The cell cycle during amphibian limb regeneration

The specialist in regeneration-the Axolotl-a suitable model to study bone healing?

While the axolotl's ability to completely regenerate amputated limbs is well known and studied, the mechanism of axolotl bone fracture healing remains poorly understood. One reason might be the lack of a standardized fracture fixation in axolotl. We present a surgical technique to stabilize the osteotomized axolotl femur with a fixator plate and compare it to a non-stabilized osteotomy and to limb amputation. The healing outcome was evaluated 3 weeks, 3, 6 and 9 months post-surgery by microcomputer tomography, histology and immunohistochemistry. Plate-fixated femurs regained bone integrity more efficiently in comparison to the non-fixated osteotomized bone, where larger callus formed, possibly to compensate for the bone fragment misalignment. The healing of a non-critical osteotomy in axolotl was incomplete after 9 months, while amputated limbs efficiently restored bone length and structure. In axolotl amputated limbs, plate-fixated and non-fixated fractures, we observed accumulation of PCNA+ proliferating cells at 3 weeks post-injury similar to mouse. Additionally, as in mouse, SOX9-expressing cells appeared in the early phase of fracture healing and amputated limb regeneration in axolotl, preceding cartilage formation. This implicates endochondral ossification to be the probable mechanism of bone healing in axolotls. Altogether, the surgery with a standardized fixation technique demonstrated here allows for controlled axolotl bone healing experiments, facilitating their comparison to mammals (mice).

Local mechanical stimuli correlate with tissue growth in axolotl salamander joint morphogenesis

Movement-induced forces are critical to correct joint formation, but it is unclear how cells sense and respond to these mechanical cues. To study the role of mechanical stimuli in the shaping of the joint, we combined experiments on regenerating axolotl (Ambystoma mexicanum) forelimbs with a poroelastic model of bone rudiment growth. Animals either regrew forelimbs normally (control) or were injected with a transient receptor potential vanilloid 4 (TRPV4) agonist during joint morphogenesis. We quantified growth and shape in regrown humeri from whole-mount light sheet fluorescence images of the regenerated limbs. Results revealed significant differences in morphology and cell proliferation between groups, indicating that TRPV4 desensitization has an effect on joint shape. To link TRPV4 desensitization with impaired mechanosensitivity, we developed a finite element model of a regenerating humerus. Local tissue growth was the sum of a biological contribution proportional to chondrocyte density, which was constant, and a mechanical contribution proportional to fluid pressure. Computational predictions of growth agreed with experimental outcomes of joint shape, suggesting that interstitial pressure driven from cyclic mechanical stimuli promotes local tissue growth. Predictive computational models informed by experimental findings allow us to explore potential physical mechanisms involved in tissue growth to advance our understanding of the mechanobiology of joint morphogenesis.

Tgf-β superfamily and limb regeneration: Tgf-β to start and Bmp to end

Axolotls represent a popular model to study how nature solved the problem of regenerating lost appendages in tetrapods. Our work over many years focused on trying to understand how these animals can achieve such a feat and not end up with a scarred up stump. The Tgf-β superfamily represents an interesting family to target since they are involved in wound healing in adults and pattern formation during development. This family is large and comprises Tgf-β, Bmps, activins and GDFs. In this review, we present work from us and others on Tgf-β & Bmps and highlight interesting observations between these two sub-families. Tgf-β is important for the preparation phase of regeneration and Bmps for the redevelopment phase and they do not overlap with one another. We present novel data showing that the Tgf-β non-canonical pathway is also not active during redevelopment. Finally, we propose a molecular model to explain how Tgf-β and Bmps maintain distinct windows of expression during regeneration in axolotls.

