NEET Biology Organisms and population Repair and Regeneration

Repair and Regeneration

(i) 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.

(ii) Capacity for regeneration: Among animals, power of regeneration was first discovered in Hydra by Tremble, 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. The skin cells and epithelial cells lining the respiratory and digestive tracts are rapidly replaced. The turn-over time for skin cells is 1–2 weeks and for intestinal cells is only 2 or 3 days. Blood corpuscles have a limited life span and are continuously replaced. Other tissues, such as liver, pancreas and thyroid, can also repair damaged parts. The cells of the central nervous system are incapable of regeneration if damaged or lost. The inability of complex animals to regenerate the lost parts is the price of their specialization.

(iii) Types of regeneration: Regeneration is of two main type – Reparative and Restorative.

(a) 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.

Healing of a bone fracture, a skin wound, or a muscle tear are instances of reparative regeneration. This shows that fully differentiated cells retain the developmental potential. Maximum reparative regeneration is found in the liver of mammals. If a part of liver is surgically removed, then the cells of the remaining part undergo repeated mitotic divisions and original volume of the liver is maintained. Similarly, if one kidney of man is lost, the other kidney enlarges to take over the function of the missing kidney and is called compensatory regeneration.

(b) 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. 

(1) Autonomy 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 autonomy. 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.
• Autonomy is a special adaptation for escaping the danger of attack by enemy or predator.

(2) 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.

Differences between reparative and restorative regeneration

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

(a) 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. If hydra or planaria is cut transversely into two or several parts, each part develops into a complete organism. Hydra may arise from a fragment as small as 0.004 mm. Fragmentation and regeneration are usual forms of asexual reproduction in several animals.

(b) 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. The mechanism of regeneration can be studies from limb regeneration in salamander.

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.

A number of evidences have supported that the mitotically proliferating cells in a regenerating limb are derived by a regressive process called transformation or dedifferentiation of specialized or fully differentiated cells of the skin, muscles, bones, etc.

Differences between regeneration in Hydra and of limb in salamander.

Differences between Morphallaxis and Epimorphosis

(v) 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.

(a) 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. It was proposed that wound epithelium stimulates the secretion of certain proteinous growth factors like epidermal growth factor (EGF) and fibroblast growth factor (FGF). EGF stimulates the mitotic division of epithelial cells under the scabe of a skinned knee while FGF stimulates the mitotic division of endothelial cells to heal the injured blood vessels.

(b) 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. This confirmed that nerve supply is required for the initiation of regeneration but not for its completion. It is proposed that the neurons release some trophic factors which are essential for the regeneration of amphibian limb. Following neural trophic factors have been identified:

(1) Glial growth factor (GGF)

(2) Fibroblast growth factors (FGFs)

(3) Insulin-like growth factors (IGFs)

(4) Transferrin.

Out of these, IGFs are necessary for growth and cartilage differentiation in the regeneration blastema, while GGF and FGFs are small peptide growth factors with multiple biological functions and determine patterning growth and differentiation. Transferrin is an iron-transport protein which is necessary for mitosis in all the dividing cells.

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

(vi) 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.

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

(a) 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.

(b) 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.

H.V. Wilson (1907) reported that if the body of Scypha is dissociated into individual cells and cells are filtered through a fine silk cloth, then the individual cells by amoeboid movements aggregate into cell masses and each cell mass develops into a small functional sponge in culture medium.

(c) Coelentrates: Colenterates 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. This is called repetitive regeneration. It has made Hydra virtually immortal. In Hydra, the body parts usually retain their original polarity. If only the head is split into two, a peculiar two-headed Hydra is formed.

(d) 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.

Power of regeneration, like the metabolic rate, is highest at the head end and progressively decreases toward the tail end. This variation is called axial gradient. A section of planaria's body grows head on its anterior side where metabolic activity is greatest and tail on its posterior side where metabolic rate is lowest. This called polarity is regeneration.

It was also found that if only the head of Planaria is cut into two parts, then a two-headed monster is formed. In this way, many headed Planaria, called heteromorph may be produced.

Recent work on regeneration suggests that certain chemicals released at the cut surface attract free cells, the neoblast from the mesenchyme. These gather to form a region of growth called blastema. The latter gives rise to the new part.

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

(f) 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. Number of segments repaired is species specific e.g. in earthworm if more than fifteen anterior segments are lost, no anterior end is reformed. Among annelids, a very interesting regenerative phenomenon is found is Eunice (Polaloworm) in which posterior sexual part called epitoke is regenerated a number of times to increase the chances of sexual reproduction.

(g) 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. If only a part of the eye-stalk is cut off, normal regeneration will occur, but if the entire eye-stalk is removed, an antenna-like structure replaces it. Regeneration that produces a part different from the lost part is called heteromorphosis. Among the crustaceans, prawns have a peculiar power of losing their limbs in self defence and the lost limbs are regenerated. This power of self-amputation (or mutilation) is called the autotomy. It is a defensive mechanism and helps the animal to escape from the predators.

(h) 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.

(i) Echinodermates: The power of regeneration is high in the echinoderms. Almost all the echinoderms have good power of autonomy and regeneration e.g. starfish can lose and regenerate up to 4 arms; Holothurion (sea cucumber) can lose its respiratory tree and visceral organs (called evisceration) in self defence. Later, these visceral organs are regenerated. Some star fishes reproduce asexually by fragmentation in which an isolated arm with a part of central disc can develop into a complete organism by morphallactic regeneration.

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

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

(2) 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.

(3) 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. The regenerated tail differs from the original one in shape, absence of vertebrae and the kind of scales covering it. This is an another case of heteromorphosis.

(4) Birds: Certain birds may regenerate beak.

(5) 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. The regenerated liver resembles the original liver in volume but not in shape. Similarly, if one kidney is lost, the other enlarges and takes over the function of the missing kidney also. Such a reparative regeneration is known as compensatory hypertrophy.

From the above observation, it is evident that the power of regeneration is more in those animal groups which have simple body organisation, with less specialization and differentiation than the higher animal groups which have higher degree of specialization so less reserve cells are available for regeneration. The inability of complex animals to regenerate the lost parts is at the cost of their specilization. It is so because the lower organisms retain more of their embryonic organisation in their adult stage.

Different animal groups and their regenerative body parts.

(vii) Regeneration and embryonic development: The process of regeneration can be described as a special kind of embryonic development because of the following similarities:

(1) In both cases, the unspecialized cells undergo repeated divisions and finally undergo differentiation to form specialized cells.

(2) Like embryogenesis, there is mass migration of cells comparable to morphogenetic movements during gastrulation.

(3)  Cell differentiation occurring in the blastula leading to morphogenesis is comparable to cell interaction and cell differentiation as in developing embryo. This shows that developmental capacity is retained even in the adult organisms. The differentiated cells can undergo dedifferentiation and then dedifferentiation. However, regeneration is regulated by various hormones and the nervous system of the organism, whereas in embryogenesis the regulation of development may be through some other mechanisms.

Differences between regeneration and embryonic development.