Regeneration (zoology)

Regeneration is a process by which some organisms replace damaged or missing tissue using living cells adjacent to the affected area. The phenomenon is not well understood, but several animal systems have enabled developmental geneticists to develop strong models describing the process.

The replacement of tissue is a common occurrence in fungi and plants. Regeneration can also occur in animals, although the capacity for regeneration progressively declines with increasing complexity in the animal species. Among primitive invertebrates (animal species lacking an internal skeleton), regeneration frequently occurs. A planarium can be split symmetrically into right and left halves. Each half will regenerate its missing mirror image, resulting in two planaria, each a clone (exact genetic copy) of the other.

In higher invertebrates, regeneration occurs in echinoderms (such as starfish) and arthropods (such as insects and crustaceans). A radially symmetrical starfish can regenerate one or several of its five arms. Regeneration of appendages (limbs, wings, and antennae) occurs in insects such as cockroaches, fruit flies, and locusts. Similar processes operate in crustaceans such as lobsters, crabs, and crayfish. Limb regeneration extends even to lower vertebrate species (species having an internal skeleton), such as amphibians and reptiles, although on a very limited basis.

In amphibians, the newt can replace a lost leg. In reptiles, some lizards can lose their tails when captured by a predator, thus assisting their escape. If the lizard is young, the tail can regenerate. The tail breaks because of a breakage plane in the tail, which severs upon hormonal activation. The glass-snake lizard is such a species.

Principles of Regeneration

Regeneration in these organisms is based upon two principles: the symmetrical organization of cells in the organism and the reversal of determination and differentiation in the surviving cells, termed blastema, adjacent to the missing tissue. These two factors are fundamental to the development of the organism.

In animal systems, two major body symmetries emerge—radial symmetry and bilateral symmetry. In radially symmetric organisms, including plants and primitive invertebrates such as hydra, jellyfish, and starfish, body tissues are arranged in a circular orientation about a central axis. Appendages may also be present that likewise orient in a circular pattern of cells. In bilaterally symmetrical organisms, including animal species such as planaria, arthropods, fish, amphibians, reptiles, birds, and mammals, the body is oriented into mirror-image halves about a central body plane, resulting in its having right and left equivalent structures along each half.

Body symmetry is critical for tissue regeneration because of positional information. The cells of an individual organize into a specific pattern during development. Cell-to-cell interactions and chemical messengers between cells provide the cells of a given tissue with information signals directing the cells to grow in a particular direction or pattern. The loss of a tissue portion may stimulate the remaining cells to carry out a programmed growth, that is, to complete a specified pattern.

Determination and differentiation both play an important role in the genetic basis for development. All cells of an organism contain the same genetic material; that is, they all contain the same deoxyribonucleic acid (DNA) the same genes. In a specialized organism (one having different tissue types—eyes, ears, skin, and nerves), these cells must behave differently, even though they contain the same genetic information. The process by which identical cells with the same genetic information give rise to different tissues is termed differentiation.

What causes similar cells to differentiate to form different tissues is a process called determination. Prior to differentiation, cells become determined, meaning that some genes in these cells are turned on, making certain proteins, while other genes are turned off, not making other proteins. All cells of a specific tissue have the same genes that are turned on or off and, therefore, make the same proteins (for example, all red blood cells manufacture hemoglobin). Cells of other tissue types have different genes that are turned on or off and make other proteins (for example, epidermis cells manufacture keratin, not hemoglobin). How cells determine and differentiate depends upon chemical signals (hormones). Hormones signal different cells that receive chemically coded information based on their location in the organism, which is based on the organism’s symmetry.

Cockroaches, Newts, and Fruit Flies

Three principal animal regenerative systems have been studied: cockroach limb regeneration, newt limb regeneration, and fruit fly imaginal disk regeneration. In all animal systems, regeneration occurs primarily in younger individuals undergoing metamorphosis, which is a change in development involving considerable alterations in body size and physical appearance. Adult regeneration is incomplete or does not occur.

Severing a cockroach limb results in distal regeneration; the remaining leg part regenerates the lost portion. If the middle portion of a leg is removed and the remaining leg parts are grafted together, complete regeneration of the missing middle portion occurs precisely between the grafted parts, but grafting requires the correct orientation of the body parts. If the leg parts are grafted backward, regeneration will be distorted, resulting in a malformed limb, sometimes with multiple leg structures sprouting from one limb. Virtually identical results have been obtained for newt limb regeneration. Furthermore, limb regeneration in the presence of certain chemicals, such as retinol palmitate, causes complete limb regeneration, including undamaged regions. The net result is a severely deformed limb.