The constrained architecture of mammalian Hox gene clusters

Vertebrate Hox genes are clustered. This organization has a functional relevance, as the transcription of each gene in time and space depends upon its relative position within the gene cluster. Hox clusters display a high organization, and all genes are transcribed from the same DNA strand. Here, we investigate the importance of this uniform transcriptional polarity by engineering alleles where one or several transcription units are inverted, with or without a CTCF site. We observe that inversions are likely detrimental to the proper implementation of this genetic system. We propose that the enhanced organization of Hox clusters in vertebrates evolved in conjunction with the emergence of global gene regulation to optimize a coordinated response of selected subsets of target genes.

In many animal species with a bilateral symmetry, Hox genes are clustered either at one or at several genomic loci. This organization has a functional relevance, as the transcriptional control applied to each gene depends upon its relative position within the gene cluster. It was previously noted that vertebrate Hox clusters display a much higher level of genomic organization than their invertebrate counterparts. The former are always more compact than the latter, they are generally devoid of repeats and of interspersed genes, and all genes are transcribed by the same DNA strand, suggesting that particular factors constrained these clusters toward a tighter structure during the evolution of the vertebrate lineage. Here, we investigate the importance of uniform transcriptional orientation by engineering several alleles within the HoxD cluster, such as to invert one or several transcription units, with or without a neighboring CTCF site. We observe that the association between the tight structure of mammalian Hox clusters and their regulation makes inversions likely detrimental to the proper implementation of this complex genetic system. We propose that the consolidation of Hox clusters in vertebrates, including transcriptional polarity, evolved in conjunction with the emergence of global gene regulation via the flanking regulatory landscapes, to optimize a coordinated response of selected subsets of target genes in cis.

Ablation of BMP signaling hampers the blastema formation in Poecilia latipinna by dysregulating the extracellular matrix remodeling and cell cycle turnover

Bone morphogenetic proteins play a pivotal role in the epimorphic regeneration in vertebrates. Blastema formation is central to the epimorphic regeneration and crucially determines its fate. Despite an elaborate understanding of importance of Bone morphogenetic protein signaling in regeneration, its specific role during the blastema formation remains to be addressed. Regulatory role of BMP signaling during blastema formation was investigated using LDN193189, a potent inhibitor of BMP receptors. The study involved morphological observation, in vivo proliferation assay by incorporation of BrdU, comet assay, qRT-PCR and western blot. Blastemal outgrowth was seen reduced due to LDN193189 treatment, typified by dimensional differences, reduced number of proliferating cells and decreased levels of PCNA. Additionally, proapoptotic markers were found to be upregulated signifying a skewed cellular turnover. Further, the cell migration was seen obstructed and ECM remodeling was disturbed as well. These findings were marked by differential transcript as well as protein expressions of the key signaling and regulatory components, their altered enzymatic activities and other microscopic as well as molecular characterizations. Our results signify, for the first time, that BMP signaling manifests its effect on blastema formation by controlling the pivotal cellular processes possibly via PI3K/AKT. Our results indicate the pleiotropic role of BMPs specifically during blastema formation in regulating cell migration, cell proliferation and apoptosis, and lead to the generation of a molecular regulatory map of determinative molecules.

Single-cell analysis uncovers convergence of cell identities during axolotl limb regeneration

Amputation of the axolotl forelimb results in the formation of a blastema, a transient tissue where progenitor cells accumulate prior to limb regeneration. However, the molecular understanding of blastema formation had previously been hampered by the inability to identify and isolate blastema precursor cells in the adult tissue. We have used a combination of Cre-loxP reporter lineage tracking and single-cell messenger RNA sequencing (scRNA-seq) to molecularly track mature connective tissue (CT) cell heterogeneity and its transition to a limb blastema state. We have uncovered a multiphasic molecular program where CT cell types found in the uninjured adult limb revert to a relatively homogenous progenitor state that recapitulates an embryonic limb bud-like phenotype including multipotency within the CT lineage. Together, our data illuminate molecular and cellular reprogramming during complex organ regeneration in a vertebrate.

