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Do it again: Round-up of regenerating animals

10:59 13 July 2011

The handy trick of regenerating lost tissues varies widely in animals. We humans can regenerate the tips of our fingers, but some animals can regrow their entire bodies from just a few cells. New Scientist brings you a round-up of some of the more dramatic examples.


Cephalopods

Being a male octopus isn't so great after sex.

In male cephalopods, an arm called the hectocotylus is adapted to deliver sperm to the female. Roy Caldwell of the University of California, Berkeley, studies the complex reproductive behaviour of Indo-Pacific octopuses. In this picture taken off Sulawesi, Indonesia, a male Abdopus aculeatus on the left is inserting his hectocotylus into the female on the right – just beneath her eyes – to deliver a spermatophore. This arm breaks off from the male during copulation and stays lodged in the female.

The hectocotylus regenerates after the mating episode.

(Image: Roy L. Caldwell, University of California, Berkeley)




Red-spotted newts

As early as 1768, physician Lazzaro Spallanzani studied the regeneration of limbs in newts. Like all salamanders, they can regenerate their tails, eyes and even entire limbs.

When a newt loses a limb, cells in the region of the wound de-differentiate into stem cells, which then form a mass of cells called a blastema, from which the limb regrows.

In a 2007 study on red-spotted newts (Notophthalmus viridescens), Anoop Kumar and colleagues from University College London showed that a protein called nAG – secreted by skin and nerve cells – is key to the formation of the blastema.

(Image: AAAS/Science))


Starfish

Like sea cucumbers, starfish are echinoderms and as such have a distributed system of nerves – the nerve plexus – within and below the skin. There is also a central ring of nerves around the mouth.

Some starfish can divide this central disc in two, with each half generating a new starfish. Others need an arm and a small part of this central ring to regenerate. The Indo-Pacific starfish genus Linckia is a master: it can regenerate its entire body from a single arm. If the arm or nerve ring is damaged it can impair regeneration, though.

The arms of the seven-armed starfish (Luidia ciliaris) pictured here show various stages of regeneration. They can grow to over 40 centimetres across and are voracious predators on shellfish and other starfish.

(Image: Paul Kay/SplashdownDirect/Rex Features)



Gecko

Predator got your tail? No worries if you're a gecko – just let it drop off.

Tail autotomy – the ability to drop the tail – has evolved in several groups of geckos, other lizards, and salamanders.

These animals have points of weakness called "fracture planes", spaced regularly along the tail, either between or in the middle of vertebrae. To drop the tail, specialised muscles contract at the fracture planes, shattering vertebrae. Further muscles then contract around the caudal vertebra to minimise bleeding.

Lizards that have evolved this defence strategy often have brightly coloured tails which continue to move after separation, to attract a predator's attention and give the lizard time to escape.

The tail grows back with a rod of cartilage in place of the lost bones of the spine.

In the case of the giant day gecko (Phelsuma madagascariensis grandis) pictured here, the tail has grown two ends from one break point.

(Image: Joel Sartore/Getty)



Planarians

Cut a planarian in half and you get two planarians.

These non-parasitic flatworms live in water or moist environments on land. There are thousands of species, ranging in size from less than a millimetre to the 60-centimetre predatory flatworm Bipalium kewense.

Planarians regenerate their tissues by cell proliferation and by remodelling existing tissues with undifferentiated stem cells called neoblasts that are distributed throughout the animal's body.

Pieces from almost any part of a planarian called Stenostomum, for example, can develop into completely new worms. This makes them an excellent model organism to study regeneration in the lab.

The freshwater planarian Dugesia, pictured here, has been experimentally split with a scalpel from the head to the pharynx, resulting in a two-headed animal.

(Image: Tom Adams/Getty)



Sea cucumber

This sea cucumber (Bohadschia argus) – pictured in the Celebes Sea off Sulawesi, Indonesia – has a dramatic response to predators. It eviscerates itself, shooting its internal organs out of its rear end.

Regeneration usually starts with the digestive tract, and it can take weeks for the organs to regrow.

(Image: Jeff Rotman/Getty)



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