How do we get a perfect complement of digits?

doi:10.1016/j.gde.2008.06.009

Current Opinion in Genetics & Development

Volume 18, Issue 4, August 2008, Pages 374-380

https://doi.org/10.1016/j.gde.2008.06.009 Get rights and content

A crucial issue in limb development is how a correct set of precisely shaped digits forms in the digital plate. This process relies on patterning across the anterior–posterior axis of the limb bud, which is under the control of Sonic hedgehog emanating from the zone of polarizing activity. Recently, Sonic hedgehog function in the limb bud has been shown to have a dual character controlling both growth and patterning of the digital field. This finding has prompted the proposal of new models of how these two functions are achieved, and this will be discussed in this review.

Introduction

The distal segment of the vertebrate limb or autopod is characterized by the presence of digits, crucial elements for the function of the limb. The digits can be considered serial structures arranged along the antero-posterior (AP) axis, thumb to little finger, but with each one having its own readily distinguishable identity (Figure 1). Most current models propose Sonic hedgehog (Shh) as the primary organizer of AP patterning and digit identity in the amniote limb. This review focuses on recent progress in the understanding on how SHH achieves this function.

Section snippets

A spatial gradient of SHH generates anterior–posterior polarity in the limb bud

The earliest insights on how the AP polarity of the limb bud is achieved came with the discovery of the zone of polarizing activity (ZPA), which was operationally defined by its capacity to produce remarkable mirror image duplications of the digits when grafted to the anterior limb bud [1, 2]. A myriad of subsequent experiments, primarily performed in the developing chick limb, showed that the ZPA cells provided the cues for AP patterning [2]. The interpretation of this process was that ZPA

Time is an additional parameter for antero-posterior identity

The analysis of the time course in which extra digits were induced by the application of SHH to the anterior chick wing mesoderm showed that their induction occurred sequentially with anterior digits being specified first and then promoted to more posterior fates as exposure time increased [13]. High concentrations of SHH required more time to specify posterior than anterior digits while low concentrations did not specify posterior digits even if maintained for longer periods. There was a

A dual SHH function in specification and growth

Most interestingly, the use of the Hoxb6CreER^T line has recently permitted the successful manipulation of the time of Shh transcription in the limb bud in a controlled and precise way and without disturbing the initiation, level or domain of transcription [33^••]. The premature arrest in Shh transcription is followed by a reduction in the number of digits proportional to the stage at which it is performed. Because the morphology of the remaining digits is sufficiently well preserved, it is

Conclusions

Considerable progress in understanding the mechanism directing digit identity has been produced recently; particularly the finding of the dual SHH function on specification and growth. However, the evidence is far from complete and some of the data appear difficult to reconcile, maybe because they have been generated in different model systems and with very different approaches.

Probably the strongest discrepancy between the models is whether SHH functions in growth and specification can in fact

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• of special interest
•• of outstanding interest

Acknowledgements

We are very grateful to John Fallon, Susan Mackem, Cheryll Tickle, and Mat Towers for helpful discussions. Supported by grant BFU2005-09309-CO2-01 from the Spanish Ministry of Education and Science.

References (49)

L. Wolpert
Positional information and the spatial pattern of cellular differentiation
J Theor Biol
(1969)
R.D. Riddle et al.
Sonic hedgehog mediates the polarizing activity of the ZPA
Cell
(1993)
A. Gritli-Linde et al.
The whereabouts of a morphogen: direct evidence for short- and graded long-range activity of hedgehog signaling peptides
Dev Biol
(2001)
P.M. Lewis et al.
Cholesterol modification of Sonic hedgehog is required for long-range signaling activity and effective modulation of signaling by Ptc1
Cell
(2001)
V. Marigo et al.
Sonic hedgehog differentially regulates expression of GLI and GLI3 during limb development
Dev Biol
(1996)
K.A. Platt et al.
Expression of the mouse Gli and Ptc genes is adjacent to embryonic sources of hedgehog signals suggesting a conservation of pathways between flies and mice
Mech Dev
(1997)
A. Lopez-Martinez et al.
Limb-patterning activity and restricted posterior localization of the amino-terminal product of Sonic hedgehog cleavage
Curr Biol
(1995)
C. Chiang et al.
Manifestation of the limb prepattern: limb development in the absence of sonic hedgehog function
Dev Biol
(2001)
P. Kraus et al.
Some distal limb structures develop in mice lacking Sonic hedgehog signaling
Mech Dev
(2001)
B. Wang et al.
Hedgehog-regulated processing of Gli3 produces an anterior/posterior repressor gradient in the developing vertebrate limb
Cell
(2000)

B.D. Harfe et al.

Evidence for an expansion-based temporal Shh gradient in specifying vertebrate digit identities

Cell

(2004)

P.J. Scherz et al.

Extended exposure to Sonic hedgehog is required for patterning the posterior digits of the vertebrate limb

Dev Biol

(2007)

P. Hasson et al.

Tbx5 is dispensable for forelimb outgrowth

Development

(2007)

H. Masuya et al.

A series of ENU-induced single-base substitutions in a long-range cis-element altering sonic hedgehog expression in the developing mouse limb bud

Genomics

(2007)

M.C. Welten et al.

