285:, an important signaling molecule needed throughout embryogenesis, acts through the Hox genes. It was originally postulated that retinoic acid acts to induce the Hoxb-8 gene, but this hypothesis has not been supported by genetic studies in mouse embryos lacking retinoic acid synthesis that still express Hoxb-8 in the limb. Hoxb-8 signaling is active in the early embryo, beginning at the posterior end of the lateral plate mesoderm and extending to the anterior region. As Hoxb-8 spreads to more anterior regions, Shh is induced in the area that will become the ZPA. Shh is only induced in the anterior region because of signals from the AER. Experiments done by Heikinheimo et al. show that when the AER is removed, beads that express FGF are sufficient to induce Shh signaling in the ZPA. Thus, the likely signaling factor from the AER is FGF.
26:
321:
266:
116:(AER). Research by Saunders and Gasseling in 1948 identified the AER and its subsequent involvement in proximal distal outgrowth. Twenty years later, the same group did transplantation studies in chick limb bud and identified the ZPA. It wasn't until 1993 that Todt and Fallon showed that the AER and ZPA are dependent on each other.
171:
342:. Without Shh, Gli2 and Gli3 are processed to a repressor form and travel to the nucleus to repress the Shh response. But when Shh is present, unprocessed Gli2 and Gli3 are able to pass into the nucleus and stimulate expression of Shh target genes, including Gli1. Studies in mice show that Gli3 knockouts have
290:
356:
193:
346:
digits. Fundamentally, Shh acts to remove repression of Gli3. When Shh diffuses from the ZPA, it predominates in the posterior region of the limb bud, activating Gli3 in the posterior region, while the repressor is still active in the anterior region. This leads to activation of other genes such as
271:
Results showed digit duplications, with the most common being 4-3-3-4, with digit 2 missing. Though there was variability, it was clearly consistent with anterior to posterior positional patterning. Variations were due to the amount of tissue grafted, and the location of the graft. These findings
240:
The idea that Shh is required for proper ZPA signaling and anterior/posterior limb formation needed to be tested. Riddle et al. took
Saunders and Gasselings findings to the next step and proved that Shh is the morphogen within the ZPA that is required for anterior posterior patterning. By isolating
211:
In 1968, Saunders and
Gasseling did transplantation studies using tissue from chick limb bud. Removing cells from the posterior region of the limb, they transplanted them to the anterior region and noticed that extra digits formed in the anterior area and these digits were mirror images to the normal
260:
to insert the cDNA into chick cells. There are unique types of this retroviral vector that only infect specific strains of avian species. Therefore, this group used a retroviral vector termed RCAS-E, which lacks a type E envelope protein, and is able to infect certain chick embryo fibroblasts with
361:
In particular, the Hox genes A and D are likely to be controlled by Shh within the ZPA. Three phases of activation of the Hox genes results in patterning of the limb parallel to the expression of the Hox genes in a nested pattern. Activation of these genes results in a new limb axis that ultimately
187:
Saunders and
Gasseling published data in the Journal of Experimental Biology in 1948, showing that reference marks inserted near the rim of the apical border of the wing bud are dispersed throughout the whole forearm of the wing. This led them to believe that the apical ectoderm may play a role in
225:
is placed in the anterior margin of the limb bud, mirror image duplications result. However, concentrations of retinoic acid that cause mirror image duplications induce high levels of a downstream gene, retinoic acid receptor Beta, which is not seen in the posterior region. It is now known that
295:
Additionally when the AER is removed, Shh is no longer expressed, and the ZPA can no longer be maintained. Acting in a positive feedback mechanism, FGF-4 is expressed near the ZPA. FGF-4 acts to maintain Shh expression, while Shh acts to maintain FGF-4 expression. At the same time, Wnt-7a is
145:
230:
acts permissively prior to limb bud initiation to allow the budding process to begin, and that the specific morphogen, hypothesized to be Shh, is normally expressed independently of retinoic acid in the posterior region of the limb bud. By looking at signaling homologs of other organisms, the
188:
forming parts of the wing. To test this, they removed apical ectoderm from wing buds which yielded deformed wings. When they removed dorsal ectoderm, normal wings formed. These results showed that the cells of the apical ectoderm have a precise fate to form specific regions of the wing.
362:
results in digit development, possibly interpreting gene expression to assign digit identity. Overall, the molecular ZPA requires input for several signaling centers, but acts as an organizer itself, inducing anterior-posterior pattering of the chick limb bud.
220:
causes mesenchyme to form on the posterior side, while low concentrations induces mesenchyme to form on the anterior end. Identifying this morphogen was the next crucial step. The first hypothesis came from Tickle et al. who showed that when
280:
Shh may be a critical signal regulating ZPA function, but the genes involved in Shh signaling are under the control of several other factors that are needed for ZPA maintenance and function including Hand2 and Hoxb-8.
