectoderm with the DV boundary coincident with the position of the AER (Altabef et al., 1997). In the mouse, very thorough and elegant studies carried out by the group led by Alex Joyner revealed that AER formation is coordi- nated by two lineage boundaries, the dorsal and the middle boundaries (Kimmel et al., 2000). The dorsal border is located along the dorsal margin of the pre-AER domain and the middle border along its middle DV extension, within the AER itself. The middle border was also identified in chick in experiments using quail/chick grafts (Michaud et al., 1997). Kimmel et al. (Kimmel et al., 2000) proposed a model for AER formation in which all the AER precursors cells are pulled towards the dorsal margin do- main (the zip model) and, in addition, bidirectional pulling toward the middle border generates the elevation of the AER. Impor- tantly, AER morphogenesis and gene expression depend on cell- cell interactions at both borders, which are regulated, at least in part, by Wnt7a and En1. Interestingly, the middle boundary is transient and its disappearance has been proposed to contribute to the regression of the AER morphology (Kimmel et al., 2000).

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Mechanism of pectoral fin outgrowth in zebrafish development

Mechanism of pectoral fin outgrowth in zebrafish development

Fins and limbs, which are considered to be homologous paired vertebrate appendages, have obvious morphological differences that arise during development. One major difference in their development is that the AER (apical ectodermal ridge), which organizes fin/limb development, transitions into a different, elongated organizing structure in the fin bud, the AF (apical fold). Although the role of AER in limb development has been clarified in many studies, little is known about the role of AF in fin development. Here, we investigated AF-driven morphogenesis in the pectoral fin of zebrafish. After the AER-AF transition at ~36 hours post-fertilization, the AF was identifiable distal to the circumferential blood vessel of the fin bud. Moreover, the AF was divisible into two regions: the proximal AF (pAF) and the distal AF (dAF). Removing the AF caused the AER and a new AF to re-form. Interestingly, repeatedly removing the AF led to excessive elongation of the fin mesenchyme, suggesting that prolonged exposure to AER signals results in elongation of mesenchyme region for endoskeleton. Removal of the dAF affected outgrowth of the pAF region, suggesting that dAF signals act on the pAF. We also found that the elongation of the AF was caused by morphological changes in ectodermal cells. Our results suggest that the timing of the AER-AF transition mediates the differences between fins and limbs, and that the acquisition of a mechanism to maintain the AER was a crucial evolutionary step in the development of tetrapod limbs.

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Genetic review of ectodermal dysplasia

Genetic review of ectodermal dysplasia

EDs are divided into two groups: According to Freire- Maia’s classification: Group A which consists of all the entities with defects in two or more of the standard structures Group B comprises of those with disturbances in only one of these structures plus another ectodermal defect. 11 subgroups have been included in Group A depending on the involved structures: 1-2-3-4 (hair-teeth-nails-sweat glands); 1-2-3 (hair- teeth-nails); 1-2-4 (hair-teeth-sweat glands); 1-3-4(hair-nails- sweat glands); 2-3-4 (teeth-nails-sweat glands); 1-2 (hair- teeth); 1-3 (hair-nails); 1-4 (hair-sweat glands); 2-3 (teeth nails); 2-4 (teeth-sweat glands); 3-4 (nails-sweat glands). Similarly, Group B is classified into four subgroups with number 5 added at the end, depicting that another ectodermal anomaly is present: 1-5, 2-5, 3-5, and 4-5. Other ectodermally derived structures like mammary glands, thyroid gland, thymus, anterior pituitary, adrenal medulla, central nervous system, external ear, melanocytes, cornea, conjunctiva, lacrimal gland and lacrimal duct that may also be involved in EDs (Chokshi et al., 2015).