Beryllium nitrate inhibits fibroblast migration to disrupt epimorphic regeneration

Epimorphic regeneration proceeds with or without formation of a blastema, as observed for the limb and skin, respectively. Inhibition of epimorphic regeneration provides a means to interrogate the cellular and molecular mechanisms that regulate it. In this study, we show that exposing amputated limbs to beryllium nitrate disrupts blastema formation and causes severe patterning defects in limb regeneration. In contrast, exposing full-thickness skin wounds to beryllium only causes a delay in skin regeneration. By transplanting full-thickness skin from ubiquitous GFP-expressing axolotls to wild-type hosts, we demonstrate that beryllium inhibits fibroblast migration during limb and skin regeneration in vivo Moreover, we show that beryllium also inhibits cell migration in vitro using axolotl and human fibroblasts. Interestingly, beryllium did not act as an immunostimulatory agent as it does in Anurans and mammals, nor did it affect keratinocyte migration, proliferation or re-epithelialization, suggesting that the effect of beryllium is cell type-specific. While we did not detect an increase in cell death during regeneration in response to beryllium, it did disrupt cell proliferation in mesenchymal cells. Taken together, our data show that normal blastema organogenesis cannot occur without timely infiltration of local fibroblasts and highlights the importance of positional information to instruct pattern formation during regeneration. In contrast, non-blastemal-based skin regeneration can occur despite early inhibition of fibroblast migration and cell proliferation.

Positional plasticity in regenerating Amybstoma mexicanum limbs is associated with cell proliferation and pathways of cellular differentiation

Background

The endogenous ability to dedifferentiate, re-pattern, and re-differentiate adult cells to repair or replace damaged or missing structures is exclusive to only a few tetrapod species. The Mexican axolotl is one example of these species, having the capacity to regenerate multiple adult structures including their limbs by generating a group of progenitor cells, known as the blastema, which acquire pattern and differentiate into the missing tissues. The formation of a limb regenerate is dependent on cells in the connective tissues that retain memory of their original position in the limb, and use this information to generate the pattern of the missing structure. Observations from recent and historic studies suggest that blastema cells vary in their potential to pattern distal structures during the regeneration process; some cells are plastic and can be reprogrammed to obtain new positional information while others are stable. Our previous studies showed that positional information has temporal and spatial components of variation; early bud (EB) and apical late bud (LB) blastema cells are plastic while basal-LB cells are stable. To identify the potential cellular and molecular basis of this variation, we compared these three cell populations using histological and transcriptional approaches.

Results

Histologically, the basal-LB sample showed greater tissue organization than the EB and apical-LB samples. We also observed that cell proliferation was more abundant in EB and apical-LB tissue when compared to basal-LB and mature stump tissue. Lastly, we found that genes associated with cellular differentiation were expressed more highly in the basal-LB samples.

Conclusions

Our results characterize histological and transcriptional differences between EB and apical-LB tissue compared to basal-LB tissue. Combined with our results from a previous study, we hypothesize that the stability of positional information is associated with tissue organization, cell proliferation, and pathways of cellular differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0095-4) contains supplementary material, which is available to authorized users.

Regulation of Axolotl (Ambystoma mexicanum) Limb Blastema Cell Proliferation by Nerves and BMP2 in Organotypic Slice Culture

We have modified and optimized the technique of organotypic slice culture in order to study the mechanisms regulating growth and pattern formation in regenerating axolotl limb blastemas. Blastema cells maintain many of the behaviors that are characteristic of blastemas in vivo when cultured as slices in vitro, including rates of proliferation that are comparable to what has been reported in vivo. Because the blastema slices can be cultured in basal medium without fetal bovine serum, it was possible to test the response of blastema cells to signaling molecules present in serum, as well as those produced by nerves. We also were able to investigate the response of blastema cells to experimentally regulated changes in BMP signaling. Blastema cells responded to all of these signals by increasing the rate of proliferation and the level of expression of the blastema marker gene, Prrx-1. The organotypic slice culture model provides the opportunity to identify and characterize the spatial and temporal co-regulation of pathways in order to induce and enhance a regenerative response.

