#surface ectoderm

Eye Lens Induction -l

What happens when Pax6 is overexpressed in surface ectoderm?

Pax6 functions as sufficient master regulator for lens program. Misexpression experiments using PAX6 cDNA electroporated into embryonic chick surface ectoderm outside normal eye region, or promoter driven in frog epidermis, activates endogenous Sox2, FoxE3, Maf genes and delta-crystallin, culminating in formation of ectopic lens placodes and lens vesicles with fiber differentiation. This demonstrates that providing Pax6 competence factor can bypass requirement for optic vesicle signal in regions that already possess low levels of BMP and FGF. Thus overexpression converts non-lens ectoderm toward lens fate, confirming key competence role.

Ref: Altmann et al. 1997; Gilbert, Developmental Biology Chapter: Pax6 overexpression induces ectopic lens formation.

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Eye Lens Induction -l

Which experiment demonstrated that Pax6 is required in surface ectoderm for lens formation?

Competence requirement was proven by tissue recombination. Fujiwara et al. used rSey rats carrying Pax6 mutation. Wild-type optic vesicle grafted onto wild-type head ectoderm induces lens efficiently. When same wild-type vesicle is apposed to Pax6 mutant ectoderm, no lens forms despite normal inductive signals. Reverse combination, mutant optic vesicle with wild-type ectoderm, still induces lens, showing Pax6 needed in responding tissue not inducer. This demonstrates transcription factor acts cell-autonomously in ectoderm to confer ability to interpret BMP and FGF cues, rather than affecting signaling source or neural crest components.

Ref: Fujiwara et al. 1994; NCBI Bookshelf, Developmental Biology Chapter: Induction and Competence experiment - Pax6 in surface ectoderm.

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Eye Lens Induction -ll

What happens when Pax6 is overexpressed in surface ectoderm?

Overexpression of Pax6 in competent head surface ectoderm results in ectopic lens formation at sites outside normal eye region. Transgenic misexpression or electroporation of Pax6 into head ectoderm induces ectopic Sox2, Six3, L-Maf, and crystallin expression forming additional lens vesicles that may recruit surrounding mesenchyme and attempt optic cup-like differentiation. No suppression of FGF signaling or inhibition of Sox2 occurs; instead synergistic activation. Surface ectoderm normally expresses low Pax6, elevating levels confers increased competence recapitulating master regulatory property previously described for eyeless gene in Drosophila eye induction and vertebrates.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Pax6 overexpression inducing ectopic lens formation.

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Eye Lens Induction -ll

Which experiment demonstrated that Pax6 is required in surface ectoderm for lens formation?

Requirement of Pax6 in surface ectoderm for lens formation was demonstrated by tissue recombination: grafting wild-type optic vesicle onto Pax6-deficient head ectoderm from Small eye mutant fails to induce lens placode or crystallin expression, whereas Pax6-deficient optic vesicle grafted onto wild-type ectoderm still induces lens. Knockout of Pax6 in optic vesicle alone does not block lens, overexpression in cup does not address ectodermal necessity, neural crest knockout irrelevant. Mosaic recombinant experiment proves cell-autonomous competence role of Pax6 within responding surface ectoderm, not inducing tissue itself.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Pax6 tissue recombination – requirement in surface ectoderm.

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