There are two types of regeneration in human, physiological and reparative. Physiological regeneration replaces cells during homeostatic maintenance, the process by which an organism maintains a stable internal environment.

In animal, there are four types of tissues, epithelial, connective, muscular, and nervous. Epithelial tissue, the tissue that covers the surfaces of an organ, is most capable of regeneration.

Human body routinely sheds epithelial cells from the skin, the respiratory tract, and the digestive tract. Blood is a connective tissue. The red blood cells have a turnover time about 180 days and are routinely regenerated and destroyed. The skeletal muscle can regenerate itself after an exercise. This is how bodybuilders build up their muscles. The nervous tissue, however, is a special case. The brain and the spinal cord form the central nervous system (CNS). The rest of the nerves form the peripheral nervous system (PNS). The CNS cannot regenerate by itself while the PNS can.

However, regeneration is limited in human. The human body can only regenerate from certain degree of injury. For example, if a limb is removed from the body, it cannot be regenerated. In this regard, skin has the most potential for regeneration. Surgeons often harvest the top layers of the skin from the thigh of a patient to prepare a skin graft to repair wounds in other places. The harvest site will slowly grow back. The harvested skin will also form blood vessels with the wound site. Besides the skin, liver is the most studied model for organ regeneration. If the liver is partially removed, the remaining organ will regrow to the original weight. However, the original shape is not restored.

Fortunately, regenerative medicine is under actively development. The bladder can be grown in a lab and already been used in patients. Many other organs can be regrown in the lab. Perhaps in the near future, they can be used to treat patients.