12th Class Biology Organisms and population Repair and Regeneration

Definition : It is that post-embryonic morphogenetic phenomenon which when temporarily stimulated brings about repair of the damaged cells/Tissues, or replacement or redevelopment of severed body parts or reconstruction of whole body from a small body fragment.

Capacity for regeneration : Among animals, power of regeneration was first discovered in Hydra by Trambley, in 1740. The capacity of repeated regeneration, though, present throughout the animal kingdom, but to varying degree. It is more marked in the lower animal than in the higher animals. Among invertebrates, protozoans, sponges and coelenterates, the regeneration capacity is very high. In higher animals, regenerative ability is much greater in the embryonic and larval stages than in the adult. In man, it is restricted to healing of injured tissues such as skin, muscles, bones, blood vessels and nerves; the lost organs cannot be regenerated.

Types of regeneration : Regeneration is of two main type - Reparative and Restorative.

(1) Reparative regeneration : In this, multicellular organism has the power only to repair certain damaged cells of the body. It is a common phenomenon observed in both invertebrates as well as the vertebrates.

(2) Restorative regeneration : In this, a multicellular organism can redevelop the severed body parts or the whole body can be formed from a body segment. It is very common in invertebrates. It may occur by epimorphosis or morphallaxis. The power of restorative regeneration varies in different groups of organisms e.g., 

(i) Autotomy power in some animals, some part of the body is broken off the body on being threatened by the enemy or predator. This phenomenon of self mutilation of body is called autotomy. The lost part may be tail, limb, viscera or arm e.g.,,

• Crabs break of their leg on approaching the enemy.
• Lizards throw off their tail.
• Holothurians (Echinoderm) throw off their internal viscera (respiratory tree etc.). It is called Evisceration.
• Starfish (Echinoderm) can regenerate the whole arm.
• Autotomy is a special adaptation for escaping the danger of attack by enemy or predator.

(ii) The climax of regeneration in which whole body can be developed from a body fragment is found in Hydra among the coelenterates; Scypha among the sponges and Planaria among flat worms.

Mechanism of regeneration : T.H. Morgan recognized two primary mechanism of regeneration in animals.

(1) Morphallaxis : It is the reconstruction of an entire animal from a small fragment by reorganizing the existing cells. The regenerated animal is far smaller than the original one after the completion of the process. It grows to attain the normal size. e.g., Hydra

(2) Epimorphosis : It replaces a lost organ of the body by proliferating new cells from the surface of the injured part. Regeneration of an appendage in an arthropod, arm in a starfish, limb in a salamander and tail in a lizard occurs in this manner.

Regeneration of a limb of a newt or salamander : Newt/salamander has very high power of regenerating their lost limb by the process of restorative regeneration. It involves the following steps :

(1) Wound healing : The epidermal cells from the edges of the cut migrate and spread over the exposed surface. This is known as wound healing.

(2) Blastema formation : A few days after the healing of the cut, the undifferentiated cells accumulate inside the epidermis. Due to this cellular aggregation, a stumpy outgrowth or bulge is formed. This is known as regeneration bud or blastema.

(3) Redifferentiation and morphogenesis : The blastema develops rudiments of digits by indentation at the free edge. These grow out into new digits.

(4) Growth : The regenerated limb increases till it attains the size of a normal limb.

Control of regeneration : Though exact control mechanism for the regeneration of a lost limb in a salamander / newt is not known but a number of experiments have confirmed its dependency upon nerves, hormones and epithelial cover.

(1) Epithelium : C.S. Thornton (1960) has shown that the presence of the wound epithelium which covers the amputated surface is necessary for blastema formation. This epidermal cap acts as a stimulus for the aggregation of blastemal cells of the mesenchyme. This establishes an epithelium-mesenchymal interaction. It was reported that if the epidermal cap is placed eccentrically, an eccentric blastema is formed while if it is continually removed, blastema formation can be prevented.

(2) Neural trophic factor : It has been shown that if a limb is first denervated and then amputated, or if nerves are by any means blocked from penetrating the epidermis, no regenerative blastema is formed. But if the amputated limb is denervated after the initiation of blastema formation, regenerative process continues and a new limb is formed.

(3) Hormones : Adrenal glands and pituitary gland have been found to influence the regenerative process considerably.

Examples of regeneration in different animal groups : The regeneration was first discovered in Hydra by trembley in 1740. Later, it was also found in other animal groups but to a varying degree.

(1) Invertebrate : Power of regeneration are found in following phylum of invertebrate.

(i) Protozoa : Among the protozoans, very high power of regeneration was found in Amoeba and it was confirmed that the presence of nucleus is essential for regeneration as anucleate part finally dies.

(ii) Sponges : Sponges have remarkable power of regeneration. Any part of the body injured or cut off is readily repaired or replaced. Small fragments of sponges grow into complete individuals.

(iii) Coelentrates : Coelenterates too have a remarkable power of regeneration. Hydra shows regeneration to an amazing degree. Trambley (1740) reported that if Hydra is cut transversely into two or more parts, then each fragment, as small as 0.004 mm, can grow into a complete organism. In Hydra, regeneration occurs by morphallaxis. Hydra has a unique capacity of regenerating its hypostome (oral end) again and again.

(iv) Flatworms (Platyhelminthes) : Very high power of regeneration has been reported in planaria among the flat worms. Like Hydra, a small fragment of Planaria can also develop into a complete animal though of smaller size than the parental animal. Internal organs used up during starvation are also fully regenerated if food becomes available.

(v) Nematodes : The power of regeneration is poor in nematodes. Superficial wounds are, however, healed up.

(vi) Annelids : Annelids have less power of regeneration than the planarians. If an earthworm or other oligochaete is cut in two halves, then each half may regenerate the lost parts. But in majority of the annelids, the regeneration power is restricted and only 4 or 5 segments at either end or both ends of the body can be regenerated.

(vii) Arthropods : Certain insects, crabs, lobsters and spiders can regenerate a lost leg. Crayfish regenerates any of the appendages and the eyes when removed. Regeneration is faster in the young than in the adult. The regenerated part may not always be similar to the lost one.

(viii) Molluscs : Molluscs have low power of regeneration. Gastropods are capable of regenerating certain body parts only like eyes, eye stalks, the parts of head and foot. The cephalopods (e.g., cuttlefish) can also regenerate their arms only.

(ix) Echinodermates : The power of regeneration is high in the echinoderms. Almost all the echinoderms have good power of autotomy and regeneration e.g., starfish can lose and regenerate upto 4 arms; Holothurion (sea cucumber) can lose its respiratory tree and visceral organs (called evisceration) in self defence.

(2) Vertebrates : Many vertebrates also possess a good power of regeneration.

(i) Fishes : The ammocoetes larva of lamprey can regenerate the lost tail. Some fishes are known to regenerate parts of fins.

(ii) Amphibians : The salamanders, newts and axolotl larvae are outstanding in their regenerative capacity among the vertebrates. They can regenerate a severed arm or leg. They can also regrow tail, jaws, external gills, intestine and retina. Tadpole of frog and toad can regenerate tail and hind limbs. Adult frog and toad are unable to regenerate limbs.

(iii) Reptiles : Certain lizards can regenerate a lost tail. The wall lizard, when threatened, can sever its tail near the base, leaving the moving tail to detract the predator while it escapes, and later regenerates a new tail.

(iv) Birds : Certain birds may regenerate beak.

(v) Mammals : Mammals are unable to regenerate any of the external parts, but can readily regenerate the liver. This organ has the maximum capacity of regeneration. Removal of over half of the liver is fully replaced.

Different animal groups and their regenerative body parts