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Morphological and functional classification of ion absorbing mitochondria rich cells in the gills of Mozambique tilapia

Morphological and functional classification of ion absorbing mitochondria rich cells in the gills of Mozambique tilapia

MR cells in the gills were in contact with the external environment through their apical membrane, which was located at the boundary of pavement cells with ridge structures on their surface. The external structure of the apical membrane of MR cells varied greatly among the four experimental groups (Fig. 1). The apical structures of MR cells found in the gill filaments were classified into the following three types: (1) a small apical pit; (2) a concave apical surface; and (3) a convex apical surface. Small apical pits were narrow and deep, so that little or no internal structure could be observed. Concave apical surfaces were slightly dented, or sometimes flat, with a mesh-like structure on their surface. Convex apical surfaces were equipped with microvilli, presenting a convex rough surface. Both concave and convex apical surfaces varied greatly in size (Fig. 1). Concave and convex apical surfaces predominantly developed in the gills of tilapia acclimated to LowNa and LowCl, respectively, whereas small apical pits predominated in Control fish (Fig. 1A–C). Both concave and convex surfaces were frequently observed in the LowNa/LowCl group (Fig. 1D). However, convex apical surfaces were more enlarged in LowCl than in LowNa/LowCl groups (Fig. 1B,D). The quantitative analysis showed that the frequency of the three types of MR cells differed greatly among the experimental groups (Fig. 2). The density of small apical pits was highest in the Control and lowest in LowNa/LowCl. The concave apical surfaces were more frequently observed in fish in media with low Na + (LowNa and LowNa/LowCl) than in those with

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Prosthodontic rehabilitation of an ectodermal dysplasia patient

Prosthodontic rehabilitation of an ectodermal dysplasia patient

Ectodermal Dysplasias comprise a large, heterogenous group of inherted disorders that are defined by primary defects in the development of two or more tissues derived from embryonic ectoderm. A multidisciplinary approach to dental treatment is required. This clinical report attempts to describe the prosthodontic management of a 13 year old girl affected by ectodermal dysplasia. Treatment included overdenture and a mandibular removable partial denture to improve

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Quantitative analysis of digitopalmar dermatoglyphics in thirty female cerebral palsy children

Quantitative analysis of digitopalmar dermatoglyphics in thirty female cerebral palsy children

By the one of genetic method, quantitative analysis of digitopalmar dermatoglyphics, we have made have made research in thirty female cerebral palsy children, in the prevention have found statistically significant differences to significance the sense of increased number finger (FRD5), on all five fin finger (TFRCD), between triradii c-d (c-d rcD), triradii ((TFRCD), d, all together (TPR rcD), and in Atd angle increased in seven variables on the left hand and fingers: fingersal gether (TPRCL), between triradii b-c (b-c rcL) rcL), b-c (b-c rcL) and c-d (c-d rcL) all together (TPR cL) and Atd angle in degrees (AtdL) And on the end, in threeon both han And on in three on both hands an fingers: ten fingers all together (TFRC), be- tween triradii a-b (b drcL) all together (TPRC) and Atd an- gles in degree both palm (ATDDL).The obtained data indicate a hypothetical genetic impact that simultaneously damaged uite possible due to common ectodermal

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Hypohidrotic ectodermal dysplasia – a case series

Hypohidrotic ectodermal dysplasia – a case series

hidrotic, where sweat glands are normal and the condition is inherited as autosomal dominant (Clouston’s syndrome).1,5 The clinical manifestation of ectodermal dysplasia syndrome includes anomalies in the dentition and hair being affected similarly in both types of ED, but the hereditary patterns and nail and sweat gland manifestations tend to differ.9 Christ- Siemens-Touraine syndrome, with X-linked recessive inheritance, is the most common and frequently reported manifestation of ectodermal dysplasia.2,7,9based on the severity of clinical manifestations, Christ-Siemens-Touraine syndrome can further be classified into hypohidrotic or anhidrotic ectodermal dysplasia.8 Oral traits of ectodermal dysplasia (ED) may be expressed as anodontia or hypodontia, with or without a cleft lip and palate. Anodontia also manifests itself by a lack of alveolar ridge development; 7 as a result, the vertical dimension of the lower third of the face is reduced, the vermilion border disappears,existing teeth are malformed, the oral mucosa becomes dry, and the lips become prominent. The face of an affected child usually has the appearance of old age which is mainly due to the wasting of the jaw muscles and hypoplasia of the jaws and also loss of mechanical stress due to anodontia or hypodontia.7, 9 Gene studies regarding the etiology of ED reveal that the mutations in the ectodysplasin-A and ectodysplasin-A receptor genes are responsible for X- linked and autosomal hypohidrotic ectodermal dysplasia.The diagnosis is based on clinical ,structural and biochemical characteristics of hair, skin biopsy, and characteristics of Article History:

