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UNT BIOL 4330 - Tetrapod Limb Formation
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BIOL 4330 1st Edition Lecture 12 Outline of Last Lecture I. Neural Crest CellsII. Regionalization of the Neural CrestIII. Bone FormationIV. Intramembraneous BonesOutline of Current Lecture I. InductionII. The 3D Coordinate System of the LimbIII. Roles of AERIV. Limb FormationCurrent LectureTetrapod limb formation is pattern formation. Pattern formation is the set of processes by which embryonic cells form ordered spatial arrangements of differentiated tissues.It is one thing to differentiate chondroblasts from osteoblasts that synthesize the cartilage and bone matrices – it is another to produce those cells in a temporal-spatial orientation that generates a functional boneInduction – the presence of one tissue is required for the formation of a structure in another- Instructive interaction: the inducing tissue informs the fate of the responding tissue.- Permissive: the inducing tissue is necessary for the response to occur but not sufficient to specifyultimate cell fates (Cell fate information is in the responding tissue)The 3D Coordinate System of the Limb1st – proximal-distal axis (“close-far” shoulder-finger or hip-toe) – the bones of the limb are formed by endochondral ossification – initially cartilaginous & replaced by bone.2nd – anterior- posterior axis (thumb-pinkie)3rd – dorsal-ventral axis – our plams (ventral) are readily distinguishable from our knuckles (dorsal)These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.The proximal-distal axis (shoulder-finger; hip-toe) appears to be regulated by protein of the fibroblast growth factor (FGF) family.The anterior-posterior axis (thumb-pinkie) seemed to be regulated by Sonic hedgehog.The dorsal-ventral axis (knuckle-palm) is regulated in part by Wnt7a.Fgf10 stimulates Wnt proteins in the surface ectoderm and these Wnt proteins instruct the surface ectoderm to make Fgf8.Once Fgf8is made in the surface ectoderm, the surface ectoderm elongates to become the apical ectodermal ridege (AER).The AER is the structure that is the source of Fgf’s for limb outgrowth and patterning. AER tells the mesenchyme cells directly beneath is to make Fgf10 – creating a positive feedback loop. Mesodermal Fgf10 tells the surface ectoderm to make Fgf8 and the surface ectoderm tells the underlying mesoderm to make Fgf10.Hox gene in lateral plat mesoderm activates Tbx, which induces Fgf10 from lateral plate mesoderm & Fgf10 induces Wnts in surface ectoderm. These Wnts induces FGFs in the surface ectoderm – causing ectoderm to become AER-FGFs and Wnts from AER stimulate production of Fgf10 in the mesoderm beneath it.Roles of AER1) Maintaining mesenchyme beneath in a plastic/proliferating state to enable the linear (Prox to distal or shoulder to finger) growth of limb2) Maintain expression of those molecules that generate the ant-post (thumb-pinkie) axis3) Interacting with the proteins specifying the ant-post and dorsal-central (knuckle-palm) axes so that each cell is given instructions how to differentiateAER is required for outgrowth P/D axis. If AER removed, limb development ceases.AER is not responsible for specification A/P axis. If extra AER is grafted supernummary, structures are formed near the distal end.If leg mesenchyme is placed directly beneath the wing AER, toes develop.AER is permissive – mesoderm is instructive. If limb mesenchyme is replaced by non-limb mesenchyme beneath the AER regresses and development ceases. Since AER is replaced by FGF bead, development can continue.Sequential removal of the AER results in a progressively less disruption of development in a Proximal/Distal fashion.Model for limb patterning, whereby the proximal-distal axis is generated by opposing gradients of retionoic acid (RA) from the proximal flank and FGFs and Wnts from the distal AER.The polarizing region specifies position along the limb’s antero-posterior axis.Sonic hedgehog produced by the polarizing region is likely to be the primary morphogen patterning the anterior-posterior axis of the limb.Ectopic expression of mouse sonic hedgehog in the anterior limb causes extra digit formation. The dorsal-ventral axis of the limb is controlled by the ectoderm. Wnt-7a expressed in the dorsal but not the ventral ectoderm induces Lim1.It is crucial that patterning all 3 limb axes is intergrated so that the correct anatomy of the limb is generated. The signals controlling limb-bud outgrowth and patterning such as FGF and Shh are the same in the chickwing and leg, but they are interpreted differently.Hox genes establish the polarizing region and also provide a code for limb patterning. Hox genes specify position along the antero-posterior axis of the vertebrate body.Limb muscle is patterned by the connective tissue. Limb muscle cells have a different origin from the limb connective tissue cells, including muscle-associated connective tissue cells.The cells that give rise to the limb muscles migrate into the limb bud from the somites as myoblasts at a very early stage. After migrated the myoblasts multiply and initially form dorsal and ventral blocks of presumptive muscle.The muscle pattern is determined by the connective tissue into which the myoblasts migrate rather than the myoblasts


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UNT BIOL 4330 - Tetrapod Limb Formation

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