Reversibility of the Differentiated State: Regeneration in Amphibians

In contrast to mammals, some fish and amphibians have retained the ability to regenerate complex body structures or organs, such as the limb, tail, eye lens, or even parts of the heart. One major difference in the response to injury is the appearance of a mesenchymal growth zone or blastema in these regenerative species instead of the scarring seen in mammals. This blastema is thought to largely derive from the dedifferentiation of various functional cell types, such as skeletal muscle, dermis, and cartilage. In the case of multinucleated skeletal muscle fibers, cell cycle reentry into S-phase as well as fragmentation into mononucleated progenitors is observed both in vitro and in vivo.

Hair cell regeneration in sensory epithelia from the inner ear of a urodele amphibian

The capacity of urodele amphibians to regenerate a variety of body parts is providing insight into mechanisms of tissue regeneration in vertebrates. In this study the ability of the newt, Notophthalmus viridescens, to regenerate inner ear hair cells in vitro was examined. Intact otic capsules were maintained in organotypic culture. Incubation in 2 mM gentamicin for 48 hours resulted in ablation of all hair cells from the saccular maculae. Thus, any hair cell recovery was not due to repair of damaged hair cells. Immature hair cells were subsequently observed at approximately 12 days posttreatment. Their number increased over the following 7-14 days to reach approximately 30% of the normal number. Following incubation of damaged tissue with bromodeoxyuridine (BrdU), labeled nuclei were confined strictly within regions of hair cell loss, indicating that supporting cells entered S-phase. Double labeling of tissue with two different hair cell markers and three different antibodies to BrdU in various combinations, however, all showed that the nuclei of cells that labeled with hair cell markers did not label for BrdU. This suggested that the new hair cells were not derived from those cells that had undergone mitosis. When mitosis was blocked with aphidicolin, new hair cells were still generated. The results suggest that direct phenotypic conversion of supporting cells into hair cells without an intervening mitotic event is a major mechanism of hair cell regeneration in the newt. A similar mechanism has been proposed for the hair cell recovery phenomenon observed in the vestibular organs of mammals.

Heterogeneous proliferative potential in regenerative adult newt cardiomyocytes

Adult newt cardiomyocytes, in contrast to their mammalian counterparts, can proliferate after injury and contribute to the functional regeneration of the heart. In order to understand the mechanisms underlying this plasticity we performed longitudinal studies on single cardiomyocytes in culture. We find that the majority of cardiomyocytes can enter S phase, a process that occurs in response to serum-activated pathways and is dependent on the phosphorylation of the retinoblastoma protein. However, more than half of these cells stably arrest at either entry to mitosis or during cytokinesis, thus resembling the behaviour observed in mammalian cardiomyocytes. Approximately a third of the cells progress through mitosis and may enter successive cell divisions. When cardiomyocytes divided more than once, the proliferative behaviour of sister cells was significantly correlated, in terms of whether they underwent a subsequent cell cycle, and if so, the duration of that cycle. These observations suggest a mechanism whereby newt heart regeneration depends on the retention of proliferative potential in a subset of cardiomyocytes. The regulation of the remaining newt cardiomyocytes is similar to that described for their mammalian counterparts, as they arrest during mitosis or cytokinesis. Understanding the nature of this block and why it arises in some but not other newt cardiomyocytes may lead to an augmentation of the regenerative potential in the mammalian heart.

