Regeneration in animals

Regeneration

Regeneration can be defined as the natural ability of living organisms to replace worn out  or lost parts, repair or renew damaged or lost parts of the body, or to reconstitute the whole body from a small fragment during the post embryonic life of an organism. Regeneration is therefor also a developmental process that involves growth, morphogenesis and differentiation.

If the tail of a wall lizard is cut, the missing part develops again from the remaining part of the tail. In some cases, regeneration is so advanced that an entire multicellular body of an organism is reconstructed from a small fragment of tissue. Huamn body spontaneously loses cells from the surface of the skin and replaced by newly formed cells by regeneration

1.Physiological Regeneration
There is a constant loss of many kinds of cells due to wear and tear caused by day-to-day activities. The replacement of these cells is known as physiological regeneration

Example:

Replacement of R.B.C's in humans:

The worn out R.B.C's are deposited in the spleen and new R.B.C's regularly produced from the bone marrow cells, since the life span of R.B.C's is only 120days.

Replacement of Epidermal Cells of the Skin in animals

The cells from the outer layers of epidermis are habitually peeled off by wear and tear. These are continuously being replaced by new cells added by the malpighian layer of the skin.

2.Reparative Regeneration

This is the replacement of lost parts or repair of damaged body organs in an organism. In this type of regeneration, wound is repaired or closed by the expansion of the adjoining epidermal tissue over the wound.

Example:

Regeneration of limbs in salamanders

Regeneration of lost tail in lizard

Healing of wound

Replacement of damaged cells

3.Heteromorphosis : In certain animals, when a particular organ is amputated, the remaining portion may develop into a different organ. This process is called heteromorphosis or heteromorphic

regeneration. In the shrimp, palinurus, the eye may regenerate after being removed' rf the eye arone is removed, it w ill regenerate soon. If the eye is removed along with the optic ganglion, the eye will not be

regenerated. However, a regeneration blastema is formed. Instead of the lost eye, it develops into an antenna like organ. Heteromorphosis is also exhibited by phasmids (stick insects) and hydroids.

4.Super regeneration : The development of superfluous number of organs or parts of the body (head, tail, limbs etc) as a result of regeneration, is known as super regeneration . In planarian, earthworm or other lower animals, small  wound or incision on the body may cause the development of a head or a tail.

if the incision or wound is near the head, an additional head is regenerated. if it is near the tail, an additional tail is formed. If the wound is at the middle of the body, an additional head and a tail are formed. In these cases, the regenerated parts are additional and therefore superfluous to the animal.

5.Autotomy:In some animals like starfish and wall lizaard, some part of the body is broken off on being threatened by a predator. This phenomenon of self-mutilation of the body is called autotomy

Example:

Crabs break off their leg on approaching of the enemy

Holothurians throw off their internal viscera

Starfish breaks off an arm

Types of Regeneration based on Cellular Mechanism

Based on cellular mechanisms regeneration can be of two types:

1) Morphallaxis
In this type, regeneration occurs mainly by the remodelling of existing tissues and the re-establishment of boundaries, thus involving very little new growth. As a result, the regenerated individual is much smaller initially. It subsequently increases its size and becomes normal after feeding. This type of regeneration is known as morphallaxis or morphallactic regeneration.

Example: Regeneration of hydra from a small fragment of its body.

2) Epimorphosis

In this type, regeneration involves dedifferentiation of adult structures in order to form an undifferentiated mass of cells. They are highly proliferating and accumulate under the epidermis, which has already expanded. Within two days, bulge transforms into a conical hump. This lump of dedifferentiated cells along with the epidermal covering is called regeneration bud or regeneration blastema. The dedifferentiated cells continue to proliferate and finally dedifferentiate to form a rudiment of the limb. The rudiment eventually transforms into a limb. This type of regeneration is known as epimorphosis or epimorphic regeneration.

Example: Limb regeneration in amphibians.

An internal type which occurs between the above two types of regeneration is compensatory regeneration. Here the cells divide but do not form an undifferentiated mass of cells. Instead they produce cells similar to themselves.

Regenerative capacity in Animal Group
The capacity of regeneration varies in its extent in various animal groups. Regenerative capacity is very high among the protozoan, sponges and coelenterates.

Invertebrates

In sponges, the entire body can be reconstructed from isolated body cells. The cells rearrange and reorganize to form bilayered sponge body wall.

Regeneration was first discovered in hydra by Tremble (1740). Even 1/1000^th part of the body regenerate into new organisms.

In hydra and planaria, small fragments of the body can give rise to a whole animal. When a hydra or a planaria is cut into many pieces, each individual part regenerates into a whole individual.

Some annelids like earthworms are able to regenerate some segments removed from the anterior and posterior ends of the body.

Some molluscs can regenerate only the eyes and heads while squids can also regenerate their arms.

Many arthropods (e.g., spiders, crustaceans, insect larvae, etc) can regenerate limbs only. Regeneration is faster in the young than in the adults. Regenerated part may not always be similar to the part lost. This type of regeneration is called heteromorphosis.

Echinoderms (like starfish, brittle star, sea lilly)exhibit autotomy. They can regenerate arms and parts of the body.

Vertebrates
Fishes: Lamprey can regenerate its lost tail. Some fishes have the ability to regenerate parts of its fins.

Amphibians: The regeneration power is well marked in urodele amphibians like salamanders, newts and their axolotl larvae. They can regenerate limbs, tail, external gills, jaws, parts of eye like lens and retina. Tail and limb regeneration is found in the larval stages of frogs and toads.

Reptiles: Lizards exhibit autotomy. When threatened, the lizard detaches its tail near the base to confuse its predator and later regenerates a new tail. The new tail differs from the old one in its shape, absence of vertebrae and the kind of scales covering it.

