Adult body is a specialized machine with each part perfected for its intended function. Most cells in the adult body are irreversibly differentiated to form parts of this machine. Such as the heart, where the myocardial cells form an interconnected bundle that can contract in unison upon receiving instructions from the cardiac pacemaker. Or the brain, which contains millions of neurons connected through synapses designed for this command centre to integrate and control all the activities of the organism from digestion to perspiration to locomotion to reproduction to introspection. The life of every animal starts off from a single cell known as the zygote. This single cell divides prolifically to give rise to the embryo, where progressively cells exit cell cycle, specialize for a particular function and differentiate. However, there are organs in the body that undergo regular wear and tear and require continuous repair and replacement of lost cells. Examples include the...