Cell proliferation during blastema formation in the regenerating teleost fin

Epimorphic regeneration in teleost fins occurs through the establishment of a balanced growth state in which a blastema gives rise to all the mesenchymal cells, whereas definite areas of the epidermis proliferate leading to its extension, thus, allowing the enlargement of the whole structure. This type of regeneration involves specific mechanisms that temporally and spatially regulate cell proliferation. To understand how the blastema is formed and how this growth situation is set up, we investigated cell proliferation patterns in the regenerating fin of the goldfish Carassius auratus from the time of amputation to that of blastema formation by using proliferating cell nuclear antigen immunostaining and bromodeoxyuridine labeling. Wound closure and apical epidermal cap formation took place by epidermal migration and re-arrangement, without the contribution of cell proliferation. As soon as the apical cap had formed, the epidermis started to proliferate at its lateral surfaces, in which all layers maintained cycling for the duration of the studied process. The distal epidermal cap, on the contrary, presented very few cycling cells, and its cytoarchitecture was indicative of continuous remodeling due to ray growth. The basal layer of this epidermal cap showed a typical morphology and remained nonproliferative whilst in contact with the proliferating blastema. Proliferation in the mesenchymal compartment of the ray started far from the amputation plane. Subsequently, cycling cells approached that location, until they formed the blastema in contact with the apical epidermal cap. Differences observed between the epidermis and mesenchyma, regarding activation of the cell cycle and the establishment of proliferative patterns, suggest that differential mechanisms regulate cell proliferation in each of these compartments during the initial stages of regeneration.

Plasticity of retrovirus-labelled myotubes in the newt limb regeneration blastema

Two important indices of myogenic differentiation are the formation of syncytial myotubes and the postmitotic arrest from the cell cycle, both of which occur after fusion of mononucleate cells. We show here that these indices are reversed in the environment of the urodele limb regeneration blastema. In order to introduce an integrated (genetic) marker into newt myotubes, we infected mononucleate cells in culture with a pseudotyped retrovirus expressing human placental alkaline phosphatase (AP). After fusion the myotubes expressed AP and could be purified by sieving and micromanipulation so as to remove all mononucleate cells. When such purified retrovirus-labelled myotubes were implanted into a limb blastema they gave rise to mononucleate progeny with high efficiency. Purified myotubes labelled with fluorescent lipophilic cell tracker dye also gave rise to mononucleate cells; myotubes which were double labelled with the tracker dye and a nuclear stain gave rise to double-labelled mononucleate progeny. Nuclei within retrovirus-labelled myotubes entered S phase as evidenced by widespread labelling after injection of implanted newts with BrdU. The relation between the two aspects of plasticity is a critical further question.

Newt myotubes reenter the cell cycle by phosphorylation of the retinoblastoma protein

Withdrawal from the cell cycle is an essential aspect of vertebrate muscle differentiation and requires the retinoblastoma (Rb) protein that inhibits expression of genes needed for cell cycle entry. It was shown recently that cultured myotubes derived from the Rb-/- mouse reenter the cell cycle after serum stimulation (Schneider, J.W., W. Gu, L. Zhu, V. Mahdavi, and B. Nadal-Ginard. 1994. Science (Wash. DC). 264:1467-1471). In contrast with other vertebrates, adult urodele amphibians such as the newt can regenerate their limbs, a process involving cell cycle reentry and local reversal of differentiation. Here we show that myotubes formed in culture from newt limb cells are refractory to several growth factors, but they undergo S phase after serum stimulation and accumulate 4N nuclei. This response to serum is inhibited by contact with mononucleate cells. Despite the phenotypic parallel with Rb-/- mouse myotubes, Rb is expressed in the newt myotubes, and its phosphorylation via cyclin-dependent kinase 4/6 is required for cell cycle reentry. Thus, the postmitotic arrest of urodele myotubes, although intact in certain respects, can be undermined by a pathway that is inactive in other vertebrates. This may be important for the regenerative ability of these animals.