Birds: Regeneration is restricted to parts of the beak.

Mammals: Regeneration is restricted to tissues only. External parts are not regenerated. Skin and skeletal tissues possess great power of regeneration. The liver has the maximum capacity of regeneration. If one kidney is damaged or removed, the other enlarges to compensate the lost kidney. This is called as compensatory hypertrophy.

Regeneration is an usual form of asexual reproduction in several lower groups of animals.

Events in Regeneration

Regeneration process in higher animals like newts and salamanders involves a series of complicated histological transformations. Needham (1952) recognized the following events in

the regeneration of amputated limbs.

(i) Wound Healing: Amputation or injury to an organ results in exposure of the interior tissues to the outside. Some cells in the injured region get destroyed. There may be bleeding at the injured surface. The blood soon clots and blocks further flow of blood to the outside. Then the epidermal cells proliferate and migrate from all sides towards the centre of the wound . Thus the epithelium forms a complete covering

over the injured tissues underneath the blood clot. The time needed for such a healing process depends on the size of the wound, size of the regenerating animals and external temperature

(ii) Demolition and Defence:The next event is the destruction and removal of the damaged tissues and the foreign elements such as germs. These are removed by autolysis and phagocytosis by the blood cells. This is favored by the increased blood supply to this area

(iii) De-differentiation:De-differentiation refers to the reversion of differentiated cells to the embryonic totipotent condition. Cells from the adjacent epidermis, muscles, nerves, cartilages, connective tissue, etc. undergo dedifferentiation. The intercellular matrix of bone and cartilage becomes dissolved and the osteoblasts and chondroblasts are set free. They dedifferentiate into totipotent cells. Similar dedifferentiation occurs in muscle cells, connective tissue fibers  and nerve cells. All these cells revert to embryonic totipotent state.

(iv) Blastema Fomation:Next step is the formation of the blastema or regeneration bud. This is due to the accumulation of de-differentiated cells under the epithelial covering of the wound. As more and more cells aggregate below the epidermis, it becomes pushed out and a conical projection appears. This bud like region consisting of an outer cap of epidermis

and central core of de-differentiated cells is called blastema or regeneration bud.

(v) Growth:The blastema then grows in size. This is caused by the mitotic division of the blastema cells.

(vi) Re-differentiation :After attaining sufficient size, the blastema passes into reditferentiation stage. The bone cells, cartilage cells, muscle cells, nerve cells and connective tissue cells are again re differentiated. They give rise to the bones, muscles, nerves and connective tissues of the organ which is to be regenerated A blastema undergoes re-differentiation exactly like a limb-bud in the embryo

Physiological changes involved in regeneration

Great deal of physiological changes occur during regeneration. Some of the notable changes are:During regeneration two periods with different types of metabolism have been noted

1.Catabolic Phase-During this phase

a.the proteolytic activity increases to a considerable extent. This is mainly due to the increase in the activity of the enzymes, cathepsin and dipeptidases. As a result, the amount ol free amino acids increases. This occurs during the destructive phase of regeneration. The activity of proteolytic enzymes help in destroying the damaged tissues.

b. Anaerobic glycolysis is accelerated during regeneration. It results in an accumulation of lactic acid in the tissues, thus lowering the pH in the blastema.

c. The Respiratory Quotient falls abruptly immediately after regeneration. This may be due to incomplete oxidation in the tissues.

2.Anabolic Phase-During this phase

a.Increase in oxidation

b..RQ Increases

c.Level  of lactic acid in the tissues decreases and  pH returns to normal

d. Large amounts of RNAs are synthesized.

e. The size of the nucleolus increases in the regenerating cells

Factors affecting regeneration.

(i) Temperature : The rate of regeneration is controlled, to a certain extent, by temperature. lf the temperature is very low, the regeneration process either does not occur or it becomes much slower.The increase in temperature upto a certain limit accelerates the regeneration. But a too high temperature is lethal for all regeneration processes. ln Planaria larva, for example, regeneration is impossible at 3oC. Regeneration is most rapid at 29.70C. A temperature of 320C or above is proved to be lethal for the regeneration process and the animal as a whole.

(ii) Oxygen supply :The amount of oxygen supply affects the rate of regeneration. ln hydroid coionies, the length of regeneration blastema increases with increased concentration of oxygen.

(iii) X-rays: X-rays inhibit regeneration, because they are known to suppress mitotic activity of cells. The inhibiting effect of X-rays depends upon the amount of irradiation administered. Once treated with X-rays, an animal retains the damaging effect for a long time, sometimes until the death of the animal.

(iv) Nervous system : Nervous system has a definite role in controlling the regeneration process. ln amphibians, the early stages of regeneration cannot proceed normally in the absence of an adequate nerve supply to the region of the wound. lf the nerves supplying the leg or the arm of a newt are destroyed simultaneously with the amputation of the limb, the regeneration of a new limb is arrested. The influence of nervous system on regeneration has also been demonstrated in annelids. ln earthworms, for example if lhe nerve cord is excised some distance away from the level at which an amputation is made, no regeneration will occur at the cut surface. It has been postulated that the cut ends of the nerves release some regeneration promoting chemical substances called trophic agents. These trophic agents stimulate the regeneration process.

(v)Growth facters:Glial growth factor,transferring ,Epidermal growth factor,fibroblast growth factors.

(vi)Proteins and chemicals:Transferrin,retinoucacid etc

(viiAge:The power of regeneration usually diminishes with increasing age.It is well known fact that bone fractures and wound heal much more rapidly in child than in an elderly person.