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Study of Ridge Based and Image Based Approach for Fingerprint Gender Classification

Study of Ridge Based and Image Based Approach for Fingerprint Gender Classification

In ridge count based gender classification males have a slightly higher ridge count than the females. To calculate the ridge count of any fingerprint find delta, place a point at farthest corner of it and place dot at core of fingerprint and join a line to count the number of ridges. Number of ridges between delta and core is the ridge count of that fingerprint. For an arch pattern, the ridge count is zero. For a whorl pattern a ridge count is made from each delta to the centre of the fingerprint, and only the higher of the two possible counts is used (Figure 3).

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Ridge expansion in deficient alveolar ridge with immediate implant  placement: a case report

Ridge expansion in deficient alveolar ridge with immediate implant placement: a case report

The technique of ridge split or ridge expansion was introduced in early 1970s by Hilt Tatum for horizontal ridge augmentation while maintaining the periosteal attachment by carefully expanding the cortical plates to improve atrophic alveolar ridge for implant placement. This technique has an added advantage of augmentation and implant placement in a single sitting. Ridge splitting techniques are useful for managing narrow edentulous ridge (>3.5 mm) for implant placement with a predictable outcome (Summers, 1995). Tantum inserted >5000 maxillary anterior implants using ridge splitting before 1985 wherein, he expanded atrophic ridges >3 mm for simultaneous implant placement and augmentation keeping the periosteum intact. Later, Summers and Scipioni et al. in 1994 revived and published articles on edentulous ridge expansion with 98.8% implant survival rate for over 5 years (Scipioni et al., 1997). The ridge deficiencies can be horizontal, vertical or combination of both as described by Siberts classes A, B and C, respectively. Dental implant devices are placed into edentulous ridges where an appropriate bone width is available to support removable or fixed type dental restorations. A minimum of 1.0 – 1.5 mm of bone width/thickness on the facial and lingual aspects of the implants is necessary making an average of 6 mm buccal/lingual ridge thickness necessary for a commonly desired 4 mm diameter implant. This creates a major challenge in implant dentistry since alveolar atrophy always occurs subsequent to tooth extraction which limits the use of endosseous implants to restore oral function. With the emergence of implant dentistry and introduction of microsaws, piezosaws, and specific ridge split osteotomes this technique has become an integral part of implant dentistry, wherein primarily bone expansion techniques were indicated in regions of division B bone volume and density of D3 or D4. Bone due to its dynamic viscoelastic nature, thinner ridges (<3.5 mm) can be expanded with better controlled instrumentation with less risk of fracture, trauma and bone perforations. The softer the trabacular bone quality, the lower the elastic modulus and greater the viscoelastic nature of the ridge. Therefore, less dense the bone, the easier and more predictable is the bone expansion (Misch, 1999).

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Analysis of scanning electron microscopic examination of the apical zone of teeth with chronic apical periodontitis - an ex vivo study

Analysis of scanning electron microscopic examination of the apical zone of teeth with chronic apical periodontitis - an ex vivo study

