How do you make a limb? Vertebrate limbs are classic models in organogenesis, and we know a fair bit about the molecular events involved. Limbs are induced at particular boundaries of axial Hox gene expression, and the first recognizable sign of their formation is the appearance of a thickened epithelial bump, the apical ectodermal ridge (AER). The AER is a signaling center that produces, in particular, a set of growth factors such as Fgf4 and Fgf8 that trigger the growth of the underlying tissue, causing the growing limb to protrude. In addition, there's another signaling center that forms on the posterior side of the growing limb, and which secretes Sonic Hedgehog and defines the polarity of the limb—this center is called the Zone of Polarizing Activity, or ZPA. The activity of these two centers together define two axes of the limb, the proximo-distal and the anterior-posterior. There are other genes involved, of course—this is no simple process—but that's a very short overview of what's involved in the early stages of making arms and legs.
Now, gentlemen, examine your torso below the neck. You can probably count five protuberances emerging from it; my description above accounts for four of them. What about that fifth one? (Not to leave the ladies out, of course—you've also got the same fifth bump, it's just not quite as obvious, and it's usually much more tidily tucked away.)
Continue reading "Generic bumps and recycled genetic cascades" (on Pharyngula)