Question

Which of the following genes is expressed in the zone of polarizing activity and whose concentration gradient is responsible for anterior-posterior patterning?

Group of answer choices

a. Fgf8

b. Fgf10

c. Tbx5

d. Tbx4

e. Shh

Answer 1

The zone of polarizing activity (ZPA) is a space of mesenchyme which contains signals that instruct the developing limb bud to form along the anterior or posterior axis. Limb bud is covered by ectoderm and is an undifferentiated mesenchyme. Eventually, the limb develops into bones, tendons, muscles and joints. Limb bud development involves different genes, signals and a unique region of ectoderm called apical ectodermal ridge.

Fgf8 gene-(fibroblast growth factor-8), Fgf10 gene and Shh (sonic hedge hog)genes expressed in the zone of polarizing activity.

Patterning along the limb bud requires signals from many sources and involved protein called transcriptional factors. It helps to control the rate at which a gene is transcribed. The limb bud expresses a transcriptional factor (TF) called ALX4 at the anterior part of the mesoderm, with the TF - HOXB8 being expressed at the posterior portion. The ZPA forms where the Hox8 region joins the apical ectodermal ridge. The apical ectodermal ridge expresses FGF8 which induces- shh expression in the posterior mesoderm. Then shh (Sonic hedgehog gene) stimulates FGF4 to be expressed in the posterior part of apical ectodermal ridge. After, there is co-dependence between FGF4 and shh for their subsequent expression and maintenance. The sonic hedge hog (shh) gene is expressed in the cells of the polarizing region.