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#limb regeneration

13 public questions tagged with this topic.

Regeneration -l

What is the role of SHH in limb regeneration?

Sonic hedgehog family member secreted from posterior mesenchyme establishes anterior-posterior pattern in developing and regenerating limbs. During salamander and chick limb regeneration, SHH expressed in posterior blastema activates downstream Gli transcription factors, with gradient high posteriorly specifying digit five to four to three identities and low anterior leading to thumb. Ectopic SHH beads anteriorly duplicate digits mirror-symmetrically, while SHH blockade produces digit loss and shortened limbs. Beyond digit specification SHH promotes cell cycle progression and survival of blastema cells, regulates blood vessel formation secondarily, but canonical role is promoting digit formation and posterior pattern rather than suppressing growth.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: SHH in limb regeneration - ZPA and digit specification.

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Positional identity highest along proximal-distal axis is at:

Proximal-distal positional identity is graded with highest positional value at proximal base decreasing toward distal tip. Classical experiments measuring Prod1 concentration, Meis1/2 expression, and cell sorting affinities show proximal blastema cells possess highest adhesion and displace distal cells in mixed aggregates, reflecting maximal positional information. Retinoic acid concentration highest proximally reinforces this gradient. Consequently proximal cells can generate entire distal pattern via intercalation, while distal cells cannot generate proximal beyond origin unless proximalized by RA. Hence proximal base represents reference point of greatest positional value establishing polarity for regeneration and development.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Positional identity highest at proximal base of limb.

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Salamander limb regeneration occurs by:

Salamander limb regeneration occurs through epimorphosis characterized by histolysis, dedifferentiation, formation of regeneration blastema, extensive proliferation, and re-differentiation recapitulating developmental programs. After amputation wound epidermis forms apical epidermal cap expressing FGF8, Wnt, BMP modulators, nerves provide trophic factors inducing MMP degradation and cell-cycle re-entry. Blastema accumulates multipotent progenitors restoring missing portion distal to cut. This contrasts morphallaxis where existing tissues repattern without growth, compensatory hypertrophy, or simple stem-cell renewal. Molecularly re-activation of FGF10-FGF8, SHH-Gremlin loops indicates redeployment of limb development pathways during epimorphic regeneration.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Salamander limb regeneration as epimorphic mechanism.

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FGFs alone in axolotl limb regeneration promote:

In axolotl limb regeneration studies, fibroblast growth factors supplied alone via beads or transgenes promote distal outgrowth. FGF8, FGF10 secreted by apical epidermal cap and nerves maintain distal blastema identity expressing HoxA13 and distal proliferation while failing to activate proximal program Meis. Therefore implantation of FGF alone leads to regeneration of distal structures only, such as hand elements, with poor proximal restoration unless exogenous retinoic acid provides proximalizing cue. Combined RA plus FGF signaling is required for complete limb pattern, but FGFs alone distalize, mirroring developmental AER FGF role in maintaining distal progressive zone during outgrowth.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: FGFs promoting distal structures in axolotl limb regeneration.

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In newt limb regeneration, retinoic acid typically:

In newt forelimb regeneration retinoic acid typically proximalizes positional identity reprogramming distal blastema toward proximal fate. Applied topically or via implanted beads, RA activates RAR-dependent transcription of Meis1/2 and proximal Hox9 genes, increases Prod1 expression level characteristic of proximal cells, suppresses distal HoxA13. Resulting regenerate duplicates proximal structures, converting wrist blastema into complete limb. Endogenous gradient with high RA proximally shapes normal pattern. Dose-dependent transformations demonstrated by Maden confirm proximalizing action; thus RA functions as respecifying morphogen resetting positional memory rather than eliminating identity or having no effect.

Ref: NCBI Bookshelf, Limb Regeneration: RA proximalization, Meis, Prod1 regulation in newt limb regeneration.

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Salamander limb regeneration proliferation is dependent on:

Axolotl limb blastema proliferation and formation depend absolutely on nerve presence providing trophic factors. Denervated limbs after amputation fail to form blastema, undergo fibrosis. Nerves supply FGF2, neuregulin, BMP, anterior gradient protein nAG, maintain apical epidermal cap expression of FGF8 and Wnt5a, prevent differentiation. Accessory limb model demonstrates sufficient nerve fibers diverted to wound plus skin graft induces ectopic blastema without amputation. Molecularly nerve signals activate blastema cell cycle, sustain Myc and maintain MMP activity degrading matrix. Hence limb regeneration proliferation is nerve-dependent distinguishing salamanders from nerve-independent regeneration in other species.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Nerve dependence of salamander limb regeneration and proliferation.

