Apical ectodermal ridge

Last updated
Apical ectodermal ridge
Limb bud diagram.jpg
The apical ectodermal ridge is a region of thickened epithelium at the most distal end of the limb bud. The zone of polarising activity (ZPA) is at the posterior part of the limb bud.
Details
Identifiers
Latin crista ectodermalis apicalis
TE ectodermal ridge_by_E5.0.3.0.0.3.4 E5.0.3.0.0.3.4
Anatomical terminology

The apical ectodermal ridge (AER) is a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AER and limb mesenchyme—including the zone of polarizing activity (ZPA)—continue to communicate with each other to direct further limb development. [1]

Contents

The position of the limb bud, and hence the AER, is specified by the expression boundaries of Hox genes in the embryonic trunk. At these positions, the induction of cell outgrowth is thought to be mediated by a positive feedback loop of fibroblast growth factors (FGFs) between the intermediate mesoderm, the lateral plate mesoderm and the surface ectoderm. FGF8 in the intermediate mesoderm signals to the lateral mesoderm, restricting the expression of FGF10 through intermediate Wnt signals. Then, FGF10 in the lateral plate mesoderm signals to the surface ectoderm to create the AER, which expresses FGF8. [2]

The AER is known to express FGF2, FGF4, FGF8, and FGF9, while the limb bud mesenchyme expresses FGF2 and FGF10. Embryo manipulation experiments have shown that some of these FGFs alone are sufficient for mimicking the AER. [3]

Structure

Morphologically, the AER emerges as a thickening of the ectoderm at the distal rim of the limb bud. This distinct structure runs along the anterior-posterior axis of the limb bud and subsequently separates the dorsal side of the limb from its ventral side.

In the wing bud in chick embryos, the AER becomes anatomically distinguishable at the late stage of development 18HH (corresponding to 3 day-old embryos), when the distal ectodermal cells of the bud acquire a columnar shape distinguishing them from the cuboidal ectoderm. At stage 20HH (corresponding to 3.5 day-old embryos), the AER appears as a strip of pseudostratified epithelium which is maintained until 23-24HH (corresponding to 4-4.5 day-old embryos). Afterwards, the AER progressively decreases in height and eventually regresses. [4]

In mouse embryos, the ventral ectoderm of the emerging forelimb at E9.5 (embryonic day 9.5 [5] ) already appears thicker in comparison to the dorsal ectoderm and it corresponds to the early AER. [6] [7] By E10, this thickening is more noticeable since the epithelium now consists of two layers and becomes confined to the ventral-distal margin of the bud although it is not detectable in living specimens using light microscope or by scanning electron microscopy (SEM). [8] Between E10.5-11, a linear and compact AER with a polystratified epithelial structure (3-4 layers) has formed and positioned itself at the distal dorso-ventral boundary of the bud. [6] [8] [9] [10] After reaching its maximum height, the AER in mouse limb buds flattens and eventually become indistinguishable from the dorsal and ventral ectoderm. [8] [11] [12] The structure of the human AER is similar to the mouse AER. [13]

In addition to wings in chicks and forelimbs in mice, pectoral fins in zebrafish serve as a model to study vertebrate limb formation. Despite fin and limb developmental processes share many similarities, [14] they exhibit significant differences, one of which is the AER maintenance. While in birds and mammals the limb AER persists until the end of digit-patterning stage and eventually regresses, the fin AER transforms into an extended structure, named the apical ectodermal fold (AEF). [15] After the AER-AEF transition at 36 hours post fertilization, the AEF is located distal to the circumferential blood vessels of the fin bud. The AEF potentially functions as an inhibitor to fin outgrowth since removing the AEF results in the formation of a new AER and subsequently a new AEF. In addition, repeated AF removal leads to excessive elongation of the fin mesenchyme, potentially because of prolonged exposure of AER signals to the fin mesenchyme. [16] Recently, the AER, which has long been thought to consist of only ectodermal cells, in fact composes of both mesodermal and ectodermal cells in zebrafish. [17]

Associated molecules

Associated molecules include: [1]

Development

FGF10 secretions from the mesenchyme cells of the limb field interact with the ectodermal cells above, and induce the formation of the AER on the distal end of the developing limb. The presence of a dorsal-ventral ectodermal boundary is crucial for AER formation – the AER can only form at that divide. [1]

Function

The AER acts to: [1]

