Regulation and limb regeneration: the effect of partial irradiation

The regulative ability of the regeneration blastema has been studied by the technique of partial irradiation. Longitudinal halves of fore- and hindlimbs were irradiated and the product of regeneration from the unirradiated halves characterized. Such half irradiated limbs could regulate only to a very limited extent. In the upper arms the anterior halves merely produced a few autopodial elements whereas complete limbs regenerated from the posterior halves. This striking regenerative inequality was less marked in the lower arm where the two halves each produced regenerates of two to three digits. In the hindlimb dorsal halves produced more than ventral halves and thus it was concluded that the posterior-dorsal quadrant has some special regenerative property. These results highlight the inadequacy of current models of limb regeneration and are discussed with reference to the arrangement of positional values in the limb and the question of the totipotency of blastemal cells.

Neurotrophic and X-ray blocks in the blastemal cell cycle

Using microdensitometry techniques the points in the cycle where blastemal cells become blocked after X-irradiation or denervation of the regenerating amphibian limb have been identified. X-irradiation blocks the cells in both G1 and G2 and those cells that were in S at the time of irradiation presumably proceed to G1. After denervation, however, cells accumulate only in G1 and those that were in S or G1 continue through the cycle to the next G1. The latter results are clearly contradictory to a recent theory proposing a G1 neurotrophic control of blastemal cells and a solution to the contradiction is presented in the light of recent results.

The role of irradiated tissue during pattern formation in the regenerating limb

The amphibian limb regeneration blastema is used here to examine whether irradiated, non-dividing tissue can participate in the development of new pattern of morphogenesis. Irradiated blastemas were rotated 180 degrees on normal stumps and normal blastemas rotated on irradiated stumps. In both cases supernumerary elements developed from the unirradiated tissue. The supernumeraries were defective but this did not seem to be due to a lack of tissue. Rather it suggested that this could be a realization of compartments in vertebrate development or simply reflect the limited regulative ability of the blastema. The results are also discussed in relation to a recent model of pattern formation.

Neurotrophic and x-ray blocks in the blastemal cell cycle

Using microdensitometry techniques the points in the cycle where blastemal cells become blocked after X-irradiation or denervation of the regenerating amphibian limb have been identified. X-irradiation blocks the cells in both G1 and G2 and those cells that were in S at the time of irradiation presumably proceed to G2. After denervation, however, cells accumulate only in G1 and those that were in S or G2 continue through the cycle to the next G1. The latter results are clearly contradictory to a recent theory proposing a G2 neurotrophic control of blastemal cells and a solution to the contradiction is presented in the light of recent results.

The role of irradiated tissue during pattern formation in the regenerating limb

The amphibian limb regeneration blastema is used here to examine whether irradiated, non-dividing tissue can participate in the development of new patterns of morphogenesis. Irradiated blastemas were rotated 180° on normal stumps and normal blastemas rotated on irradiated stumps. In both cases supernumerary elements developed from the unirradiated tissue. The supernumeraries were defective but this did not seem to be due to a lack of tissue. Rather it suggested that this could be a realization of compartments in vertebrate development or simply reflect the limited regulative ability of the blastema. The results are also discussed in relation to a recent model of pattern formation.

Neurotrophic control of the cell cycle during amphibian limb regeneration

It is shown here that amputated and denervated limbs of larval axolotls dedifferentiate and a proportion of the cells released undergo DNA synthesis and mitosis. When the limb is denervated prior to amputation fewer cells go through the cell cycle, implying the existence of a pool of trophic factor in the limb. Recent work has demonstrated that denervated blastemal cells accumulate in the G1 phase of the cycle. These results strongly argue against the theory that the trophic factor controls the G2 phase. Rather, it is proposed that this factor regulated either the total number of cells cycling or the rate at which they cycle by varying the length of the G1 phase.

Neurotrophic control of the cell cycle during amphibian limb regeneration

It is shown here that amputated and denervated limbs of larval axolotls dedifferentiate and a proportion of the cells released undergo DNA synthesis and mitosis. When the limb is denervated prior to amputation fewer cells go through the cell cycle, implying the existence of a pool of trophic factor in the limb. Recent work has demonstrated that denervated blastemal cells accumulate in the G1 phase of the cycle. These results strongly argue against the theory that the trophic factor controls the G2 phase. Rather, it is proposed that this factor regulates either the total number of cells cycling or the rate at which they cycle by varying the length of the G1 phase.

The role of Schwann cells in paradoxical regeneration in the axolotl

The experiments described here examine further the conditions under which paradoxical regeneration occurs and provide support for the hypothesis that a proximal migration of Schwann cells is responsible for the phenomenon. When only the hand is shielded from irradiation and the limb is denervated, amputation through the forearm or upper arm sometimes results in regeneration. The effects of variation in the time interval between denervation and amputation, the level of amputation and the method and number of denervations on the incidence of regeneration were investigated. The presence or absecce of viable Schwann cells at the amputation plane was deduced from the remyelination of nerves under conditions which do or do not permit paradoxical regeneration. The nerves of totally irradiated and denervated limbs remain unmyelinated following regrowth of axons and such limbs do not regenerate after amputation. When only the hand was shielded from irradiation before the limbs were denervated, the new axons became completely remyelinated and some of these limbs regenerated when amputated. It is suggested that under these conditions Schwann cells can migrate proximally and can then proliferate further to form a blastema, since they would be the only unirradiated tissue present at the amputation plane.