For the fruit fly Drosophila melanogaster, the period from egg to adult is approximately ten days at 25 degrees Celsius (77 degrees Fahrenheit). The period includes roughly seven days, during which the organism proceeds through three larval (maggot) stages, followed by a three-day immobile pupal stage, during which metamorphosis occurs. Metamorphosis involves the replacement and modification of larval body structures with adult body structures (eyes, legs, and wings). The cells that are to become the adult structures are present but dormant in the larval stages. These special cells, called imaginal disks, are determined to become specific adult structures but remain undifferentiated until activated by the hormone ecdysone during metamorphosis.

There is one imaginal disk for each future body structure (two eye imaginal disks, six leg imaginal disks, for example). Gerold Schubiger and other geneticists have determined “fate maps” for each imaginal disk. determined what each cell group on each disk would become in the adult. For example, the male genital disk has been mapped so that specific cell groups are associated with the formation of the specific adult structures of, for example, the heart, penis, and testes.

Experiments by Peter J. Bryant and Schubiger focused on removing parts of leg and wing imaginal disks. The remaining surviving cells either regenerated the missing tissue or duplicated themselves. Whether regeneration or duplication occurred depended upon the amount of tissue lost and the position of the surviving imaginal disk tissue. If a small portion of a disk or a particular region of the disk was lost, then the remaining disk cells regenerated the missing part, thus producing a complete disk and ultimately a normal adult structure. If a large section of a disk or a sensitive region of it was lost, then the remaining cells duplicated a mirror image of themselves, giving rise to a useless adult structure.

Models of Regeneration

These collective studies, especially those involving the Drosophila imaginal disks, have produced two comprehensive regeneration models: the gradient regeneration model and the polar coordinate model. The gradient regeneration model, proposed by Victor French, explains the regenerative capacity of a given tissue by arranging the cells of the tissue along a gradient of regenerative capacity. Cells are arranged in order of high regenerative capacity to low regenerative capacity. This high-to-low regenerative gradient is directly correlated to the positional information of each cell. Cells located proximal (near) to the main body axis have high regenerative capacity. Cells located distally (far) from the main body axis have progressively lower regenerative capacities. Removal of distal cells results in their replacement by regeneration of the proximal highly regenerative cells. The removed distal cells, which lack regenerative information because they are at the low end of the gradient, cannot regenerate the proximal cells.

The gradient regeneration model can best be visualized as a right triangle with its hypotenuse (longest side) being a downward slope. Highly regenerative cells (proximal to the main body axis) are at the top of the slope. They contain positional information for themselves plus information for all cells below them on the slope. Higher cells can replace lost lower cells (located distal to the main body axis). Distal cells low on the slope have considerably less positional information and, therefore, can only duplicate themselves.

The polar coordinate model, proposed by French with Peter J. Bryant and Susan V. Bryant, is a more elaborate version of the gradient model that explains not only the Drosophila imaginal-disk experiments but also the cockroach and newt regeneration experiments. This model is a three-dimensional gradient that covers regeneration not only in a proximate to distal direction but also from the exterior to the interior. The polar coordinate model can best be visualized as a cone. The circular end of the cone represents proximal tissue, whereas the tapered tip represents distal tissue, thus simulating the proximal-to-distal regeneration gradient.

On the circular (proximal) base of the cone, imagine a bull’s-eye. The outermost circle represents exterior tissue, whereas the circle center (bull’s-eye) represents the most interior tissue. There is thus a three-dimensional regeneration gradient—proximal-to-distal and exterior-to-interior. The circle is furthermore subdivided clockwise into twelve regions, completing the polar coordinate model of tissue regeneration capacity.

The tissue pattern of regeneration will again favor those cells located at high gradient positions, namely proximal (cone base) and exterior (outside circle). These cells possess positional information for regeneration of lower gradient tissue. Lower gradient cells, located distally (cone tip) and interiorly (circle center), will have limited positional information and will be capable only of duplicating themselves.

For clockwise regeneration, the polar coordinate model operates by the shortest intercalation route; that is, if a small tissue section is lost, the remaining large section will regenerate the lost piece based upon positional information. If a large tissue section is lost, however, the remaining small section will lack sufficient regenerative information and will be capable only of duplicating a mirror image of itself.