In vitro and in vivo characterization of blastemal cells from regenerating newt limbs

To better characterize the cells involved in newt limb regeneration, blastemal cells from accumulation and differentiation phase blastemas were grown in dissociated cell culture, and their morphology and antigenic phenotype determined using a variety of antibodies directed against intermediate filaments, cell adhesion molecules, and extracellular matrix molecules. In addition to previously described blastemal cell morphologies, many of the cells in these cultures had a round cell body, with an eccentrically placed nucleus and a cytoplasm filled with autofluorescent granules. The majority of accumulation phase blastemal cells labeled with antibodies against GFAP, vimentin, 22/18 as well as with antibodies against NCAM, L-1, laminin, and fibronectin. The majority of differentiation phase blastemal cells had a similar phenotype but lacked expression of vimentin and fibronectin. Comparison of the blastemal phenotype in vitro and in vivo showed similar expression characteristics. However, in differentiation phase blastemas, laminin immunoreactivity was concentrated in specific locations. In addition, the proliferation of cultured blastemal cells is stimulated by the addition of a crude brain extract, consistent with previous studies in vivo and in vitro. Taken together, these observations suggest that dissociated cultures of newt limb blastemal cells provide a suitable model for the analysis of the cell and molecular mechanisms involved in limb regeneration.

A test of the punctuated-cycling hypothesis in Ambystoma forelimb regenerates: the roles of animal size, limb innervation, and the aneurogenic condition

The punctuated-cycling (PC) hypothesis [39] predicts that the proportion of actively cycling (AC) cells within the blastema influences the rate of limb regeneration in urodele amphibians. To test this, we compared the rate of regeneration and the parameters of the PC hypothesis in small and large Ambystoma mexicanum larvae and in aneurogenic limbs of Ambystoma maculatum. Aneurogenic limbs regenerated more slowly than limbs of small axolotls, but considerably faster than limbs of large axolotls. Regardless of regeneration rates, virtually all blastema cells were in the proliferative fraction (Pf) (ranging from 92.3% +/- 4.2% to 96.2% +/- 3.4%). As predicted, in the blastemata of more rapidly regenerating small axolotls, 86% of the proliferative fraction was actively cycling, but as regeneration slowed, the proportion of the proliferative fraction that was actively cycling decreased (the AC of aneurogenic limbs being 69.5%, and that of large axolotl limbs being 57.3%) and the proportion of transiently quiescent cells increased. The parameters of the PC hypothesis were also examined in small axolotls at two different times during regeneration. During dedifferentiation and initial blastema formation, 61% of the cells in the proliferative fraction were actively cycling and 34% were transiently quiescent. During the rapid-growth phase of the blastema, 88% of the cells in the proliferative fraction were actively cycling and only 7% of the cells were transiently quiescent. It therefore appears that dedifferentiated cells do not immediately begin active cycling and that the transiently quiescent population is relatively large; however, during the period of rapid growth the proportion of transiently quiescent cells is small. In amputated/denervated limbs of small axolotls, the size of the proliferative fraction decreased as the length of the denervation interval increased. Furthermore, with prolonged denervation the total proportion of actively cycling blastema cells also declined (to about 15%). The failure of denervated limbs to regenerate was correlated with an increased nonproliferative fraction and a reduced proportion of actively cycling cells.

Muscle and cartilage differentiation in axolotl limb regeneration blastema cultures

A tissue culture system is described for explants of mesenchyme from Ambystoma mexicanum limb regeneration blastemas. Explants were cultured on collagen substrate for 3 weeks in minimal essential medium supplemented with the hormones insulin, thyroxine, somatotropin, and hydrocortisone, plus beef embryo extract (EE), 2%. This medium supported extensive cell migration onto the substrate followed by cell proliferation and differentiation of both cartilage matrix and myotubes. Cultures on plastic substrate, rather than on collagen, displayed similar cell outgrowth and cartilage formation, but relatively little myotube formation. Differentiation in EE-supplemented medium was compared with that in two defined media: Explants in medium containing only the hormones showed little outgrowth or cartilage development and never formed myotubes; medium containing the hormones plus fibroblast growth factor, 50 ng/ml, supported an intermediate degree of outgrowth and cartilage development and occasional myotube formation. Explant size was also a factor: Smaller explants survived and formed myotubes less frequently, even when on collagen in EE-supplemented medium.