Relationship between Hox gene expression and limb patterning

The Hox genes, which initially establish the anterior-posterior axis of the entire embryo, continue to participate in the dynamic regulation of limb development even after the AER and ZPA have been established. Complex communication ensues as AER-secreted FGFs and ZPA-secreted Shh initiate and regulate Hox gene expression in the developing limb bud. Though many of the finer details remain to be resolved, a number of significant connections between Hox gene expression and the impact on limb development have been discovered. The pattern of Hox gene expression can be divided up into three phases throughout limb bud development, which corresponds to three key boundaries in proximal-distal limb development. The transition from the first phase to the second phase is marked by the introduction of Shh from the ZPA. The transition into the third phase is then marked by changes in how the limb bud mesenchyme responds to Shh signaling. This means that although Shh signaling is required, its effects change over time as the mesoderm is primed to respond to it differently. These three phases of regulation reveal a mechanism by which natural selection can independently modify each of the three limb segments – the stylopod, the zeugopod, and the autopod. [18]

The Hox genes are “physically linked in four chromosomal clusters (Hoxa, Hoxb, Hoxc, Hoxd), [18] and their physical position on the chromosome seems to correlate with the time and place of expression. For example, the most 3’ HOXC genes (HOXC4, HOXC5) are expressed only in the anterior limbs (wings) in chickens, while the more 5’ genes (HOXC9, HOXC10, HOXC11) are expressed only in the posterior limbs (legs). The intermediate genes (HOXC6, HOXC8) are expressed in both the upper and lower limbs. Within the limb bud, expression also varies as a function of the position along the anterior-posterior axis. Such is the case with HOXB9, which is most highly expressed next to the AER, and decreases when moving anteriorly to posteriorly, resulting in the least HOXB9 expression next to the posterior ZPA. HOXB9 expression is inversely proportional to the level of Shh expression, which makes sense, as the ZPA secretes Shh. HOXA and HOXD genes for the most part follow nested expression domains, in which they are activated uniformly along the anterior-posterior axis of the limb itself, but not the anterior-posterior axis of the entire body. Whereas HOXC and HOXB genes tend to be restricted to specific limbs, HOXA and HOXD are usually expressed in all limbs. HOXD9 and HOXD10 are expressed in the developing limb throughout the entire anterior-posterior axis, followed by HOXD11, HOXD12, HOXD13, which are each expressed in more posterior regions, with HOXD13 being restricted to only the most posterior regions of the limb bud. As a result, HOXD expression clusters around the posterior ZPA (where HOXD9, 10, 11, 12, and 13 are all expressed), while less expression occurs around the AER, where only HOXD9 and HOXD10 are expressed. [18]

Transplantation experiments

Results overview

AER maintains limb outgrowth through FGF secretion, mesenchyme cells determine identity [1]

These experiments reveal that the limb mesenchyme contains the necessary information concerning limb identity, but the AER is needed to stimulate the mesenchyme to live up to its destiny (of becoming an arm, leg, etc.)

  1. When the AER is removed, limb development halts. If an FGF bead is added in the AER’s place, normal limb development proceeds.
  2. When an extra AER is added, two limbs form.
  3. When forelimb mesenchyme is replaced with hindlimb mesenchyme, a hindlimb grows.
  4. When forelimb mesenchyme is replaced with non-limb mesenchyme, the AER regresses, and limb development halts.
  5. When the AER from a late limb bud is transplanted to an earlier limb bud, the limb forms normally. The converse – transplantation of an early limb bud to a late limb bud – also results in normal limb development. However, the underlying mesoderm in the progress zone ‘’is’’ fate specified. If progress zone mesoderm is transplanted along with the AER, then additional finger/toes are formed (for an early-->late transplantation) or the finger/toes are formed too early (for a late-->early transplantation).
AER formation relies on dorsal-ventral boundary [1]

The precise microenvironmental cues present at the D-V boundary are crucial for AER formation. When the limb bud is dorsalized - in limbless mutants, for example - and no dorsal-ventral boundary exists, the AER is unable to form and limb development halts.

Removal/addition of AER

The removal of the AER results in truncated limbs where only the stylopod is present. [20] The transplantation of an additional AER results in the duplication of limb structures, usually as a mirror image next to the already developing limb. The mirror image reflection is a result of the transplanted AER obeying signals from the existing ZPA.

