#Sonic Hedgehog

Eye Lens Induction -l

Which experiment demonstrated the role of Shh in separating the eye field?

Evidence for Shh involvement in eye field separation came from loss-of-function genetics. Chiang and colleagues generated Shh null mice by targeted disruption and observed single forebrain vesicle, absence of ventral diencephalon and fusion of optic primordia into single cyclopic eye located ventrally. Molecular analysis revealed ectopic expansion of Pax6 and loss of Pax2 across midline, confirming Shh normally represses Pax6 medially. This phenotype recapitulated human holoprosencephaly associated with SHH mutations, establishing experimental proof that midline-derived Shh signal bisects single eye field into bilateral domains essential for two eyes and nose formation.

Ref: Chiang et al. Nature 1996; Gilbert, Chapter 12: Demonstration of Shh role in eye field separation - knockout cyclopia.

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Eye Lens Induction -l

What happens in a Sonic Hedgehog (Shh) mutant embryo?

Shh knockout illustrates midline patterning defects. Normally Shh from prechordal plate suppresses Pax6 medially and promotes hypothalamic and optic stalk identities, splitting single eye field. In Shh null mice described by Chiang et al., failure of this suppression leaves Pax6 expressed across midline, preventing invagination of two vesicles. Forebrain also fails to divide, resulting in holoprosencephaly spectrum with most severe manifestation as cyclopia, single median eye with fused retina and single lens, alongside loss of ventral forebrain structures, confirming Shh requirement for bilateralization of eye primordia.

Ref: Chiang et al., Nature 1996; Gilbert, Developmental Biology, Chapter 12: Shh mutant phenotype - cyclopia and holoprosencephaly.

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Eye Lens Induction -ll

Which experiment demonstrated the role of Shh in separating the eye field?

Role of Sonic hedgehog in splitting single eye field into bilateral domains was demonstrated by knockout resulting in cyclopia. Shh-null mice, zebrafish cyclops mutants deficient in Nodal upstream of Shh, and pharmacologic cyclopamine inhibition of Smoothened all exhibit failure to suppress midline Pax6, retaining fused eye field leading to single median eye cup, absent hypothalamus, and holoprosencephaly. Overexpression of Pax6 in midline does not split field, optic vesicle graft to trunk tests lens competence not midline, L-Maf overexpression drives crystallin. Cyclopia after SHH loss directly illustrates midline inhibitory function and organizer role.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Shh knockout cyclopia demonstrating midline split function.

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Eye Lens Induction -ll

What happens in a Sonic Hedgehog (Shh) mutant embryo?

Sonic hedgehog mutant embryos exhibit severe midline defects including cyclopia due to failure to suppress Pax6 centrally and split eye field. In SHH-null mouse or zebrafish cyclops mutants, two optic vesicles remain fused, resulting in single median eye chamber with merged retinae and absence of optic stalk and hypothalamus. SHH normally promotes ventral forebrain identity and proximal optic stalk Nkx2 expression. Dorsalization of neural tube reflects BMP excess, optic vesicle still forms but unfused, neural crest migration defect unrelated. Cyclopia thus hallmark of SHH pathway disruption during forebrain patterning and midline development.

Ref: NCBI Bookshelf: Shh mutant cyclopia – failure of eye field separation and holoprosencephaly.

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Eye Lens Induction -ll

What is the primary function of Sonic Hedgehog (Shh) in eye development?

Sonic hedgehog produced by prechordal mesendoderm and ventral forebrain plays crucial role in separating single eye field into two bilateral fields. Early eye field expresses Rx and Pax6 broadly; SHH induces midline repression of Pax6, promotes proximal optic stalk character, and allows lateral optic vesicle outgrowth retaining Pax6. This midline clearance and proximal-distal patterning prevent cyclopia. SHH does not directly induce lens, activate Pax6 broadly, or drive neural crest migration, but patterns forebrain midline essential for paired eye formation and bilateral optic vesicle evagination.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Eye field separation – SHH suppression of Pax6 midline.

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Limb Development -l

Which experiment demonstrated that Shh signaling is necessary for limb outgrowth?

Necessity of sonic hedgehog signaling for limb outgrowth and anteroposterior patterning was demonstrated through pharmacologic and genetic inhibition approaches. Treatment with cyclopamine blocking Smoothened signal transduction or Shh-null mouse embryos exhibited severely truncated limbs lacking posterior digits 3-5 and premature downregulation of Gremlin1 resulting in increased BMP activity that represses apical ridge FGF expression. Blocking retinoic acid synthesis shifts axial boundaries, ridge removal tests maintenance, Tbx5 overexpression alters identity, but specific SHH inhibition uniquely reveals dual requirement for growth and patterning in tetrapod limbs.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: SHH inhibition – cyclopamine and Shh mutants affecting outgrowth.

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Limb Development -lll

Which factor plays a major role in regulating digit identity by controlling Sonic Hedgehog (Shh) expression?

Digit identity along anterior-posterior axis depends on graded Sonic Hedgehog from zone of polarizing activity. SHH induces Gremlin1, which antagonizes BMPs, thereby sustaining AER FGFs and forming SHH-Gremlin-FGF feedback loop that prolongs SHH exposure and expands progenitors. Higher and longer SHH promotes posterior digit fates through sustained Hoxd13 and Gli activator activity and repression of Gli3 repressor. Interruption of Gremlin alters SHH duration and range, changing digit number and morphology. TBX5 specifies forelimb field, BMP4 drives apoptosis, Wnt3a induces AER.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: ZPA SHH-Gremlin loop and digit identity.

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Fertilization in mammals-lll

Shh signaling blocked by:

Sonic hedgehog signaling involves cholesterol-modified ligand binding Patched relieving inhibition of Smoothened GPCR-like protein, activating GLI transcription for ventral patterning, limb bud and organogenesis. Cyclopamine is steroidal alkaloid from Veratrum californicum acting as Smoothened antagonist, binding transmembrane domain preventing downstream GLI activation. Ingestion during pregnancy causes holoprosencephaly and cyclopia in lambs. Alcohol, BPA and retinoic acid teratogens act via different pathways: alcohol disrupts L1CAM and Shh indirectly, BPA via estrogen receptors, retinoic acid via HOX dysregulation. Cyclopamine is canonical specific chemical probe that experimentally blocks Shh, used to model Shh deficiency disorders and study morphogen gradients.

Ref: Chen et al., Genes Dev 2002: Cyclopamine inhibition of Smoothened blocks Sonic hedgehog signaling in developmental patterning.

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