Gene expression and digit homology in the chicken embryo wing

Evol Dev

(2005)

G.F. Stopper et al.

Inhibition of sonic hedgehog signaling leads to posterior digit loss in Ambystoma mexicanum: parallels to natural digit reduction in urodeles

Dev Dyn

(2007)

J.W. Saunders et al.

Ectodermal–mesenchymal interactions in the origin of limb symmetry

C. Tickle

The early history of the polarizing region: from classical embryology to molecular biology

Int J Dev Biol

(2002)

X. Zeng et al.

A freely diffusible form of Sonic hedgehog mediates long-range signalling

Nature

(2001)

M.H. Chen et al.

Palmitoylation is required for the production of a soluble multimeric Hedgehog protein complex and long-range signaling in vertebrates

Genes Dev

(2004)

Y. Li et al.

Cholesterol modification restricts the spread of Shh gradient in the limb bud

Proc Natl Acad Sci U S A

(2006)

A. Callejo et al.

Patched, the receptor of Hedgehog, is a lipoprotein receptor

Proc Natl Acad Sci U S A

(2008)

Y. Yang et al.

Relationship between dose, distance and time in Sonic Hedgehog-mediated regulation of anteroposterior polarity in the chick limb

Development

(1997)

M.A. Ros et al.

The chick oligozeugodactyly (ozd) mutant lacks sonic hedgehog function in the limb

Development

(2003)

Cited by (22)

