Regeneration

An important process in which an organism regenerates itself this process, if an organism is cut into one or more pieces, then it can get back to its original state with the help of specialized cells known as stem cells. For regeneration, an organism should have a very simple structure with a few specialized cells. It can occur in animals, plants, insects, birds, humans, etc. For example, Lizards, hydra, starfish, mice, planaria, etc

Types of Regeneration

• Reparative regeneration: This regeneration repairs certain cells but only in selected situations. This regeneration includes wound healing and the repair of damaged tissues. When an injury or trauma occurs, only in that case will reparative regeneration work. This regeneration is generally observed in both vertebrates and invertebrates. Example: Part of the human liver regenerates after its removal.
• Restorative regeneration: This regeneration regenerates the whole body or lost body parts. This regeneration includes the replacement of lost tissues with specialized tissues. This regeneration is generally more observed in invertebrates as compared to vertebrates. Example-
  • Regeneration of planaria (whole-body regeneration)
  • Lizard tail regeneration (regeneration of a lost body part).

Mechanism of Regeneration

According to T.H. Morgan, there are four mechanisms observed in animals are-

Morphallaxis

• This mechanism includes the regeneration of the whole animal body from small pieces by reorganizing the existing cells., in morphallaxis, the regeneration takes place via the re-patterning of current tissues and there’s little growth. 
• It occurs in various plants, animals, and even humans. 
• various factors affect morphallaxis such as temperature, pH, excretory products, etc.  
• Example: Hydra

Mechanism of Morphosis

Regenerative tissue remodeling is used in the mechanism. Both fresh tissue growth and the redevelopment of the body’s organs into altered proportions are made possible by this. The creation of the proper head and foot regeneration is regulated by a series of gradients seen in hydras. The foot gradient allows the basal disc to form only in one location, while the head gradient only allows the head to form in one location. The hydra’s polarity acts as the gradient’s driver. The head-region hypostome prevents the development of another hypostome. Because of this, a hydra cannot grow two heads.

Morphosis in Hydra

About 0.5 cm long freshwater Cnidarians are referred to as hydras. A hydra’s body is short and tubular. Hydras have a foot made up of a basal disc and a head made up of a hypostome region. The mouth and tentacles, which are located on the head section of the hydra, are used for grabbing and consuming prey. The basal disc, which enables the hydra to adhere to rocks and other objects, is located in the foot section of the hydra. The foot component of a hydra can regenerate and produce a new head with the hypostome area, while the head portion can renew and form a new foot with the basal disc. A hydra’s head and foot would still develop if it were cut into smaller parts at the appropriate places on the hydra, according to the middle pieces. This doesn’t involve cellular division and leads to a smaller hydra that was recreated through morphallaxis.

Epimorphosis

• This mechanism includes the replacement of lost organs in the body by generating new cells from the injured part of the body.
• Dedifferentiation of the adult structures creates an undifferentiated mass of cells, which is later re-differentiated to create new structures.
• It occurs in both vertebrates and invertebrates.
• Examples: Starfish, Planaria, Amphibian limbs, etc.

Epimorphosis in vertebrates

The process of vertebrate epimorphosis, which is shown by salamanders’ limb regeneration, is based on blastema development, which allows cells to multiply into new tissues.

Salamanders’ Regeneration of Limbs

The epidermal cells that are present close to the wounded area first migrate to the wound and cover it. The mesenchymal tissues in the limb stump release MMP, aiding in the thickening of wound tissues (Apical Ectodermal Cap), which causes the creation of the stump tip. The progress zone re-establishes because the nerves close to the wound degenerate. Cells such as those in bones, cartilage, and other tissues dedifferentiate as a result of this. The release of the fibroblast growth factor by the apical ectodermal cap aids in resetting the development of the limbs to the embryonic stage. Only cartilage cells can regenerate new cartilage tissue, and only bone cells can create new bone tissue, and the same rule applies to all other types of tissues close to the incision. An incarnation occurs when the distal portion of a limb develops and starts to interact with the originating portion to create the intermediate portion of the limb. The new limb is created in the area of the amputated limb once all of these procedures are finished.

A detailed explanation of Salamanders’ Regeneration of Limbs

• Salamanders regenerate their limbs in a series of interconnected phases. 
• Epidermal cells in the vicinity of the wound area are recruited a few hours to a day after limb amputation to cover the wound. The wound area must have enough nerves to carry out this operation. When this circumstance occurs, a structure known as the wound epidermis develops. 
• The wound epidermis’ main function is to stop debris from getting into the wound.
• The apical epithelial cap (AEC), which resembles a bud on the surface of the amputation site, is a structure that the wound epidermis eventually creates as it grows. Because it secretes different growth hormones that promote limb growth, the AEC is crucial to regeneration. Fibroblast growth factors (FGFs), which are present in many creatures, including humans, and are frequently engaged in tissue healing in adult organisms, are among these elements.
• Schwann cells, local cartilage, muscles, and other tissues found below the wound epidermis then have histolysis, which results in cell dedifferentiation. The reverse of differentiation, cell dedifferentiation is the process by which cells that have previously transformed into a specialized cell type are returned to an undifferentiated form.
• Salamanders regenerate from mesenchymal stem cells, which can develop into a variety of cell types like bone, fat, and cartilage. The process of cell dedifferentiation produces a population of mesenchymal stem cells, which migrate to the surface of the wound and congeal into the regeneration blastema, a cone-shaped mass of cells.
• Due to its intricacy and the fact that it differs for various tissue types, the dedifferentiation process is still not fully understood. The down-regulation of differentiation-promoting genes and the up-regulation of genes specific to the embryo and regeneration, however, are common characteristics observed in all dedifferentiation events.
• In addition, matrix metalloproteinases cause these cells to undergo histolysis before dedifferentiation (MMPs). MMPs are enzymes that can break down extracellular matrix proteins like collagen, which give neighboring cells their structural support. Additionally, MPPs help to maintain and develop the blastema overall by preventing the production of scars.
• Until the limb has fully recovered, the blastema expands distally over time as a result of mesenchymal stem cell proliferation. The AEC’s released substances assist in mediating this. 
• Neurotrophic factors produced by blastema cells during this phase aid in the regeneration of sensory and motor neurons. As the blastema grows, its cells start to re-differentiate into tissue cells unique to the limb that is recovering, and limb structural repatterning moves forward.
• It is believed that the original limb cells at the location of the damage have positional memory of their placement along the three axes of the limb, which is passed on to cells of the blastema when they re-differentiate, allowing them to migrate to the correct position in the new limb.
• The proximal-distal, anterior-posterior, and dorsal-ventral axes are among these three axes. The regeneration of the limb structure is guided by the re-expression of several developmental genes, including HOX genes, which ensures appropriate differentiation of blastemal cells until it is fully reconstructed.

