Question No.  19562 :
Why does a lizard's tail grow again after it is cut off? Are there any other animals which can grow lost body parts again?

In developmental biology, regeneration is the process by which some organisms replace or restore lost or amputated body parts. Most organisms have a capacity for regeneration, although the extent of their abilities varies from that of simple planarian flatworms, which can grow an entire new body from a small strip of tissue, to the limited capacity of higher vertebrates to regenerate epidermal and other tissues in wound healing. A number of regenerative mechanisms have been evolved by different species. Autotomy, the spontaneous loss and replacement of a body part, occurs in many insects and crustaceans, and enables them to shed a crippled leg or claw. The discarded body part usually breaks off at a predetermined site; in some, the new part is an exact replica of the lost structure, while in others, the new part is functionally similar but anatomically different from the lost part. Many lizards can escape from their enemies by breaking off the end of their tail. Later they grow a new one, but the new tail does not include all of the tissues and structures of the original one. The developmental abilities of archaeocytes give sponges remarkable powers of regeneration. Even if large parts of a sponge's body are lost or damaged, they may be replaced or repaired. Many starfish can drop off arms as a defensive reaction. They can then regenerate (grow again) new arms to replace the old ones. If a starfish is cut in two, each of the pieces may regenerate into a new animal. Most starfish live for three to five years, but some may live longer. Regeneration is most common in invertebrates, occurring in almost all coelenterates and planarians, most annelids (segmented worms), and many insects. Among vertebrates, limited regeneration of limbs occurs in most fishes and salamanders, and tail regeneration takes place in larval frogs and toads (but not adults) and several reptile species. Although feather replacement may be considered a form of regeneration, few birds have the capacity to regenerate more complex structures. No mammals have the ability to regrow lost limbs or tails, but some species can regenerate other peripheral appendages, (e.g. a deer's antlers) or internal organs (e.g. the human liver). Regenerating tissues apparently follow a strict polarity, growing back in the proper orientation to the rest of the body. Since the most commonly lost structures are limbs and tails, the pattern of growth is usually outward from the body, suggesting that tissues more proximal than the injury contain all the necessary information to replace the lost part, but not those closer to the main trunk of the body. In some cases, however, as in fish fins, regeneration may occur in both directions. Regeneration is at the frontier of developmental biology and involves many unresolved problems. Scientists are trying hard to understand the molecular basis of regeneration. Question Asked By:

Name: Tham

Age Group: 13 to 20

Occupation Type: Student

Education Level: Others