FGF-soaked beads can mimic the AER

Implantation of a plastic bead soaked in FGF-4 or FGF-2 will induce formation of a limb bud in an embryo, but proliferation will cease prematurely unless additional beads are added to maintain appropriate levels of the FGF. Implantation of sufficient beads can induce formation of a 'normal' additional limb at an arbitrary location in the embryo. [21] [22]

Ectopic limb formation

Transplantation of the AER to flank mesoderm between the normal limb buds results in ectopic limbs. If the AER is transplanted closer to the forelimb bud, the ectopic limb develops like a forelimb. If the AER is transplanted closer to the hindlimb bud, the ectopic limb develops like a hindlimb. [23] If the AER is transplanted near the middle, the ectopic limb has both forelimb and hindlimb features. [24]

AER does not specify limb identity

Transplantation of an AER that would give rise to an arm (or wing, as these experiments are commonly performed on chicken embryos) to a limb field developing into a leg does not produce an arm and leg at the same location, but rather two legs. In contrast, transplantation of cells from the progress zone of a developing arm to replace the progress zone of a developing leg will produce a limb with leg structures proximally (femur, knee) and arm structures distally (hand, fingers). Thus it is the mesodermal cells of the progress zone, not the ectodermal cells of the AER, that control the identity of the limb. [25]

AER timing does not specify underlying mesoderm fate

AER timing does not regulate the fate specification of the underlying mesoderm, as shown by one set of experiments. When the AER from a late limb bud is transplanted to an earlier limb bud, the limb forms normally. The converse – transplantation of an early limb bud to a late limb bud – also results in normal limb development. However, the underlying mesoderm in the progress zone is fate specified. If progress zone mesoderm is transplanted along with the AER, then additional finger/toes are formed (for an early → late transplantation) or the finger/toes are formed too early (for a late → early transplantation). [20]

Related Research Articles

Mesoderm Middle germ layer that forms muscle, bone, blood vessels and more

The mesoderm is the middle layer of the three germ layers that develop during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.

Ectoderm

The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm and endoderm. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma meaning "skin".

Somitogenesis

Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelium, and dermis.

The primitive node is the organizer for gastrulation in the vertebrate embryo. The organizer is determined by the Nieuwkoop center in amphibians or the Posterior Marginal zone in amniotes.

Intermediate mesoderm Layer of cells in mammalian embryos

Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm located between the paraxial mesoderm and the lateral plate of the developing embryo. The intermediate mesoderm develops into vital parts of the urogenital system, as well as the reproductive system.

Lateral plate mesoderm

The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow region of intermediate mesoderm. The mesoderm is the middle layer of the three germ layers, between the outer ectoderm and inner endoderm.

Somatopleuric mesenchyme

In the anatomy of an embryo, the somatopleure is a structure created during embryogenesis when the lateral plate mesoderm splits into two layers. The outer layer becomes applied to the inner surface of the ectoderm, and with it (partially) forms the somatopleure.

Eye development Formation of the eye during embryonic development

Eye formation in the human embryo begins at approximately three weeks into embryonic development and continues through the tenth week. Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.

Limb development

Limb development in vertebrates is an area of active research in both developmental and evolutionary biology, with much of the latter work focused on the transition from fin to limb.

The limb bud is a structure formed early in vertebrate limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development. In the development of the human embryo the upper limb bud appears in the third week and the lower limb bud appears four days later.

In the field of developmental biology, regional differentiation is the process by which different areas are identified in the development of the early embryo. The process by which the cells become specified differs between organisms.

Hypoblast Embryonic inner cell mass tissue that forms the yolk sac and, later, chorion

In amniote embryology, the hypoblast, is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to the chorion.

FGF8

Fibroblast growth factor 8 is a protein that in humans is encoded by the FGF8 gene.

Cheryll Anne Tickle is a distinguished British scientist, known for her work in developmental biology and specifically for her research into the process by which vertebrate limbs develop ab ovo. She is an Emeritus Professor at the University of Bath.

Zone of polarizing activity

The zone of polarizing activity (ZPA) is an area of mesenchyme that contains signals which instruct the developing limb bud to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an ectoderm 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 apical ectodermal ridge (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.

Diplopodia is a congenital anomaly in tetrapods that involves duplication of elements of the foot on the hind limb. It comes from the Greek roots diplo = "double" and pod = "foot". Diplopodia is often found in conjunction with other structural abnormalities and can be lethal. It is more extreme than polydactyly, the presence of extra digits.

The development of the digestive system in the human embryo concerns the epithelium of the digestive system and the parenchyma of its derivatives, which originate from the endoderm. Connective tissue, muscular components, and peritoneal components originate in the mesoderm. Different regions of the gut tube such as the esophagus, stomach, duodenum, etc. are specified by a retinoic acid gradient that causes transcription factors unique to each region to be expressed. Differentiation of the gut and its derivatives depends upon reciprocal interactions between the gut endoderm and its surrounding mesoderm. Hox genes in the mesoderm are induced by a Hedgehog signaling pathway secreted by gut endoderm and regulate the craniocaudal organization of the gut and its derivatives. The gut system extends from the oropharyngeal membrane to the cloacal membrane and is divided into the foregut, midgut, and hindgut.