The French, Bryant, and Bryant polar coordinate model is a three-dimensional regenerative capacity gradient intended to model a tissue based on cell position. The model really boils down to one principle: a large, proximal (or exterior) group of cells can regenerate missing tissue; a small, distal (or interior) group of cells cannot.

Studying Regeneration

Developmental geneticists have studied regeneration in a variety of ways. Among the principal experimental techniques have been fate map determination of imaginal disks and limb regeneration, already discussed above, transdetermination of imaginal disks, and studies of simple organismal development.

Geneticists have found that under special circumstances, an imaginal disk or a portion of an imaginal disk can change its pattern of determination; that is, it transdetermines. A wing occasionally grows from an eye, for example, or a leg from a wing. Cells that are programmed to follow one developmental route follow another route instead. The cause of transdetermination is unknown, but the process does follow specific patterns. For example, a genital imaginal disk can be transdetermined to form an antenna or leg, but not vice versa. An antenna disk can be transdetermined to produce an eye, wing, or leg, but the wing and leg disks cannot transdetermine to an antenna.

Further regenerative studies involve the model developmental systems, including the cellular slime mold Dictyostelium discoideum. In the presence of adequate food, this exists as single, amoeba-like cells. If the cells are starved, they release a chemical attractant (chemotaxic) substance, cyclic AMP, that attracts the cells to one another. The resulting cellular mass moves as a single unit until the organism finds a suitable food source, upon which the cells differentiate and specialize to produce and release spores, each of which subsequently gives rise to a new amoeba-like stage. Similarly, scientists have found that salamanders that have their limbs amputated in the lab regenerate better-quality new limbs than those that lose limbs in the wild. Such studies are necessary because regeneration ultimately involves changes in the determination and differentiation of cells.

From an evolutionary perspective, scientists have found a link that may help more closely identify the origin of regeneration in the animal kingdom. A group of researchers found that when lungfish regenerate their fins, they share similar skeletal abnormalities with salamanders. This finding may indicate their cells share a common ancestor.

Future Prospects

Regeneration research presents two opportunities for further development: an understanding of higher cell differentiation and growth and prospects for replacing lost or damaged human tissues and organs. The polar coordinate model for tissue regeneration indicates that tissue replacement depends on blastemas at the damaged area, replacing tissue using positional information. Future research includes genetic and molecular studies to identify intercellular chemotaxis molecules and other information molecules that mediate cell-to-cell communication and thereby control how cells develop and grow in specified patterns. Such research will unravel important clues to cellular development and regeneration.

Scientists’ understanding of cellular differentiation and growth is historically limited to more primitive species (such as Dictyostelium discoideum and Drosophila melanogaster) but continues to grow in the twenty-first century with advanced technology and quality research in genomics, stem cells, molecular biology, and bioengineering. Scientists have gained a better understanding of the types of cells involved in regeneration and how they contribute to the process in various animals. Modern regeneration biologists also better understand the molecular pathways that facilitate regeneration in some animals.

One significant breakthrough in cell regeneration research was with the Drosophila imaginal disk studies and cockroach and new limb regeneration experiments. However, much more work remains, particularly concerning genetic and molecular analysis. The action of specific steroid and protein hormones on cellular growth and differentiation is a further avenue of research. While regeneration research has been pursued for many decades, it is still in its infancy. Further research will allow for an understanding of organismal development. To advance regenerative biology and regenerative medicine to the point of applicability to human bodies, scientists note the importance of an interdisciplinary, cross-disciplinary approach. The complexity of the questions remaining in this field requires expertise in mechanical and technical disciplines working together. For example, since the science emerged, scientists have questioned why regenerative abilities are not evenly distributed across metazoans.

Principal Terms

Chemotaxis: A process by which cells are attracted to a chemical, moving from low to high concentrations of the chemical until the cells cluster

Determination: An event in organismal development during which a particular cell becomes committed to a specific developmental pathway

Differentiation: The process by which a determined cell specializes or assumes a specific function

Fate Map: A map of determined but undifferentiated tissue by which specific cell regions can be identified as giving rise to specific adult structures

Imaginal Disk: A determined, undifferentiated tissue in fruit fly larvae that gives rise to a specific adult structure

Positional Information: A concept by which differentiating cells organize themselves to produce a particular tissue type based on cell-to-cell interactions

Stem Cell: A determined, undifferentiated cell that is hormonally activated and changes into a specific cell type

Transdetermination: An event by which a determined, undifferentiated cell changes its determination, thereby giving rise to a different tissue type

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