#Wnt7a

Limb Development -l

What happens if Wnt7a is knocked out?

Dorsal-ventral patterning mediated by Wnt7a from dorsal ectoderm specifies dorsal mesenchyme fate via induction of LIM-homeodomain factor Lmx1b. Dorsal Lmx1b-positive cells form hairy skin, nails, and extensor tendons, while ventral ectoderm expressing En1 suppresses Wnt7a allowing ventral flexor and pad differentiation. Loss of Wnt7a ablates Lmx1b expression, producing double-ventral phenotype with ventral footpads duplicated onto dorsal surface and absence of dorsal nails. Zone of polarizing activity and SHH signaling remains intact, confirming Wnt7a exclusively instructs dorsoventral polarity and dorsal identity specification.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Wnt7a knockout and double-ventral limb phenotype.

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Limb Development -lll

What happens when Wnt7a is deleted in mice?

Dorsal ectoderm Wnt7a induces Lmx1b to dorsalize limb mesenchyme and overlying skin. Knockout of Wnt7a removes dorsal cue, allowing ventral program driven by Engrailed-1 and BMP to expand dorsally, resulting in biventral limbs with ventral skin, volar foot pads on both sides, absent nails, and ventralized musculature and tendons resembling double sole phenotype. Limb still forms because outgrowth depends on FGF10-FGF8 loop, not Wnt7a. Digit fusion results from BMP antagonism defects, not Wnt7a loss. Thus absent dorsal structures characterize Wnt7a deletion, illustrating DV axis failure and Lmx1b misregulation downstream.

Ref: Parr and McMahon 1995, Gilbert Chapter 20: Wnt7a knockout double-ventral limb phenotype.

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Tetrapod

What happens when Wnt7a is knocked out in mice?

Mice deficient in Wnt7a show dorsal-to-ventral transformation demonstrating dorsalizing role convincingly. Loss of dorsal Wnt7a eliminates Lmx1b expression in dorsal mesoderm, causing dorsal paw surfaces to form ventral-type footpads, ventral-like skin, absence of nails, double-ventral pattern with duplicate pads. Additionally Wnt7a is required to maintain SHH expression posteriorly; mutants exhibit shortened posterior digits and reduced posterior patterning. Phenotype reveals dual requirement for dorsal fate and posterior development. Engrailed1 expands dorsally in absence of Wnt7a, reinforcing ventralization and demonstrating reciprocal repression system governing dorsoventral axis establishment and maintenance.

Ref: NCBI Bookshelf, Developmental Biology: Wnt7a knockout and dorsal-to-ventral limb transformation in mice.

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Tetrapod

Which gene is expressed in the dorsal ectoderm and controls limb dorsal-ventral polarity?

Dorsal-ventral limb polarity originates from non-AER ectodermal signals rather than ridge. Wnt7a transcript localized specifically to dorsal ectoderm induces LIM homeodomain transcription factor Lmx1b in dorsal mesoderm, conferring dorsal characteristics including nails, dorsal tendon pattern, hair orientation and pigmentation. Ventral ectoderm expresses Engrailed1 repressing Wnt7a and activating BMP signaling promoting ventral fates like footpads. Ectoderm rotation experiments inverting DV orientation produce inverted limb polarity, confirming ectodermal instruction. Wnt7a also contributes to maintaining SHH posteriorly, integrating axes, but primary function remains dorsalizing mesenchyme through Lmx1b induction pathway.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Wnt7a dorsal ectoderm and Lmx1b dorsalization of mesoderm.

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