Axolotls are a type of salamander that can regenerate lost limbs. This ability is due in part to an ultra-sensitive on-off switch that controls the expression of a gene called Pax7.

Pax7 is a transcription factor that is essential for the development of muscle, cartilage, and other tissues in the limbs. When Pax7 is expressed, these tissues can grow and regenerate. However, when Pax7 is turned off, these tissues stop growing and eventually die.

In axolotls, the Pax7 gene is regulated by a microRNA called miR-21. miR-21 is a small molecule that binds to the Pax7 gene and prevents it from being expressed. This means that in normal axolotls, Pax7 is only expressed in the developing limbs.

However, when an axolotl loses a limb, the expression of miR-21 is decreased. This allows Pax7 to be expressed, which in turn leads to the regrowth of the lost limb.

The ultra-sensitive nature of the Pax7 on-off switch is essential for the ability of axolotls to regenerate their limbs. If the switch were not sensitive enough, Pax7 would be expressed in too many cells and the axolotl would develop tumors. Conversely, if the switch were too sensitive, Pax7 would not be expressed in enough cells and the axolotl would not be able to regenerate its limbs.

The Pax7 on-off switch is a remarkable example of how a single gene can control a complex process like limb regeneration. Understanding how this switch works could lead to new treatments for injuries and diseases that affect the limbs.

Additional information

* Axolotls are not the only animals that can regenerate their limbs. Other animals that can regenerate their limbs include salamanders, newts, and starfish.

* The ability to regenerate limbs is thought to be a primitive trait that has been lost in most vertebrates.

* Scientists are studying the genetics and molecular biology of limb regeneration in order to develop new treatments for injuries and diseases that affect the limbs.