111:
covering. Eventually, the limb bud develops into bones, tendons, muscles and joints. Limb bud development relies not only on the ZPA, but also many different genes, signals, and a unique region of ectoderm called the
876:
Heikinheimo M, Lawshé A, Shackleford GM, Wilson DB, MacArthur CA (November 1994). "Fgf-8 expression in the post-gastrulation mouse suggests roles in the development of the face, limbs and central nervous system".
139:
being expressed at the posterior portion. The Alx4 region, the medial region, and the Hox8 expressing area meet at a proximal area where the AER develops. The ZPA forms where the Hox8 region joins the AER.
204:
127:
requires signals from many sources. Specifically, proteins called transcription factors (TF) help control the rate at which a gene is transcribed. The limb bud expresses a TF called
623:
Nohno T, Noji S, Koyama E, et al. (March 1991). "Involvement of the Chox-4 chicken homeobox genes in determination of anteroposterior axial polarity during limb development".
162:
to be expressed in the posterior part of the AER. After these events, there is a co-dependence between FGF-4 and Shh for their subsequent expression and maintenance. Additionally,
304:
The downstream targets that are activated in response to Shh pose another challenge. Genes that are targets of Shh signaling encode factors that lead to the formation of the
296:
expressed in the dorsal ectoderm, and provides further positive feedback to FGF-4 and Shh. Without this system, limbs and digits are either significantly reduced or missing.
30:
The apical ectodermal ridge is a thickened epithelium at the most distal end of the limb bud. The zone of polarizing activity is at the posterior part of the limb bud.
1044:
Litingtung Y, Dahn RD, Li Y, Fallon JF, Chiang C (August 2002). "Shh and Gli3 are dispensable for limb skeleton formation but regulate digit number and identity".
1159:
203:
564:
Tickle C, Alberts B, Wolpert L, Lee J (April 1982). "Local application of retinoic acid to the limb bond mimics the action of the polarizing region".
989:"Interaction between the signaling molecules WNT7a and SHH during vertebrate limb development: dorsal signals regulate anteroposterior patterning"
928:
Niswander L, Jeffrey S, Martin GR, Tickle C (October 1994). "A positive feedback loop coordinates growth and patterning in the vertebrate limb".
426:"Posterior apical ectodermal ridge removal in the chick wing bud triggers a series of events resulting in defective anterior pattern formation"
476:
394:
381:
Saunders JW (December 1998). "The proximo-distal sequence of origin of the parts of the chick wing and the role of the ectoderm. 1948".
1152:
839:"Retinoid signaling is required for the establishment of a ZPA and for the expression of Hoxb-8, a mediator of ZPA formation"
1145:
729:
Riddle RD, Johnson RL, Laufer E, Tabin C (December 1993). "Sonic hedgehog mediates the polarizing activity of the ZPA".
71:
348:
679:"Retinoic acid promotes limb induction through effects on body axis extension but is unnecessary for limb patterning"
150:
These regions are dependent on signaling in order for the appropriate induction events to occur. The AER expresses
351:, plays a role in limb morphology, specifically, digit positioning, but the specific regulation of BMP is unclear.
327:
257:
256:
library from stage 22 limb bud RNA. The group ectopically expressed the gene by taking advantage of a retroviral
245:
66:
213:
1202:
182:
113:
78:
782:"Quantitative effects of hedgehog and decapentaplegic activity on the patterning of the Drosophila wing"
521:
Wolpert L (October 1969). "Positional information and the spatial pattern of cellular differentiation".
506:
Saunders JW, Gasseling MT (1968). "Ectodermal-mesenchymal interactions in the origin of limb symmetry".
272:
indicate that Shh could substitute for the function of the ZPA. Thus Shh is sufficient for ZPA action.
1104:
838:
425:
1185:
1053:
937:
573:
1243:
241:
the Shh gene and implanting it into the anterior limb bud, mirror image digit duplications formed.
107:
to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an
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and mouse and involved in limb bud formation. The clone was then used as a template to screen a
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124:
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Hox genes, FGF genes and BMP genes in the posterior region, setting up digit patterning.
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941:
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ones. This posterior mesenchyme was the ZPA, which is now known to express the protein
155:
798:
781:
534:
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is expressed in the dorsal ectoderm, is needed to maintain the FGF and Shh signaling.
1237:
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10.1002/(SICI)1097-010X(19981215)282:6<677::AID-JEZ4>3.0.CO;2-F
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10.1002/(SICI)1097-010X(19981215)282:6<628::AID-JEZ2>3.0.CO;2-N
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330:. There are three Gli factors that are essential for limb development:
84:
1217:
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Activation of Gli zinc-finger transcription factors occurs through the
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1137:
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216:(Shh). One hypothesis is that at high concentrations, this unknown
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primers that correspond to sequences of Shh that are conserved in
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46:
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Lu HC, Revelli JP, Goering L, Thaller C, Eichele G (1 May 1997).
