Question

5. Limb development. Different species display characteristic digit patterning along the anterior-posterior axis. The forelimbs of bats are modified such that the posterior digits are elongated and maintain their webbing (to make the functional bat wing). Researchers observed that in addition to the normally timed expression of Sonic hedgehog (Shh) in the bat ZPA, there is a second phase of both Fgf8 and Shh expression in the developing forelimb in the interdigital tissue. Using what you know about the functions of Fgf8 and Shh in limb outgrowth in the chick that we talked about in class, how does the second phase of Fgf8 and Shh contribute to both the elongation of the forelimb digits and to the maintenance of the webbing between the digits? PLEASE EXPLAIN IN AT LEAST TWO PARAGRAPHS

Answer 1

Initially the mesenchymal cells which is about to form the limb field develops on the lateral side of the embryo called lateral plate mesoderm. They are then relocated to the anterior and posterior regions of the embryo by the wint 2b and wint 8c forming the limb field. From the limb field they first starts to develop as a bud of tissues from both the sides of the embryo and proliferate as limb mesenchyma. Along with that epidermal cell will also grow and forms a local thickening called 'apical ectodermal ridge (AER) on the tip.The fgf8 signeling from this AER acts on the limb field initiating a possitive feedback signeling from limb field as fgf 10 and Wnt. Fgf 8 also specifies the posterior mesenchyme to form the 'zone of polarizing activity' which determine the anterior posterior axis specification (thumb-pinkie). This zone secrete the 'Shh' and they form the concentration gradient from posterior to anterior axis(A-P). Wnt signaling determines the dorsal -ventral axis (knuckles-palm).

Shh maintains the skeletal development in bats. In bats hint limb digits are uniform in length but for forelimbs,posterior digits are elongated and joind by tissue along with short thumb. In the certain webbed organisms the tissues between the digits are retained, because they dont have BMP mediated apoptosis. A second phase expression of  Shh and Ptc 1 is observed at interdigital tissues of bats, they secrete 'Gremlin' a BMP inhibitor in the interdigit regions and they restrict the action of apoptosis by inhibiting BMP action at those regions. Fgf 8 expression is also observed at interdigital region before the action of Shh .So the webbing and digit expantions are maintained by fgf8 and shh.