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What structure maintains positional information during limb regeneration in salamanders?

During salamander limb regeneration blastema itself carries and maintains positional information acquired from stump level. Cells retain memory through persistent expression of Meis1/2, HoxA9, HoxA13, and cell surface protein Prod1 gradient correlating with proximal-distal origin. Transplantation of proximal blastema to distal location results in intercalation, distal to proximal does not unless proximalized by RA. Blastema sorting assays demonstrate differential adhesion based on Prod1 levels preserving positional disparity. Therefore blastema constitutes repository of positional identity integrating retinoid and FGF cues ensuring regenerate restores correct pattern with faithful proximal-distal polarity.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Blastema maintains positional information via Prod1 and Meis.

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Proximal-distal positional values in limb regeneration are influenced by gradients of:

Proximal-distal positional values determining which segments regenerates are encoded by counter-gradients of retinoic acid proximally and FGFs distally. High RA proximal limb activates Meis1/2 specifying proximal identities, distal FGF8/10 from apical cap promotes HoxA13 distal fate. Blastema cells interpret RA-to-FGF ratio through intracellular receptors regulating Prod1 adhesion molecule gradient and Hox codes. Experimental manipulation: RA treatment proximalizes, excess FGF distalizes. Affinophoresis experiments sorting proximal and distal blastema cells based on differential adhesion corroborate gradient encoding positional memory and intercalary regeneration behavior following positional disparity grafting.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: RA and FGF gradients controlling proximal-distal identity.

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Dedifferentiation to form a blastema is characteristic of:

Epimorphosis is regeneration type defined by formation of blastema mass through dedifferentiation of mature cells or mobilization of reserve progenitors forming proliferative growth zone that reconstructs lost structure. Unlike morphallaxis where existing tissue repatterned with minimal growth, epimorphosis involves extensive proliferation, histolysis of stump matrix, wound epidermis specialization. Examples include salamander limbs, zebrafish fins, lizard tails, requiring nerve trophic factors, FGF signaling, matrix metalloproteinases. Blastema later undergoes patterning and redifferentiation, restoring original architecture, thus representing growth-driven regeneration rather than remodeling or compensatory hypertrophy.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Epimorphosis and blastema formation via dedifferentiation process.

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Retinoic acid influences limb regeneration by:

Retinoic acid influences amphibian limb regeneration by resetting proximal-distal positional value toward proximal identity. Binding RAR-RXR nuclear receptors activates transcription of Meis1, Meis2, HoxA9, and surface protein Prod1 characteristic of proximal cells, while repressing distal genes like HoxA13. Distal blastema exposed to RA behaves as proximal, generating full limb duplication when grafted. Endogenous gradient formed by Raldh2 synthesis proximally and Cyp26b1 degradation distally patterns normal limb. Hence exogenous RA proximalizes positional identity, reprogramming blastema cells to adopt more proximal fate during regeneration and development processes.

Ref: Nature Reviews, Developmental Mechanisms: RA as proximalizing morphogen in limb regeneration and Meis activation.

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Salamander limb regeneration involves:

Axolotl and newt limb regeneration proceeds via epimorphosis requiring wound epidermis, nerve-derived trophic signals, immune modulation. Stump tissues including muscle, dermis, cartilage, Schwann cells undergo dedifferentiation to lineage-restricted progenitors accumulating beneath apical epidermal cap as blastema. These multipotent progenitor cells proliferate driven by FGF8, FGF10, BMP, Wnt signaling, maintain positional memory via Prod1, Meis, Hox gradients, then redifferentiate reconstructing missing limb. Nerve deviation experiments show blastema formation depends on threshold neurotrophic support, demonstrating complex orchestration of growth factors, extracellular matrix remodeling, and re-deployment of developmental programs.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Salamander limb blastema formation and multipotent progenitors.

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Axolotl limb regeneration treated with retinoic acid will:

Axolotl limb amputation at distal level normally regenerates only distal parts. Retinoic acid treatment proximalizes blastema positional memory activating proximal markers Meis1/2, HoxA9 and Prod1, converting distal blastema to proximal identity. Consequently distal wrist blastema treated with RA regenerates complete limb from stylopod through digits, representing serial duplication beyond amputation plane. Experiments by Maden and colleagues showed dose-dependent proximalization: low doses duplicate forearm, high doses duplicate entire arm. This property demonstrates RA as reprogramming agent altering positional value and Hox gene expression during epimorphic regeneration.

Ref: Alberts, Molecular Biology of the Cell, Chapter 22: RA proximalization and complete limb regeneration in axolotl.

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