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expression in the posterior mesoderm. Shh then stimulates
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of polarizing activity_by_E5.0.3.0.0.1.5 E5.0.3.0.0.1.5
1103:
Nelson CE, Morgan BA, Burke AC, et al. (1 May 1996).
463:
Pearse RV, Tabin CJ (December 1998). "The molecular ZPA".
1105:"Analysis of Hox gene expression in the chick limb bud"
103:
that contains signals which instruct the developing
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677:Zhao X, Sirbu IO, Mic FA, et al. (June 2009).
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237:, hedgehog, served as a viable candidate.
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1004:
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424:Todt WL, Fallon JF (1 November 1987).
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15:
244:Isolation was conducted by designing
7:
508:Epithelial-mesenchymal Interactions
465:The Journal of Experimental Zoology
383:The Journal of Experimental Zoology
987:Yang Y, Niswander L (March 1995).
780:Ingham PW, Fietz MJ (April 1995).
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523:Journal of Theoretical Biology
1:
799:10.1016/S0960-9822(95)00084-4
535:10.1016/S0022-5193(69)80016-0
125:Patterning along the limb bud
52:zona activitatis polarisantis
1006:10.1016/0092-8674(95)90297-X
891:10.1016/0925-4773(94)90022-1
743:10.1016/0092-8674(93)90626-2
637:10.1016/0092-8674(91)90274-3
131:at the anterior part of the
1208:Zone of polarizing activity
97:zone of polarizing activity
19:Zone of polarizing activity
1260:
328:Hedgehog signaling pathway
180:
879:Mechanisms of Development
695:10.1016/j.cub.2009.04.059
207:ZPA mouse, right forelimb
77:
23:
1203:Apical ectodermal ridge
183:Apical ectodermal ridge
177:Apical ectodermal ridge
114:apical ectodermal ridge
208:
79:Anatomical terminology
231:segmentation gene of
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1186:Triradiate cartilage
99:(ZPA) is an area of
1066:10.1038/nature01033
1058:2002Natur.418..979L
942:1994Natur.371..609N
578:1982Natur.296..564T
1223:Septum transversum
300:Downstream signals
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1191:Limb development
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936:(6498): 609–12.
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689:(12): 1050–7.
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214:sonic hedgehog
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181:Main article:
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1109:Development
843:Development
529:(1): 1–47.
430:Development
344:polydactyly
226:endogenous
41:Identifiers
1244:Embryology
1213:Sclerotome
683:Curr. Biol
366:References
250:Drosophila
234:Drosophila
120:Patterning
101:mesenchyme
58:Acronym(s)
1121:0950-1991
1074:0028-0836
1015:0092-8674
958:0028-0836
899:0925-4773
855:0950-1991
808:0960-9822
751:0092-8674
645:0092-8674
594:0028-0836
543:0022-5193
485:0022-104X
442:0950-1991
403:0022-104X
276:Mediators
218:morphogen
1238:Category
1196:Limb bud
1082:12198547
824:14426793
713:19464179
661:42393794
510:: 78–97.
314:zeugopod
310:stylopod
133:mesoderm
109:ectoderm
105:limb bud
1218:Myotome
1129:8625833
1090:4431757
1054:Bibcode
1031:7869066
1023:7697724
974:4305639
966:7935794
938:Bibcode
915:8587334
907:7873403
863:9165113
816:7627558
767:4973500
759:8269518
704:2701469
653:1672266
610:4242623
602:7070499
574:Bibcode
551:4390734
493:9846380
450:3502993
411:9846378
306:autopod
36:Details
1127:
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1072:
1046:Nature
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566:Nature
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258:vector
1086:S2CID
1027:S2CID
970:S2CID
911:S2CID
820:S2CID
763:S2CID
657:S2CID
606:S2CID
261:Shh.
164:Wnt7a
137:HOXB8
83:[
47:Latin
1170:bone
1125:PMID
1117:ISSN
1078:PMID
1070:ISSN
1019:PMID
1011:ISSN
993:Cell
962:PMID
954:ISSN
903:PMID
895:ISSN
859:PMID
851:ISSN
812:PMID
804:ISSN
755:PMID
747:ISSN
731:Cell
709:PMID
649:PMID
641:ISSN
625:Cell
598:PMID
590:ISSN
547:PMID
539:ISSN
489:PMID
481:ISSN
446:PMID
438:ISSN
407:PMID
399:ISSN
340:Gli3
338:and
336:Gli2
332:Gli1
312:and
254:cDNA
160:FGF4
152:FGF8
129:ALX4
95:The
1113:122
1062:doi
1050:418
1001:doi
946:doi
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