Hox genes in amphibians and reptiles

Hox genes play a massive role in some amphibians and reptiles in their ability to regenerate lost limbs, especially HoxA and HoxD genes.

T-box transcription factor Tbx4 is a transcription factor that belongs to T-box gene family that is involved in the regulation of embryonic developmental processes. The transcription factor is encoded by the TBX4 gene located on human chromosome 17. Tbx4 is known mostly for its role in the development of the hindlimb, but it also plays a critical role in the formation of the umbilicus. Tbx4 has been shown to be expressed in the allantois, hindlimb, lung and proctodeum.

John W. Saunders Jr. was an American scientist whose research in the field of developmental biology and zoology played an integral part in helping to understand how various vertebrate limbs develop. Saunders researched the vertebrate limb and studied the apical ectodermal ridge (AER). This research was critical in recognizing growth factors that are secreted from the AER and are important in assisting the pattern of developing vertebrate limbs.

References

  1. 1 2 3 4 5 6 Gilbert, Scott F. "Developmental Biology". 9th ed., 2010
  2. ↑ Ohuchi H, Nakagawa T, Yamamoto A, et al. (June 1997). "The mesenchymal factor, FGF10, initiates and maintains the outgrowth of the chick limb bud through interaction with FGF8, an apical ectodermal factor". Development. 124 (11): 2235–44. doi:10.1242/dev.124.11.2235. PMID   9187149.
  3. ↑ Martin GR (June 1998). "The roles of FGFs in the early development of vertebrate limbs". Genes Dev. 12 (11): 1571–86. doi: 10.1101/gad.12.11.1571 . PMID   9620845.
  4. ↑ Todt, William L.; Fallon, John F. (1984-04-01). "Development of the apical ectodermal ridge in the chick wing bud". Development. 80 (1): 21–41. doi:10.1242/dev.80.1.21. ISSN   1477-9129. PMID   6747526.
  5. ↑ "Mouse Timeline Detailed - Embryology". embryology.med.unsw.edu.au. Retrieved 2018-12-14.
  6. 1 2 Bell, Sheila M; Schreiner, Claire M; Scott, William J (June 1998). "The loss of ventral ectoderm identity correlates with the inability to form an AER in the legless hindlimb bud". Mechanisms of Development. 74 (1–2): 41–50. doi: 10.1016/s0925-4773(98)00065-3 . ISSN   0925-4773. PMID   9651475.
  7. ↑ Loomis, Cynthia A.; Harris, Esther; Michaud, Jacques; Wurst, Wolfgang; Hanks, Mark; Joyner, Alexandra L. (July 1996). "The mouse Engrailed-1 gene and ventral limb patterning". Nature. 382 (6589): 360–363. Bibcode:1996Natur.382..360L. doi:10.1038/382360a0. ISSN   0028-0836. PMID   8684466. S2CID   4326299.
  8. 1 2 3 Wanek, N.; Muneoka, K.; Holler-dinsmore, G.; Burton, R.; Bryant, S. V. (January 1989). "A staging system for mouse limb development". Journal of Experimental Zoology. 249 (1): 41–49. doi:10.1002/jez.1402490109. ISSN   0022-104X. PMID   2926360.
  9. ↑ Kelley, R. O.; Fallon, J. F. (1983). "A freeze-fracture and morphometric analysis of gap junctions of limb bud cells: initial studies on a possible mechanism for morphogenetic signalling during development". Progress in Clinical and Biological Research. 110 Pt A: 119–130. ISSN   0361-7742. PMID   6828478.
  10. ↑ Meyer, R. A.; Cohen, M. F.; Recalde, S.; Zakany, J.; Bell, S. M.; Scott, W. J.; Lo, C. W. (1997). "Developmental regulation and asymmetric expression of the gene encoding Cx43 gap junctions in the mouse limb bud". Developmental Genetics. 21 (4): 290–300. doi:10.1002/(SICI)1520-6408(1997)21:4<290::AID-DVG6>3.0.CO;2-2. ISSN   0192-253X. PMID   9438343.
  11. ↑ Jurand, A. (1965-05-18). "Ultrastructural Aspects of Early Development of the Fore-Limb Buds in the Chick and the Mouse". Proceedings of the Royal Society B: Biological Sciences. 162 (988): 387–405. Bibcode:1965RSPSB.162..387J. doi:10.1098/rspb.1965.0045. ISSN   0962-8452. S2CID   84698867.
