Apical ectodermal ridge

Apical ectodermal ridge

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.
Details
Identifiers
Latin crista ectodermalis apicalis
Code TE 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]

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]

Formation

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]

Role

The AER acts to:[1]

Relationship between Hox gene expression and limb patterning[4]

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.

The Hox genes are “physically linked in four chromosomal clusters (Hoxa, Hoxb, Hoxc, Hoxd),[4] 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.

Transplantation experiments

Removal/addition of AER

The removal of the AER results in truncated limbs where only the stylopod is present.[5] 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.[6][7]

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.[8] If the AER is transplanted near the middle, the ectopic limb has both forelimb and hindlimb features.[9]

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.[10]

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).[5]

Summary

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

  1. 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.

Associated molecules

Associated molecules include:[1]

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. 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. 1 2 3 4 Nelson, C. E.; et al. (1996). "Analysis of Hox gene expression in the chick limb bud" (PDF). Development 122 (5): 1449.
  5. 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.
  6. ↑ 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. doi:10.1126/science.7908145. PMID 7908145.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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. PMID 9389663.
  10. ↑ 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.
  11. ↑ Zhu, J.; et al. (2008). "Uncoupling Sonic hedgehog control of pattern and expansion of the developing limb bud" (PDF). Dev. Cell 14: 624–632. doi:10.1016/j.devcel.2008.01.008.

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