Epimorphosis in Invertebrates

Some species of cockroaches are invertebrate animals that can repair lost organs or body parts. Examples include the regeneration of body parts in planarians, the regeneration of posterior segments in capitellar, and the regeneration of limbs in American cockroaches.
Planaria reproduces asexually by regeneration. A planaria’s body is divided into pieces and used in this process of reproduction to regenerate into new planaria. In just a few weeks, a very small portion of planaria can regenerate into the entire organism. Due to pluripotent stem cells, a cell in the cut zone multiplies to produce a blastema and develops into new tissue. Because the stem cells in the tissues include more than 20% of the cells of an original organism, they can produce all the types of cells that new planaria organisms require. The older cells are replaced by these adult stem cells, which are the only proliferating cells in the shattered body.

Stem Cell-mediated Regeneration

• Stem cells permit an organism to regrow certain organs or tissues which have been lost. 
• Example- continuous replacement of blood cells from hematopoietic stem cells

Compensatory Regeneration

• The differentiated cells divide while keeping their distinct characteristics. 
• Both stem cells and the dedifferentiation of mature cells do not produce new cells. 
• There is no bulk of undifferentiated tissue formed, and each cell creates cells that are comparable to itself. 
• The liver of mammals regenerates in this way frequently.

Is Regeneration a Method of Reproduction?

No, regeneration is not a method of reproduction as it involves a process in which new organisms are formed from single organisms but most of the organisms will not wait to break or cut into pieces for reproduction.

Which Organisms can Regenerate?

It includes:

• Arthropods – Many arthropods can regenerate their limbs and other appendages caused by injury. 
  • Arachnids including scorpions can regenerate their venom.
  • The fruit fly Drosophila melanogaster can regenerate its gut and germline.
• Annelids – They can regenerate their posterior and anterior body parts even after latitudinal bisection.
• Echinoderms – Starfish, sea cucumber, sea urchins, and many more come under this category. They can regenerate their damaged appendages, internal organs some parts of the central nervous system.
• Planaria (Platyhelminthes) – It includes planarians that can regenerate their lost body parts.
• Amphibians – They can regenerate their tails, jaws, limbs, and retina. They include frogs and tetrapods.
• Hydra – 
  • They can regenerate their head and basal parts of the body.
  • They can regenerate the tissues that are cut from the gastric region.
  • If a hydra is cut into two equal parts then it can regenerate itself into a new hydra with approximately the same size.
• Aves (Birds) – Some birds can regenerate their feathers.
• Mammals – 
  • Humans- can regenerate their lost tissues or organs. (Only specific tissue and organs) 
  • Male deer- can regenerate their antlers annually.
  • Mice- can regenerate damaged tissues, hair follicles, fur, and skin.
• Reptiles – Lizards, crocodiles, and many more come under this category. Lizard has the highest regenerative capacity as compared to others.
  • Lizard-Tail regeneration
  • Crocodile-Maxillary bone regeneration.
• Chondrichthyes – Leopard sharks, bamboo sharks, and many more sharks come under this category.
  • Leopard sharks- regenerate their teeth after every 9-12 days because their teeth developed within the bony cavity.
  • Bamboo sharks- regenerate two-thirds of their liver.
  • Other sharks can regenerate their scales and even damage their skin.

Importance of Regeneration

• Damaged tissues of many living organisms can regenerate easily.
• Not only tissues but in the case of lizards, planaria, and many more organisms can regenerate their body parts.
• In the case of sharks, teeth can be regenerated in leopard sharks and liver in bamboo sharks.
• Many amphibians can regenerate their tails, limbs, retina, and tail.
• Even birds who have lost their features can regenerate their features through regeneration.

FAQs on Regeneration

Question 1: Define regeneration.

Answer:

An important process in which an organism regenerates itself With this process, if an organism is cut into one or more pieces, then it can get back to its original state with the help of specialized cells known as stem cells.

Question 2: In human which organ regenerate?

The liver regenerates itself when it gets damaged. 

Question 3: Which mechanisms take place in hydra?

Answer: 

Morphallaxis takes place in hydra.

Question 4: Name the organisms which can regenerate.

Answer:

Organisms that can regenerate includes starfish, planaria, starfish, lizard, mice, human, etc.

Question 5: Are snakes also regenerating their tail like a lizard?

Answer:

No, the snake is a reptile but it can’t regenerate its tail like a lizard.