  12. ↑ Guo, Qiuxia; Loomis, Cynthia; Joyner, Alexandra L (December 2003). "Fate map of mouse ventral limb ectoderm and the apical ectodermal ridge". Developmental Biology. 264 (1): 166–178. doi: 10.1016/j.ydbio.2003.08.012 . ISSN   0012-1606. PMID   14623239.
  13. ↑ Milaire, J (1965). "Aspects of limb morphogenesis in mammals". Organogenesis: 283–300.
  14. ↑ Zuniga, Aimée; López-Ríos, Javier; Zeller, Rolf (December 2009). "Vertebrate limb bud development: moving towards integrative analysis of organogenesis". Nature Reviews Genetics. 10 (12): 845–858. doi:10.1038/nrg2681. ISSN   1471-0064. PMID   19920852. S2CID   31202624.
  15. ↑ Dane, P. J.; Tucker, J. B. (June 1985). "Modulation of epidermal cell shaping and extracellular matrix during caudal fin morphogenesis in the zebra fish Brachydanio rerio". Journal of Embryology and Experimental Morphology. 87: 145–161. ISSN   0022-0752. PMID   4031750.
  16. ↑ Tamura, Koji; Kawakami, Koichi; Yokoyama, Hitoshi; Abe, Gembu; Yano, Tohru (2012-11-15). "Mechanism of pectoral fin outgrowth in zebrafish development". Development. 139 (22): 2916–25. doi: 10.1242/dev.090324 . ISSN   1477-9129. PMID   22791899.
  17. ↑ Currie, Peter D.; Hall, Thomas E.; Weidinger, Gilbert; Knopf, Franziska; Cohen, Naomi; Nguyen, Phong D.; Wood, Alasdair; Sonntag, Carmen; Berger, Silke (July 2016). "A somitic contribution to the apical ectodermal ridge is essential for fin formation". Nature. 535 (7613): 542–546. Bibcode:2016Natur.535..542M. doi:10.1038/nature18953. ISSN   1476-4687. PMID   27437584. S2CID   4462717.
  18. 1 2 3 4 5 Nelson, C. E.; et al. (1996). "Analysis of Hox gene expression in the chick limb bud" (PDF). Development. 122 (5): 1449–66. doi:10.1242/dev.122.5.1449. PMID   8625833.
  19. ↑ Zhu, Jianjian; Nakamura, Eiichiro; Nguyen, Minh-Thanh; Bao, Xiaozhong; Akiyama, Haruhiko; Mackem, Susan (2008). "Uncoupling Sonic Hedgehog Control of Pattern and Expansion of the Developing Limb Bud". Developmental Cell. 14 (4): 624–632. doi: 10.1016/j.devcel.2008.01.008 . ISSN   1534-5807. PMC   8284562 . PMID   18410737.
  20. 1 2 Rubin L, Saunders JW (May 1972). "Ectodermal-mesodermal interactions in the growth of limb buds in the chick embryo: constancy and temporal limits of the ectodermal induction". Dev. Biol. 28 (1): 94–112. doi:10.1016/0012-1606(72)90129-7. PMID   4625230.
  21. ↑ Fallon JF, López A, Ros MA, Savage MP, Olwin BB, Simandl BK (April 1994). "FGF-2: apical ectodermal ridge growth signal for chick limb development". Science. 264 (5155): 104–7. Bibcode:1994Sci...264..104F. doi:10.1126/science.7908145. PMID   7908145.
  22. ↑ Niswander L, Tickle C, Vogel A, Booth I, Martin GR (November 1993). "FGF-4 replaces the apical ectodermal ridge and directs outgrowth and patterning of the limb". Cell. 75 (3): 579–87. doi:10.1016/0092-8674(93)90391-3. PMID   8221896. S2CID   27128022.
  23. ↑ Cohn MJ, Izpisúa-Belmonte JC, Abud H, Heath JK, Tickle C (March 1995). "Fibroblast growth factors induce additional limb development from the flank of chick embryos". Cell. 80 (5): 739–46. doi: 10.1016/0092-8674(95)90352-6 . PMID   7889567.
  24. ↑ Ohuchi H, Takeuchi J, Yoshioka H, et al. (January 1998). "Correlation of wing-leg identity in ectopic FGF-induced chimeric limbs with the differential expression of chick Tbx5 and Tbx4". Development. 125 (1): 51–60. doi:10.1242/dev.125.1.51. PMID   9389663.
  25. ↑ Zwilling E (1959). "Interaction between ectoderm and mesoderm in duck-chicken limb bud chimaeras". J. Exp. Zool. 142 (1): 521–32. doi:10.1002/jez.1401420124. PMID   13789035.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.