Apical resorption is a biological phenomenon, characterized by processes of cement and/or dentine depletion, resulting from the physiological or pathological activity of resorptive cells, called dentoclasts (a subclass of the osteoclasts) [7,9, 12]. A periapical lesion with a visible radiolucency is generally accompanied by some degree of root resorption. Several authors have verified that root resorption is commonly present when pathologic tissue surrounds the tooth apex [11, 15, 18]. Studies have suggested that the permanent dentition is protected against physiological resorptive processes, but pathological resorption has been found in cases of trauma, orthodontic treatment, expansion of tumor or cystic formations, or has been largely the result of inflammatory processes in the pulp tissue, etc. [14]. It has been histologically demonstrated that in internal root resorption, normal or necrotic pulp tissues are transformed into granulation tissue with giant multinuclear cells resorbing the dentin wall in the absence of the odontoblast layer and predentine [13, 23]. Stopping the internal resorptive processes is likely to occur through removal of the pulp and granulation tissue, as well as interruption of the blood supply to these tissues, necessary for the development of resorbing cells. Key stages in the treatment of chronic apical periodontitis (CAP) are the assessment of the status of the periapical zone, effective decontamination and subsequent sealing of the root

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Mg2+ transport in plasma membrane vesicles of renal epithelium of the Mozambique tilapia (Oreochromis mossambicus)

Mg2+ transport in plasma membrane vesicles of renal epithelium of the Mozambique tilapia (Oreochromis mossambicus)

at the apical and basolateral poles of the renal tubular epithelium, apical and basolateral plasma membrane vesicle preparations were derived from kidney tissue of freshwater- and seawater-adapted Mozambique tilapia Oreochromis mossambicus. Brush-border preparations were enriched 15.8-fold in alkaline phosphatase activity and consisted almost exclusively of right-side-out membrane vesicles. Basolateral membrane preparations were enriched 7.5-fold in Na + /K + -ATPase activity and contained resealed

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Wnt9b dependent FGF signaling is crucial for outgrowth of the nasal and maxillary processes during upper jaw and lip development

Wnt9b dependent FGF signaling is crucial for outgrowth of the nasal and maxillary processes during upper jaw and lip development

Ectoderm-specific inactivation and activation of Ctnnb1 in the facial processes caused severe hypoplasia and hyperplasia of the facial processes, respectively (Reid et al., 2011; Wang et al., 2011). Mesenchyme-specific Ctnnb1 deletion in the facial processes also causes severe defects in facial development, with missing facial skeletal components accompanying significant hypoplasia in the frontofacial region and BA1 (Brault et al., 2001). Interestingly, unlike Wnt9b and Lrp6 mutants, increased apoptosis without reduced proliferation was detected in both ectodermal and mesenchymal cells of the BA1 and NPs of Ctnnb1 deletion mutants (Brault et al., 2001; Wang et al., 2011). Inactivation of Wnt9b or Lrp6 might only partially remove the activity of WNT/-catenin signaling in the facial processes, as Lrp5 and other WNT genes can provide the remaining WNT signaling activity. Different cellular defects (proliferation versus apoptosis) observed in mice lacking Ctnnb1, Lrp6 or Wnt9b also strongly suggest that multiple cellular processes are regulated by WNT/-catenin signaling within the facial processes.

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Macroglobulin complement related encodes a protein required for septate junction organization and paracellular barrier function in Drosophila

Macroglobulin complement related encodes a protein required for septate junction organization and paracellular barrier function in Drosophila

antibodies against Mcr alone (B-D), or co-stained with antibodies against Mcr (red, and in E′-J′) and Cor (green, and in E″-H″), GFP (green in I) or Fas3 (green in J, and in J″). (B-E) Mcr is associated with the membrane in stage 11 embryos (B) and by stage 13 is obviously expressed in ectodermally derived epithelia including the epidermis (ep), foregut (fg), hindgut (hg), salivary gland (sg) and trachea (tr) (C). In stage 14 embryos (D), Mcr is enriched at the apical lateral region of the membrane, but is also expressed more basolaterally (arrow). By stage 16 (E; in the hindgut), Mcr colocalizes with Cor in the region of the SJ. In third instar wing imaginal discs (F-H), Mcr colocalizes with Cor in the apical region of the lateral membrane of the disc proper cells, which can be seen in deeper sections (G) where lateral membranes lie adjacent to the folds in the epithelium (arrow), and by rendering an xz transverse section from a confocal z-series (H). A higher magnification view (outset in H) shows that Cor localization extends more basally than Mcr (arrows), and that Mcr is also expressed on the apical surface in a domain independent of Cor (arrowhead; note that staining in the peripodial epithelium can be seen above this line). (I) Confocal optical section of a He-GAL4, UAS-GFP hemocyte stained with antibodies against Mcr (red) and GFP (green) and with DAPI (blue). Mcr is expressed in larval hemocytes, but is largely found inside the cell. (J) In the ovary, Mcr is most strongly expressed in stage 1 of the germarium (arrow) and in polar follicle cells (arrowhead), where it colocalizes with Fas3. Mcr is also expressed at lower levels in the follicle cells and at the membrane in the germ cells. Scale bars: 20 μm.

