97772
| Question | Answer |
| AER | Apical ectodermal ridge: at apex of limb bud, controls rate of cell proliferation, maintains genes that control patterning of antero-posterior axis and dorso ventral axis. |
| Progress zone | Signals from AER to adjacent 200um of mesoderm tell cells to become humerus, radius/ulna or digits. |
| ZPA | Zone of polarising activity, is a small piece of tissue at posterior edge of limb bud. Expresses molecule Sonice hedgehog (shh) which is diffusible, so high conc at posterior (digit 5/little finger) and low conc at anterior (digit 1/thumb). Requires fgf (fibroblast growth factor) for Shh to be expressed. |
| Dorsal/ventral axis (back of hand/palm) | Regulated by ectoderm covering limb mesoderm. Wnt7a induces Lmx1 expression in dorsal mesenchyme. Engrailed gene is expressed on ventral side. Fibroblast growth factors (Fgf's) ensure dorsal/ventral signals are positioned correctly. |
| BMP's (Bone morphogenetic proteins) | Responsible for sculpting the limb from its paddle shape by apoptosos (programmed cell death) between digits, radius/ulna and at limb ends. This removes cells and allows skeletal elements to emerge. |
| Evolutionary differences: Fish | Do not have structures similar to digits, so have same proximal Hox gene expression but not distal. |
| Evolutionary differences: Duck | Soft tissue persists between digits so no cell death here but normal apoptosis in other regions of limb. This can be achieved in other animals by placing BMP inhibitors between digits. |
| Evolutionary differences: Snake - forelimb | Loss associated with changes in Hox gene expression as there is expansion in the expression along the axis leading to no cervical-thoracic boudary so no site for forelimb to develop. |
| Evolutionary differences: Snake - hindlimb | Some snakes show remnants of hindlimb and a pelvic girdle eg python. Hind limb buds do start to form but AER doesnt form properly. This means no P-D outgrowth or A-P pattern. Results in tiny spur like a small femur. |
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