Based on cellular mechanisms, regeneration can be divided into two broad categories : morphallaxis and epimorphosis.
Morphallaxis refers to the type of regeneration in which lost body parts are replaced by the remodelling of the remaining tissue. In this type of regeneration, little or no cellular proliferation takes place during the regeneration process. A classic example of an organism that regenerates using this mechanism is the Hydra. When a Hydra is cut into two pieces, two Hydra will be regenerated, both smaller than the parental Hydra. Once regeneration is completed, the two Hydra can continue to grow and reach the size of their original parent. In contrast to morphallaxis epimorphosis require active cellular proliferation prior to the replacement of the lost body part. For eg. Planarian, which are flatworms, regenerate using a mechanism in which pre-existing stem cells, known as neoblasts, begin to proliferate and migrate to the injured site in response to injury. These cells then form a mass of proliferating cells, known as regeneration blastema, that will later differentiate into the specialized cells that comprise the regenerated structure. Most tissue regeneration in mammals also belongs to the epimorphosis. For example, mammals can regenerate their muscle, bone epithelia of the skin and gut, blood, and some neurons by activating pre-existing stem cells or progenitor cells.