The role of Schwann cells in paradoxical regeneration in the axolotl

The experiments described here examine further the conditions under which paradoxical regeneration occurs and provide support for the hypothesis that a proximal migration of Schwann cells is responsible for the phenomenon. When only the hand is shielded from irradition and the limb is denervated, amputation through the forearm or upper arm sometimes results in regeneration. The effects of variation in the time interval between denervation and amputation, the level of amputation and the method and number of denervations on the incidence of regeneration were investigated. The presence or absence of viable Schwann cells at the amputation plane was deduced from the remyelination of nerves under conditions which do or do not permit paradoxical regeneration. The nerves of totally irradiated and denervated limbs remain unmyelinated following regrowth of axons and such limbs do not regenerate after amputation. When only the hand was shielded from irradiation before the limbs were denervated, the new axons became completely remyelinated and some of these limbs regenerated when amputated. It is suggested that under these conditions Schwann cells can migrate proximally and can then proliferate further to form a blastema, since they would be the only unirradiated tissue present at the amputation plane.

Blastemal kinetics and pattern formation during amphibian limb regeneration

To investigate whether the uniqueness of proximal and distal limb regenerates could be attributed simply to differing blastemal growth characteristics, their increase in volume, cell number and cell-cycle times were determined. With respect to these parameters proximal and distal blastemas were identical and, furthermore, no evidence could be found for the existance of separate growth zones such as an apical proliferation centre or a progress zone within the blastema. It was therefore concluded that level-specific properties of the blastemal cells play the major role in determining the structure of the regenerate, not their growth characteristics. The only discernible difference was in the cell number within the two types of blastema at the onset of cartilage redifferentiation - proximal regenerates had 60% more cells. Thus it seems that the larger the pattern to be regenerated (the more proximal the amputation plane), the larger the primordium within which that pattern first appears. These two conclusions are discussed in relation to current theories of pattern formation during limb regeneration and development and a few way of envisaging the regeneration of pattern is described.

Blastemal kinetics and pattern formation during amphibian limb regeneration

To investigate whether the uniqueness of proximal and distal limb regenerates could be attributed simply to differing blastemal growth characteristics, their increase in volume, cell number and cell-cycle times were determined. With respect to these parameters proximal and distal blastemas were identical and, furthermore, no evidence could be found for the existence of separate growth zones such as an apical proliferation centre or a progress zone within the blastema. It was therefore concluded that level-specific properties of the blastemal cells play the major role in determining the structure of the regenerate, not their growth characteristics. The only discernible difference was in the cell number within the two types of blastema at the onset of cartilage redifferentiation— proximal regenerates had 60 % more cells. Thus it seems that the larger the pattern to be regenerated (the more proximal the amputation plane), the larger the primordium within which that pattern first appears. These two conclusions are discussed in relation to current theories of pattern formation during limb regeneration and development and a new way of envisaging the regeneration of pattern is described

How x-rays inhibit amphibian limb regeneration

The effects of an inhibiting dose of 2,000 rad of X-rays on the regenerating limbs of axolotl larvae have been examined in a histological and cytological study. Particular attention was paid to the mitotic indices of normal and irradiated epidermal and blastemal cells. Both the characteristic pattern of epidermal mitotic stimulation which normally follows amputation and the later increase in blastemal mitoses are suppressed by irradiation. In most cells the effects are permanent, but in a small proportion a mitotic delay is induced and upon subsequent division chromosome damage in the form of micronuclei is revealed. Thus irradiated cells which do divide almost certainly die. These results are discussed in relation to other theories of X-ray inhibition of regeneration with particular reference to the view that irradiated cells can be reactivated.

The cell cycle during amphibian limb regeneration

The duration of the cell cycle in the blastema of regenerating limbs of axolotls has been measured by means of [3H]thymidine pulse labelling and autoradiography. A chase was required to define the pulse period. An average cell cycle at 20 degrees C takes 53 h, S-phase takes 38 h; including parts of mitosis, G1 is 10 h and G2 is 5 h long. The protracted cycle and S-phase are consonant with the large genome in axolotis and other urodeles. The rapidly growing blastema probably contains a steady population of about 5000 proliferating cells, as there is a regular withdrawal of differentiating cells from the population. The kinds of determination which exist in this population of cells, or are exerted on it, are briefly considered.

Irradiation inhibits the regeneration of aneurogenic limbs

The developing arms of axolotl larvae from the 2-digit stage onward and the aneurogenic arms of surgically denervated larvae maintained in parabiosis are able to regenerate after amputation. Such regeneration is uniformly inhibited by local irradiation of the arm, whether innervated or not. This demonstration refutes a recent hypothesis that X-rays interfere with a special activity of nerves required for regeneration, and supports the earlier concept that X-rays act directly on those cells which must proliferate to form the regenerated tissues.

Participation of cartilage grafts in amphibian limb regeneration

Irradiated axolotl arms bearing grafts of pure cartilage obtained from the non-irradiated humerus of other specimens show a variable response to amputation of the hand. Some arms grow spikes with little morphogenesis, others produce one or more digits, and some regenerate small and defective but recognizable hands. Identically irradiated control arms which carry no grafts or grafts of irradiated tissue show none of these features. The cartilage grafts must therefore be responsible for the limited amount of regeneration observed. The origin of the regenerated tissues could not be independently determined in this experiment, although grafts were made reciprocally between white and dark genotypes, as all the regenerates were either very pale or completely unpigmented. A previous demonstration that grafted tissue does not reactivate irradiated host cells, however, permits the conclusion that the grafted cartilage provides all the mesodermal tissues of these regenerates.

The grafts consisted exclusively of chondrocytes, while the regenerates contained cartilage, muscle, blood vessels, nerve sheaths and general connective tissue. It is concluded that chondrocytes dedifferentiate into pluripotent blastemal mesenchyme cells which can redifferentiate into all other internal tissues of the limb. The concepts of modulation and neoblasts (undifferentiated reserve cells) thus have no basis in amphibian limb regeneration.