Self-organization in the limb: A Turing mechanism for digit development
2015, Current Opinion in Genetics and Development
Citation Excerpt :
Our understanding of digit patterning mechanisms has traditionally focused largely on secreted morphogens and their concentration-dependent effects on positional identities across a field of cells. This work has been extensively reviewed elsewhere [1–5] and contributed to a deep understanding of the dynamic processes of pattern formation. In the background of these classical embryological and developmental genetic studies, theoreticians have long hypothesized that our fingers may have a greater connection to mathematics than our ability to count to ten.
The statistician George E. P. Box stated, ‘Essentially all models are wrong, but some are useful.’ (Box GEP, Draper NR: Empirical Model-Building and Response Surfaces. Wiley; 1987). Modeling biological processes is challenging for many of the reasons classically trained developmental biologists often resist the idea that black and white equations can explain the grayscale subtleties of living things. Although a simplified mathematical model of development will undoubtedly fall short of precision, a good model is exceedingly useful if it raises at least as many testable questions as it answers. Self-organizing Turing models that simulate the pattern of digits in the hand replicate events that have not yet been explained by classical approaches. The union of theory and experimentation has recently identified and validated the minimal components of a Turing network for digit pattern and triggered a cascade of questions that will undoubtedly be well-served by the continued merging of disciplines.
Formation of proximal and anterior limb skeleton requires early function of Irx3 and Irx5 and is negatively regulated by shh signaling
2014, Developmental Cell
Citation Excerpt :
Sonic hedgehog (Shh), a prominent regulator of skeletal pattern, is expressed by a group of cells in the posterior aspect of limb bud mesenchyme called the zone of polarizing activity (ZPA) (Riddle et al., 1993). It is a key signal for establishing the posterior aspect of the limb skeleton that includes the ulna/fibula and digits 2 through 5 (Bastida and Ros, 2008; Hill, 2007; Robert and Lallemand, 2006). In Shh null mice, most of the femur, the tibia, and digit 1 remain intact (Chiang et al., 1996, 2001).
Limb skeletal pattern relies heavily on graded Sonic hedgehog (Shh) signaling. As a morphogen and growth cue, Shh regulates identities of posterior limb elements, including the ulna/fibula and digits 2 through 5. In contrast, proximal and anterior structures, including the humerus/femur, radius/tibia, and digit 1, are regarded as Shh independent, and mechanisms governing their specification are unclear. Here, we show that patterning of the proximal and anterior limb skeleton involves two phases. Irx3 and Irx5 (Irx3/5) are essential in the initiating limb bud to specify progenitors of the femur, tibia, and digit 1. However, these skeletal elements can be restored in Irx3/5 null mice when Shh signaling is diminished, indicating that Shh negatively regulates their formation after initiation. Our data provide genetic evidence supporting the concept of early specification and progressive determination of anterior limb pattern.
Tracing the evolution of avian wing digits
2013, Current Biology
Citation Excerpt :
As a key regulator of digit number and identity, as well as a potential marker for ‘posterior’ digits, Shh is another gene whose expression is highly relevant to assessing digit homology. Some recent developmental data support I-II-III identities, including expression patterns of selected individual genes (such as Hoxd12–8, Mkp3 and Sef) as well as more recent genome-wide expression profiles comparing different digits, and lineage tracing experiments of digits arising from Sonic hedgehog (Shh)-expressing cells [11,14,34–40]. In tetrapod model organisms, the first digit uniquely lacks expression of all 5’ Hoxd genes except Hoxd13, and expresses several very anteriorly restricted genes.
It is widely accepted that birds are a subgroup of dinosaurs, but there is an apparent conflict: modern birds have been thought to possess only the middle three fingers (digits II-III-IV) of an idealized five-digit tetrapod hand based on embryological data, but their Mesozoic tetanuran dinosaur ancestors are considered to have the first three digits (I-II-III) based on fossil evidence. How could such an evolutionary quirk arise? Various hypotheses have been proposed to resolve this paradox. Adding to the confusion, some recent developmental studies support a I-II-III designation for avian wing digits whereas some recent paleontological data are consistent with a II-III-IV identification of the Mesozoic tetanuran digits. A comprehensive analysis of both paleontological and developmental data suggests that the evolution of the avian wing digits may have been driven by homeotic transformations of digit identity, which are more likely to have occurred in a partial and piecemeal manner. Additionally, recent genetic studies in mouse models showing plausible mechanisms for central digit loss invite consideration of new alternative possibilities (I-II-IV or I-III-IV) for the homologies of avian wing digits. While much progress has been made, some advances point to the complexity of the problem and a final resolution to this ongoing debate demands additional work from both paleontological and developmental perspectives, which will surely yield new insights on mechanisms of evolutionary adaptation.
Human Developmental Genetics
2013, Emery and Rimoin's Principles and Practice of Medical Genetics
Limb anterior-posterior polarity integrates activator and repressor functions of GLI2 as well as GLI3
2012, Developmental Biology
Citation Excerpt :
A long-standing question in developmental biology is how the vertebrate limb acquires a stereotyped polarity and number of skeletal elements along the anterior–posterior (AP) axis within each segment of the proximal–distal axis (stylopod, zeugopod, autopod). The secreted factor sonic hedgehog (SHH) is responsible for mediating the AP organizing function of the zone of polarizing activity (ZPA), a posterior cell-population that promotes digit formation and AP polarity (Bastida and Ros, 2008). The mechanism by which SHH signaling determines digit number in the autopod (distal to the wrist) appears to involve regulating proliferation and survival of digit precursor cells (Towers et al., 2008; Zhu et al., 2008).
Anterior–posterior (AP) limb patterning is directed by sonic hedgehog (SHH) signaling from the posteriorly located zone of polarizing activity (ZPA). GLI3 and GLI2 are the transcriptional mediators generally utilized in SHH signaling, and each can function as an activator (A) and repressor (R). Although GLI3R has been suggested to be the primary effector of SHH signaling during limb AP patterning, a role for GLI3A or GLI2 has not been fully ruled out, nor has it been determined whether Gli3 plays distinct roles in limb development at different stages. By conditionally removing Gli3 in the limb at multiple different time points, we uncovered four Gli3-mediated functions in limb development that occur at distinct but partially over-lapping time windows: AP patterning of the proximal limb, AP patterning of the distal limb, regulation of digit number and bone differentiation. Furthermore, by removing Gli2 in Gli3 temporal conditional knock-outs, we uncovered an essential role for Gli2 in providing the remaining posterior limb patterning seen in Gli3 single mutants. To test whether GLIAs or GLIRs regulate different aspects of AP limb patterning and/or digit number, we utilized a knock-in allele in which GLI1, which functions solely as an activator, is expressed in place of the bifunctional GLI2 protein. Interestingly, we found that GLIAs contribute to AP patterning specifically in the posterior limb, whereas GLIRs predominantly regulate anterior patterning and digit number. Since GLI3 is a more effective repressor, our results explain why GLI3 is required only for anterior limb patterning and why GLI2 can compensate for GLI3A in posterior limb patterning. Taken together, our data suggest that establishment of a complete range of AP positional identities in the limb requires integration of the spatial distribution, timing, and dosage of GLI2 and GLI3 activators and repressors.
Wavelength of a Turing-type mechanism regulates the morphogenesis of meshwork patterns
2021, Scientific Reports

View all citing articles on Scopus

View full text

How do we get a perfect complement of digits?

Introduction

Section snippets

A spatial gradient of SHH generates anterior–posterior polarity in the limb bud

Time is an additional parameter for antero-posterior identity

A dual SHH function in specification and growth

Conclusions

References and recommended reading

Acknowledgements

J Theor Biol

Cell

Dev Biol

Cell

Dev Biol

Mech Dev

Curr Biol

Dev Biol

Mech Dev

Cell

Cell

Dev Biol

Development

Genomics

Evol Dev

Dev Dyn

Ectodermal–mesenchymal interactions in the origin of limb symmetry

The early history of the polarizing region: from classical embryology to molecular biology

Int J Dev Biol

A freely diffusible form of Sonic hedgehog mediates long-range signalling

Nature

Palmitoylation is required for the production of a soluble multimeric Hedgehog protein complex and long-range signaling in vertebrates

Genes Dev

Cholesterol modification restricts the spread of Shh gradient in the limb bud

Proc Natl Acad Sci U S A

Patched, the receptor of Hedgehog, is a lipoprotein receptor

Proc Natl Acad Sci U S A

Relationship between dose, distance and time in Sonic Hedgehog-mediated regulation of anteroposterior polarity in the chick limb

Development

The chick oligozeugodactyly (ozd) mutant lacks sonic hedgehog function in the limb

Development