Regeneration

Morphoallaxis:  Hydra grows by loss of cells from its ends and by budding.
Hydra has a hollow tubular body (0.5 cm long), with tentacles surrounding the mouth (hypostome) and, at the other end, a basal disc (foot).
Hydra has only two germ layers, the ectoderm and the endoderm separated by the basement membrane.
Hydra undergo continuous growth and pattern formation and cells are lost at the tentacle tips and from the basal disc.
The cells continually change their position and form new structures as they move up and down the body column.
Budding occurs, 2/3 down body axis which develops a head then detaches as a small new Hydra.

Regeneration in Hydra is polarized and does not depend on growth.
When cut in two, the lower piece will develop a head & the upper will develop a foot.
A piece excised from the Hydra body will regenerate both a head and a basal disc in the same polarity.
A small fragment will produce a small Hydra that will grow after feeding.
Heavily irradiated Hydra, that cannot undergo cell division (grow) will regenerate.

The head region inhibits the formation of a nearby heads
The head region of Hydra acts as an organizing region and as an inhibitor of inappropriate head formation.
The hypostome and the basal discs act as organizing centres to give polarity and act to induce head and tail formation.
Grafts of the hypostome to the gastric region will induce a 2nd head (& eventually a new body).
Grafts of the region next to the head to the gastric region will not generate a new head unless the original head is removed but will generate  a new head in the foot region.

Head regeneration in Hydra can be accounted for in terms of two gradients:
1) a head inhibitor gradient and
2) positional information gradient (along the body axis).
Diacylglycerol, a potent 2nd messenger (i.e. phosphatidylinositol signaling) causes ectopic head formation while lithium induces ectopic feet.
Homologues of Hox genes and forkhead transcription factors act in the organizing regions of Hydra.

Epimorphosis: Vertebrate limb regeneration involves cell dedifferentiation and growth.
In postamputation newts, epidermal cells cover the wound to form a blastema.
The cells of the blastema arise from beneath the wound epidermis, dedifferentiate and start to divide.
Over weeks, these cells become cartilage, muscle and connective tissue.
Transdetermination can be seen by labling (multinucleate) muscle cells with rhodamine-dextran (a large marker dye).
Labled mononucleate cells arise that give rise to cartilage as well as muscle.
In regenerating newt cells, the Rb protein is inactivated by phosphorylation.
Limb regeneration is also dependent upon the presence of nerve cells.

The blastema gives rise to structures with positional distal values.
Regeneration always proceeds in a direction distal to the cut surface.
An amputated limb will re-establish blood supply when fused to trunk.
If the humerus is then cut, then both surfaces will regenerate distal structures.
Grafting a distal blastema to a proximal stump will induce the stump (mostly) to generate a normal limb and the distal blastema forms the wrist and hand.
This is accomplished by reestablishing positional values  by inducing intercalary growth.
While mammals cannot regenerate limbs,  many (including young children)  can regenerate the ends of their digits.

Retinoic acid can change proximo-distal values in regenerating limbs.
Retinoic acid is present in developing vertebrate limbs and can alter positional values in the chick's limb.
Exposure to retinoic acid changes the positional value of a blastema to more proximal ones, such that elements proximal to the cut as well as those distal will be generated.
Wounded epidermis is a strong source of retinoic acid.
In regenerating limbs, retinoic acid is present in a distinct pattern & is higher in concentration in more distal blastemas.

Insect limbs intercalate positional values.
When tissues of vastly different positional value are placed in conjunction, then intercalary growth  occurs to replace the missing values.
Grafting of amputated cockroach legs demonstrate intercalation.
A distal cut tibia grafted onto a proximal cut will grow to intercalate the missing pieces.
However, a proximally cut tibia, grafted onto a distally cut host will also grow by intercalation.
In the latter case, the regenerated potion is in the reverse orientation (by bristle direction).
Circumferential values can also be regenerated by intercalation.

Plant cuttings can produce complete plants by regenerating all positional values.