Healing, regenerating and shedding

The first thing an injured animal has to do is survive the damage.

In some species, a detached body part can develop into a whole new animal through asexual reproduction by fragmentation. In that case, the detached piece starts a life of its own.These processes occur in several groups of marine animals, and what they can achieve varies considerably among species.

Three processes to tell apart

1

Wound healing

Wound healing closes the wound and restores a protective barrier. It limits the exposure of internal structures to the surrounding water. A closed wound can still leave the animal without the part it lost.
2

Regeneration

Regeneration rebuilds missing tissues or anatomical structures. Depending on the species, it can replace a small piece of tissue, a limb or an entire organ.Growing more cells is only part of the work. Regrowing an arm requires muscles, nerves, any skeletal elements the arm normally contains, and in some cases sensory structures. They must form in the right places and work together.
3

Autotomy

Some animals can shed part of the body through a specialised mechanism called autotomy, usually in response to a threat or stress. Several crustaceans and echinoderms can shed an appendage in this way.Autotomy describes how the appendage is lost. Regeneration describes how it may later be rebuilt. An arm torn off by a predator can also regenerate without autotomy having occurred.

How does a body rebuild what it lost?

A sea star has no spare arms stored inside its body. A sea cucumber has no spare intestine waiting to replace the first.

Rebuilding begins with cells and tissues that survived the injury.In some cases, cells multiply and form new tissues. Some cells change what they do. A specialised cell can lose some of the features that suited it to its old role and take part in rebuilding the tissue. This is called dedifferentiation.Interactions between cells, molecular signals and the properties of the surrounding tissues guide these changes.Producing cells is only the beginning. The right structures must form in the right places.In an octopus arm, muscles have to move the new section, nerves have to make working connections, and suckers have to form along the arm. A new section is fully restored only when those parts work together again.Regeneration takes time and energy. While its tissues grow back, the animal must survive, sometimes with a reduced ability to move or feed.What grows back depends on what the animal has lost and what living tissues remain.
Picture the missing part. Look first at what stayed alive around the injury.
Two cutaway views of a sea cucumber: without an intestine, then with an intestinal tube forming.
Simplified diagram of the Caribbean sea cucumber. Above, the mesentery and the ends of the digestive tract remain after evisceration. Below, a new intestinal tube is forming. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

Rebuilding an intestine from surviving tissues

Researchers studying the Caribbean sea cucumber Holothuria glaberrima have followed the growth of a new intestine after evisceration.

Researchers found that rebuilding begins with structures that remain inside the body.The mesentery, a membrane that supported the digestive tract, stays attached to the inner body wall.Once the intestine is gone, the free edge of the membrane thickens. Cells in the tissue change state, and the swelling becomes the first outline of a new intestine.Some cells undergo changes associated with dedifferentiation. The remaining ends of the digestive tract also help re-form the lining of the new organ.As the new tube takes shape, its wall and other structures develop until it can function as an intestine.In the studies, regeneration took several weeks. The timing varied among individuals and with the conditions.The new organ begins at the edge of the very membrane that held the old one in place.These findings apply to the particular species and organs that researchers studied. The ability to regenerate an intestine does not establish that every sea cucumber can rebuild any organ.

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