Regeneration is the capacity of the body to produce the lost parts by proliferation of cells. It leads to the healing of wounds, replacing of worn out layers, and even the formation of a complete individual.
Image Source: transitionvoice.com
ADVERTISEMENTS:
Sometimes regeneration leads to reproduction when a part of organism gives rise to the whole organism. Regeneration is similar to development as both involve cell division, cell movement, cell differentiation and morphogenesis (that is, formation of organs).
I. Types of Regeneration:
1. Reparative regeneration is quite common in both invertebrate and vertebrate groups. It is confined to healing of injuries. For example, earthworms and related annelids are capable of regenerating a few body segments severed from either or both the ends of the body.
Among amphibians, salamanders and axolotl larvae are unique for their power of regenerating some organs and body parts such as limb, tail, external gills, jaws, and intestine and eye structures. Liver in mammals has the extensive power of reparative regeneration.
2. Restorative regeneration is common in some invertebrates. In sponges, the entire organism may regenerate from a group of its cells.
II. Pattern of Regeneration:
T.H. Morgan was the scientist who recognized two primary mechanisms of regeneration. These are:
1. Morphallaxis:
ADVERTISEMENTS:
It involves the re-constitution of the whole body from each of the small fragments by re-orienting and re-organising the existing body cells. The regenerated animal is smaller than the original one after the completion of the process. E.g., Hydra and Planaria.
2. Epimorphosis:
It takes place by the proliferation of the new tissue cells from the surface of the wound. Eg. Regeneration of arm in starfish, tail in lizard, etc.
III. Regenerative Ability in Different Animals:
(i) Protozoans:
Ciliates like Stentor, Blepharisma and Spirostomum require only cell cortex for regeneration. 1/100 part of body of Acetabularia can regenerate the entire body.
(ii) Sponges:
ADVERTISEMENTS:
Regenerate any injured or severed part. Isolated cells regroup and form whole body.
(iii) Coelenterates:
Hydra (discovered by Trembley) has excellent power of regeneration. If Hydra cuts into two or more sections than the lost part is regenerated with exact polarity. In the hydroids, it is the interstitial cells which are totipotent and responsible for complete regeneration.
(iv) Platyhelminthes:
Planarians stocks of undifferentiated cells called neoblasts are omnipotent. They regenerate the cut portions and lost parts.
(v) Nematodes:
Regeneration poor. Only closure of superficial wound.
(vi) Annelids:
Both polychaetes as well as oligochaetes can regenerate anterior and posterior regions of their body if amputated (cut off). Healing of the wounds occur by neoblast cells. Wound healing is mediated by proteins called growth factor such as epidermal growth factor (EGF), fibroblast growth factor (FGF), platelet derived growth factor (PDGP).
(vii) Arthropods:
Limited renewal of lost appendages as in certain crustaceans and spider. Regeneration is faster in young than in adult. Regeneration that produces a part different from the lost part is called heteromorphosis.
(viii) Molluscs:
Regeneration poor. In gastropods eye stalk with eye may be regenerated. Cephalopods may regenerate their arm.
(ix) Echinoderms:
Star fishes, brittle stars and sea lilies can regenerate arms and parts of disc.
(x) Fishes:
Poor regeneration restricted to fish fins.
(xi) Amphibians:
Wound healing urodeles (Salamander) posses’ intense capacity to regenerate limbs and ails.
(xii) Reptiles:
Lizards are known to regenerate their tails (autotomy). Autotomy is the voluntary casting of a part of body when an animal is attacked. Another example is legs in crabs.
(xiii) Birds:
Parts of beak can be regenerated otherwise ability is poor.
(xiv) Mammals:
Extremely poor regenerative capacity. Few marsupials can regenerate the limbs. Capillaries at injured places can reform. Liver, if 20% left, can regenerate itself. Similarly if one kidney is removed, the other kidney enlarges to take over the function of the removed one. This type of regeneration is called compensatory regeneration or compensatory hypertrophy (i.e. overgrowth).
IV. Control of Regeneration:
Regeneration in animals is brought about by the stimulation of nerve endings upon amputation (act of cutting of limbs) of body parts.
Nerve impulses in sensory neurons, in turn stimulate neurosecretory activity in central ganglia, and the neurosecretory materials initiate regeneration activity.
V. Advantages of Regeneration:
1. To the organisms: Repair of the accidently lost portions e.g., lizards, molluscs, starfishes, etc.
2. It is the usual form of asexual reproduction e.g. protozoans, sponges, coelenterates, flatworms, and numerous plants.
3. Escape from enemy by autotomy (e.g. in lizards) followed by regeneration of the part.
4. To the scientists: A good means of study of cell division, cell movements, cell and tissue differentiation and development of form or morphogenesis (as in Hydra and Planaria).