Availability time of tritium-labeled DNA precursors in newt eyes following intraperitoneal injection of 3H-thymidine

Following intraperitoneal injection of 3H-thymidine into host newts, iris together with a regenerating lens was transplanted from a donor eye into a lentectomized host eye at frequent intervals for 20 hours and then every 1 or 2 days for 14 days. The eyes were fixed 2 hours and 1 or 2 days after implantation and autoradiographs prepared. Following fixation 2 hours after operation, incorporation of 3H-thymidine into DNA, as evidenced by grain counts over nuclei, fell rapidly for 3.5 hours after injection and was no longer apparent after 4.5 hours. However, almost one-half of the implants were lightly labeled when they remained in the host eyes for 1 or 2 days beginning from 1 to 14 days after isotope injection. When these implanted, regenerating lenses were left in the host eyes for longer periods of time, then a light label was found over nuclei in most of the implants remaining in the eye for 3 to 24 days. When 3H-thymidine was injected from 1 to 3 days after extirpation of both lens and neural retina, before DNA synthesis had been initiated in the pigmented retinal epithelium or iris, there were numerous cases of labeled nuclei among depigmenting cells of the pigmented retinal epithelium which was regenerating a new neural retina from 2 to 25 days after isotope injection. Depigmenting cells of the dorsal iris and regenerating lens were similarly labeled. These results provide evidence for the continued availability of small amounts of tritiated DNA-precursor molecules which can be incorporated in DNA of proliferating cells long after the initial injection of 3H-thymidine.

Limb regeneration to digit stages occurs in well-fed adult newts after hypophysectomy

Experiments were designed to determine the maximum survival time and extent of limb regeneration of well-fed adult newts (Notophthalmus viridescens) after complete hypophysectomy. Adult newts were either well-fed or fasted for 3 weeks at which time hypophysectomies were performed. Forelimbs were amputated 5 days posthypophysectomy. Fasted hypophysectomized newts did not survive beyond 4 weeks, and limb regeneration was either absent or abortive. All of the well-fed newts survived to 3 weeks posthypophysectomy, and even at 6 weeks posthypophysectomy showed 30% survival. In the complete absence of the pituitary gland, many of the fed newts regenerated limbs to digit stages. It is concluded that pituitary hormones are not an absolute requirement for limb regeneration of adult newts. Newts in good nutritional condition at the time of hypophysectomy survive longer and their limbs regenerate to more advanced stages.

Studies of mitosis in excised limb regenerates of the newt, Notophthalmus viridescens

Subsequent to excision and explantation of the limb blastema into culture medium, there is an abrupt reduction in mitotic index lasting several hours. Coincident with the disappearance of mitosis, abnormal mitotic figures (AMFs), lacking the condensed chromosomal nature of normal figures, are seen. These persist until normal levels of mitotic activity are restored approximately 6 hr later. The transient loss of mitotic activity is observed in both ganglionated and nonganglionated explants. The large number of prophase figures observed at time zero were sharply reduced within minutes, without concomitant increases in the later phases. The possibility that some cells, without completing mitosis, become temporarily indistinguishable as mitotic figures, is discussed in terms of chromosomal decondensation and recondensation.

Microdensitometric analysis of denervation effects on newt limb blastema cells

This report examines the fate of cycling cells in normal and denervated blastemas of adult newts. Cells are found to accumulate in G1 in blastemas which are nerve independent. No stage specific accumulation different from controls is found in limbs with nerve-dependent blastemas.

Neurotrophic stimulation of DNA synthesis in the regenerating forelimb of the newt, Triturus

The effect of infusions of soluble newt brain extracts on deoxyribonucleic acid (DNA) synthesis in the denervated, regenerating newt blastema has been studied. When moderate early to early regenerates are denervated for 48 hours, DNA synthesis normally drops to about 40% of that found in contralateral, innervated blastemas. Infusion of brain extract halfway through the denervation period (at 24 hours) approximately doubles the rate of DNA synthesis in denervated regenerates.

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.