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Validation of the polysemen admixture on viability and acrosomal morphology of boar spermatozoa

Validation of the polysemen admixture on viability and acrosomal morphology of boar spermatozoa

The mean values for sperm acrosomal mor- phology in the five boars studies during the two seasons are presented in Table 2. There were no significant seasonal effect on acrosomal parameters studied. 3-boar semen admixture gave the highest mean NAR value during the late rainy season and the least missing apical ridge during the late rainy season, but not statistically (p>0.05) different from others except 5- boar semen admixture which was signifi- cantly (p<0.05) different from 3-boar se- men admixture. The assessment of sperm viability and acrosomal status gives reli- able estimate of fertility of the male.(15) It is apparent from the present result that the fertilizing ability of the semen from boar may be higher in late rainy season. Blom

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The Developmental Organization of Regulatory States in the Sea Urchin Larva

The Developmental Organization of Regulatory States in the Sea Urchin Larva

The transcription factor Rx plays an important role in the specification of ciliary photoreceptors in vertebrates and controls the expression of pax6 and opsin genes during the development of eyes [7, 8]. Rx is also expressed in ciliary photoreceptors of the marine annelid Platynereis dumerilii [9], indicating that this transcription factor might play a conserved role in the formation of ciliary photoreceptors. We analyzed the expression of rx in 72h larvae of the purple sea urchin Strongylocentrotus purpuratus by whole mount in situ hybridization (WMISH), showing that this gene is expressed in bilateral clusters of 2-3 cells on the oral side of the neurogenic apical organ (Figure 2.1A). The particular location of these cells, and the expression of rx, suggested that these cells may correspond to photoreceptor cells. We decided to test this hypothesis based on four requirements for functional directional photoreceptor cells: i) expression of a photosensitive Opsin, ii) neuronal cell type identity, iii) response to light stimulation, and iv) presence of shading pigments.

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Src kinases mediate the interaction of the apical determinant Bazooka/PAR3 with STAT92E and increase signalling efficiency in Drosophila ectodermal cells

Src kinases mediate the interaction of the apical determinant Bazooka/PAR3 with STAT92E and increase signalling efficiency in Drosophila ectodermal cells

