provided by Hox gene I. How Conserved is Tbx Gene Function?— i. Tbx gene expression in zebrafish is restricted to pelvic and pectoral fin buds ii. This observation and observations from comparative anatomy studies suggests that teleost pectoral and pelvic fins are homologous to tetrapod forelimbs and hindlimbs iii. Mutation of Tbx5 human homologue causes the Holt-Oram syndrome = defects in arm and heart development • Limbs can still form in absence of Tbx5, but Tbx5 is necessary for some aspects of limb formation J. Limb Bud Growth: Role of the AER— i. Once the limb bud has formed, its continued growth depends on the apical ectodermal ridge (AER) • AER = ridge running along the distal margin of the limb bud ii. Removal of AER prevents the growth of limb bud iii. Ectopic transplantation of the AER leads to the outgrowth of supernumary limb buds K. Limb Bud Growth: Role of the Progress Zone— i. Progress Zone—area of actively dividing mesenchyme located directly beneath the AER (derived from limb bud) ii. Reciprocal interaction b/w the progress zone and the AER (both talk to each other; both signals required for limb bud growth) = • Progress Zone => AER ➼ Cells of the progress zone induce and maintain the AER ❧ If limb mesenchyme is removed and replaced w/ non- limb mesenchyme beneath the AER, AER regresses and don’t get any limb growth ➼ Cells in the progress zone specify what type of limb to form = forelimb vs. hindlimb ❧ If leg mesenchyme is placed beneath the wing AER, distal hindlimb structures (leg) develop at end of wing ❧ However, if the leg mesenchyme cells are transplanted further away from the AER, these cells become wing structures • So, got no impact; there has to be close association b/w progress zone and AER for progress zone to influence AER formation • AER => Progress Zone ➼ AER keeps the cells of the progress zone in a mitotic state (cells in progress zone would stop dividing if AER removed)
Vandan Desai BIOL 442—Developmental Biology 6 L. Experiments Demonstrating the Interaction Between AER and Progress Zone— M. What Are the Molecules that Mediate AER-Progress Zone Interactions— i. FGF10 (expressed in the limb mesenchyme: progress zone ) induces the AER and synthesis of FGF8 from the AER • Occurs via a pathway involving Wnt3A in the ectoderm • Pathway = ➼ FGF10 is stabilized in hindlimb and forelimb region => ➼ FGF10 induces expression of Wnt3A in the AER (previously labeled ectoderm) => ➼ Wnt3A induces expression of FGF8 within AER => ➼ FGF8 secreted from AER will maintain FGF10 expression in mesenchyme/progress zone => ➼ FGF10 expression promotes cell division in mesenchyme IV. Specification of the Proximal-Distal Axis :: Assume AER transplanted to region where limb structures wouldn’t develop AER = provides stable FGF10 production & FGF10 promotes cell division; so, even if we don’t have AER, by inserting FGF10, we still get cell division for limb formation Not progress zone cells since not derived from limb bud • Proximal (humerus) => Distal (autopods) • Growth and differentiation of cells along proximal-distal axis is made possible by interactions b/w AER and progress zone
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