Regeneration

Regeneration is the reactivation of development in later life to restore missing tissues or to regrow severed pieces of an organism to the original state. Specialised cells called the stem cells carry out the process. It is more common in simpler organisms (like Hydra, Planaria, sponges) and in some tissues of higher organisms, but it is not limited to tissues with few specialised cells (e.g., liver regeneration in mammals involves highly specialised cells).

Types of Regeneration

Two types of regeneration are commonly found-

1. Reparative Regeneration

Includes wound healing and the repair of damaged tissues under selected conditions, like when an injury or trauma occurs; only in that case will reparative regeneration work.

2. Restorative Regeneration

Includes the replacement of lost tissues with specialised tissues to restore the whole body or lost body parts. More commonly observed among the non-chordates.

Mechanisms of Regeneration

Regeneration can occur in three major ways.

Epimorphosis

Epimorphosis involves the dedifferentiation of adult structures to form an undifferentiated mass of cells that becomes re-specified later. It is characteristic of regeneration in amphibian limbs (e.g., salamander), tail regeneration in lizards, etc.

Example: Epimorphic regeneration of salamander limbs.

When an adult salamander limb is amputated, epidermal cells from the vicinity migrate to the wound region (supplied with nerves) and form the wound epidermis.
Wound epidermis stops debris from getting into the wound.
The mesenchymal tissues in the limb stump thicken the wound epidermis to form the Apical Ectodermal Cap (AEC).
After the formation of AEC, histolysis of nerve cells, local cartilages, muscles, and other tissues close to the wound epidermis occurs. Histolysis of these tissues results in cell dedifferentiation, when the specialised cell types return to an undifferentiated form.
Cell dedifferentiation produces a population of mesenchymal stem cells, which migrate to the wound surface and harden into a cone-shaped mass of cells, the blastema, and a progress zone re-establishes.
AEC secrete various growth factors like Fibroblast growth factors (FGFs), to reset the development of the limbs to the embryonic stage.
Blastema produces neurotrophic factors to regenerate the sensory and motor neurons. Blastema expands distally over time, and the cells start to re-differentiate into tissue cells unique to the recovering limb, and the structural repatterning continues.
Re-expression of several developmental genes (like HOX) ensures appropriate differentiation of blastema and guides the regeneration of the limb structure until it is fully reconstructed.
The remaining cells around the amputated region are able to reconstruct only the missing structure of the limb or the entire amputated limb, with all its differentiated cells arranged in the proper order.

Morphallaxis

Morphallaxis involves the repatterning of the existing tissue with little new growth.

Example: Morphallactic regeneration in hydras.

Hydra is a Cnidarian having a tubular body with a distal end (the mouth or hypostome that remains surrounded by rings of tentacles) and a proximal end (the foot or the basal disc that enables the organism to anchor to substratum). They usually multiply by budding off new individuals; the buds are formed about two-thirds of the way down the body axis.
In this process, both fresh tissue growth and the redevelopment of the organs into altered proportions occur. A series of gradients regulates the regeneration of the proper head and foot. The polarity of the hydra’s body acts as the gradient driver.
When a hydra is cut in half, the half containing the head regenerates a new basal disc using the foot gradient, and the half containing the basal disc regenerates a new head using the head gradient. These gradients allow the body parts to develop at single and specific locations. If a hydra is cut into several portions, the other portions will regenerate both heads and feet at their appropriate ends. Thus, with morphallactic regeneration, a small hydra grows.

Compensatory Regeneration

In compensatory regeneration, no mass of undifferentiated tissues is formed. Similar cells to the original tissues are produced, and their differentiated functions are maintained.

Example: Compensatory regeneration in the mammalian liver.

After surgical removal (partial hepatectomy) of specific lobes of the liver, the others are intact.
The liver, as an organ, regains its original mass by proliferation of existing cells. It is cell division across the remaining liver tissue.
Regenerating liver cells do not fully de-differentiate when they re-enter the cell cycle. Rather, the five types of liver cells each begin to divide to produce more of themselves.
Each type of cell retains its cellular identity; thus, the liver retains its ability to perform all the hepatic functions.

Frequently Asked Questions (FAQs)

Q1. Are regeneration and reproduction the same?

A. Regeneration is a repair and survival mechanism that helps in the restoration of a lost body part or organ. In Planaria, regeneration of body fragments can lead to reproduction by fragmentation. This is a special case. Generally, regeneration is not equivalent to reproduction.

Q2. Does regeneration occur in plants?

A. In plants, regeneration often occurs through meristematic activity (vegetative propagation, callus culture, tissue culture). Example: A rose stem cutting, when planted, can grow into a new rose bush.