Baz is a conserved scaffold protein involved in the establishment of apico-basal polarity in many organisms (Suzuki and Ohno, 2006). Baz contains three conserved regions (CR) (Fig. 4B) with established functions (Benton and St Johnston, 2003a; Izumi et al., 1998; Joberty et al., 2000; Lin et al., 2000; Morais-de-Sá et al., 2010; von Stein et al., 2005). The N-terminal CR1 is required for homodimerization; the CR2 includes three PDZ domains and is involved in the interaction with PAR6; the C-terminal CR3 interacts with aPKC. Apart from these, Baz contains two phosphorylation sites for PAR1, which serve as binding sites for 14-3-3 adaptor proteins, and the C-terminal region interacts with membrane lipids and mediates direct interaction with the plasma membrane (Benton and St Johnston, 2003b; Krahn et al., 2010). In the wild-type embryos, Baz is not expressed in the mesoderm and STAT92E does not localize to the membrane cortex in this tissue. However, we have shown (Sotillos et al., 2008) that STAT92E is translocated towards the membrane upon ectopic expression of Baz in the mesoderm (compare Fig. 4A,C and supplementary material Fig. S3A,B). To find out which region of Baz is required for STAT92E translocation to the cortex, we expressed different fragments of the Baz protein fused to GFP in mesodermal cells and observed which fragments could affect the localization of a full-length STAT92E-Myc-tagged protein. We find that the PDZ and the aPKC-binding domains are dispensable for STAT92E membrane translocation (Fig. 4E,F). However, when we remove either the most N-terminal (amino acids 1-317) or C-terminal regions (a complete deletion of the C-terminal region, amino acids 1001-1464; or a smaller deletion, amino acids Fig. 2. STAT92E homodimerization and membrane localization domains. (A-C) Ectodermal expression of full-length STAT92E (A), or the N-terminal (B) or the C-terminal (C) half of the protein in germline clone embryos lacking the endogenous STAT92E. In stat92E-null embryos the full-length (A) and C-terminal half (C) localize to the membrane, whereas the N-terminal half (B) is unable to localize to the apical membrane. (D) In this same background, the N-terminal half regains weak apical membrane localization when co-expressed with full-length STAT92E-Myc. (A-C) GFP staining. (D) GFP on the middle and anti-Myc on the right panel with merged channels on the left. Scale bars: 10 μm. All pictures are centred in posterior spiracle region. Anterior is leftwards and dorsal is upwards.

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Cessation of gastrulation is mediated by suppression of epithelial mesenchymal transition at the ventral ectodermal ridge

Cessation of gastrulation is mediated by suppression of epithelial mesenchymal transition at the ventral ectodermal ridge

Fig. 1. Histological and fate map analysis of chick tail development. (A) A schematic illustration of chick caudal embryogenesis. cl, cloaca; cm, cloacal membrane; hg, hind gut; ps, primitive streak, tb, tailbud; ver, ventral ectodermal ridge. (B) The tailbud of chick embryos at HH stage 18. The white line in B indicates the approximate level of the section shown in C. Scale bar: 400 ␮ m. (C) Transverse section of the chick tailbud. no, notochord; nt, neural tube; psm, presomatic mesoderm; tvm, tail ventral mesoderm. Bar: 100 ␮ m. (D) High magnification of the box in C, showing the chick VER and the adjacent region. Scale bar: 50 ␮ m. (E) Immunostaining with laminin and E-cadherin of the chick VER. Bar: 50 ␮ m. (F) Fate mapping of the tailbud ectoderm including the chick VER. The ectoderm was labeled with DiI at HH stage 16 and 20 (left panels). Middle and right panels show distribution of labeling after 24 hours. The white line in the middle panels indicates the position of the sections shown on the right. hl, hind limb. Scale bars: whole mounts, 800 ␮ m, 400 ␮ m; sections, 100 ␮ m.

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Development of reaggregated chick hindlimb buds

Development of reaggregated chick hindlimb buds

Several mutants in the chicken have also pointed to an important role for the AER. Strains o f chicken th at give rise to polydactylous limbs are associated with a pre-axial extension o f the thickened AER {Po^ and Silkie - Zwilling and Hansborough, 1956; diplopodia^ SLiid talpid^ - M acC abe and A bbott, 1974) allowing extra anteroposterior outgrow th and thus extra digits. Other good evidence comes from mutants that fail to form some of their limbs. In these strains the limbs bud out normally at the appropriate stage but fail to grow such that the hatchling loses some, or all, of its limbs. Careful study showed that these early buds n ev er form an A ER. R eco m b in atio n s o f m u tan t mesenchyme into an ectodermal jacket from a wild type chicken, and thus providing it with a genetically normal AER, resulted in b etter distal developm ent, including perfectly norm al lim b outgrowth for one strain {lim bless - Fallon et al., 1983). The reciprocal experim ent with wild-type m esenchym e and m utant ectoderm yielded no normal limbs, indicating that the mutations affect the ectoderm (Zwilling, 1956b; Fallon et al., 1983).

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