Lens regeneration in axolotl: new evidence of developmental plasticity
© Suetsugu-Maki et al; licensee BioMed Central Ltd. 2012
Received: 7 November 2012
Accepted: 17 December 2012
Published: 17 December 2012
Among vertebrates lens regeneration is most pronounced in newts, which have the ability to regenerate the entire lens throughout their lives. Regeneration occurs from the dorsal iris by transdifferentiation of the pigment epithelial cells. Interestingly, the ventral iris never contributes to regeneration. Frogs have limited lens regeneration capacity elicited from the cornea during pre-metamorphic stages. The axolotl is another salamander which, like the newt, regenerates its limbs or its tail with the spinal cord, but up until now all reports have shown that it does not regenerate the lens.
Here we present a detailed analysis during different stages of axolotl development, and we show that despite previous beliefs the axolotl does regenerate the lens, however, only during a limited time after hatching. We have found that starting at stage 44 (forelimb bud stage) lens regeneration is possible for nearly two weeks. Regeneration occurs from the iris but, in contrast to the newt, regeneration can be elicited from either the dorsal or the ventral iris and, occasionally, even from both in the same eye. Similar studies in the zebra fish concluded that lens regeneration is not possible.
Regeneration of the lens is possible in the axolotl, but differs from both frogs and newts. Thus the axolotl iris provides a novel and more plastic strategy for lens regeneration.
KeywordsLens Regeneration Axolotl Plasticity
Salamanders, especially newts, are capable of regenerating tissues, organs and even body parts, such as limbs and tails throughout their adult lives [1, 2]. Among all tissues that can be regenerated, the lens is a special case. In contrast to removal of a part of limb or tail (total removal disallows regeneration), the lens can be removed in its entirety. It has also been shown that newts can regenerate the lens very faithfully no matter how many repeated lentectomies are performed or the age of the animal . Notably, the lens regenerates by another tissue, the pigment epithelial cells of the iris by the process of transdifferentiation. There is, however, a restriction: The lens can only be regenerated from the dorsal iris. Even though the ventral iris can be induced experimentally to regenerate a lens, normally it never contributes to the process . This restriction can be very instructive when it comes to investigating the mechanisms underlying the process.
The only other vertebrates that have been shown to regenerate the lens are frogs and some fish. In frogs, the process has been studied well and it has been established that regeneration occurs only in pre-metamorphic stages and that the lens is derived from the cornea by transdifferentiation . After metamorphosis, the ability for lens regeneration ceases. Another salamander, the neonate axolotl, is also capable of regenerating limbs and tails as is the newt. However, in a paper by Stone in 1967 it was concluded that this species is not able to regenerate the lens, even though no staging details were provided . We have decided to revisit this issue and have undertaken a detailed study starting with larvae at stage 44. At this stage the eye tissues including the lens have been well differentiated, while other body parts (such as limbs) are beginning to form . We find that starting at this stage axolotls, similarly to newts, can regenerate a perfect lens from the iris and that this ability persists for about two to three weeks beyond that stage. After that, the ability for lens regeneration is lost. Surprisingly, however, we find that the lens can be regenerated by either the dorsal or the ventral iris. In some cases regeneration occurred from both irises in the same eye. A similar series of experiments employing zebrafish failed to show any evidence that this species can regenerate their lens underscoring the importance of urodeles in the study of lens regeneration.
Axolotl larvae (st35 and st43, pre-hatched) were supplied from the Ambystoma Genetic Stock Center (Department of Biology, Univ. of Kentucky, KY, USA) and kept at 27°C. The hatched larvae were fed brine shrimp. Larvae were anesthetized in 0.1% ethyl 3-aminobenzoate (#E10521; Sigma, St. Louis, MO, USA) at different stages. Using a sharp-edged blade, an incision was made in the cornea, and then the lens was removed in its entirety. At different time intervals after lentectomy, animals were fixed in methanol acetic acid solution (methanol: acetic acid = 3:1) at 4°C overnight and processed for paraffin embedding. Similar series were also performed using zebrafish embryos (wild-type Ekkwill strain) at different stages. Animal care adhered to the guide lines of the Institutional Animal Care and Use Committee (IACUC), University of Dayton (axolotl) and of the IACUC, Cincinnati Children's Hospital Medical Center (zebrafish).
Hematoxylin and Eosin staining and immunohistochemistry
Paraffin sections of 15 μm were deparaffinized and used for H & E staining and immunohistochemistry. For H & E staining, hematoxylin (#26754-01; Electron Microscope Sciences, Hatfield, PA, USA) and eosin (#26762-01; Electron Microscope Sciences) were used. To examine cell proliferation in the iris after lentectomy, 5-bromo-2'- deoxyuridine (BrdU; #B5002; SIGMA, 75 ug/g body weight) was injected into larvae 3 hours before fixation. Sections from BrdU injected larvae were treated with 1N HCl for 5 minutes at room temperature, blocked in TNB buffer (0.1 M Tris-HCl, pH7.5; 0.15 M NaCl; and 0.5% blocking reagent) supplied in the TSA kit (Perkin Elmer, Waltham, MA, USA) and incubated with mouse anti-BrdU antibody (1/100 dilution, #MAB3510; Millipore, Billerica, MA, USA) overnight at 4°C, and subsequently washed and incubated with Alexa 488 conjugated anti-immunoglobulin G (IgG) (1/100 dilution, Invitrogen, Grand Island, NY, USA) for 90 minutes at room temperature. To monitor lens regeneration, sections were incubated with anti γ-crystallin rabbit antibody (1/300 dilution, source bovine crystallin) and then detected with an anti-rabbit Cy3 conjugated antibody (1/100 dilution, Millipore). Immunohistochemistry images were taken using a BX51 microscope (Olympus, Tokyo, Japan) with a CCD camera (Cool SNAP cf2; Photometrics, Tucson, AZ, USA) and imaging software (Metamorph, Molecular Devices, Eugene, OR, USA), or by confocal imaging (Olympus FV500 confocal microscope).
Results and discussion
Lens regeneration in axolotl evaluated at different stages starting with 1 day post-hatch after stage 44.
Stage at lentectomy
Lens regeneration in zebrafish at different stages of development.
Regenerated lenses at Day 4 post-lentectomy
Prim 22 (35 hpf)
High or Long Pec (42-48 hpf)
Long Pec + FGF2 (48 hpf)
Protruding mouth (72 hpf)
Our results, thus, convincingly show that lens regeneration is possible in the axolotl larvae. In this respect, there are differences and similarities when axolotl lens regeneration is compared with the same process in frogs and in newts. Similarly to frogs, regeneration is possible during a limited time window, but in contrast, the iris is the source of regeneration in the axolotl and not the cornea. Similarly to newts, regeneration takes place from the iris pigment epithelium, but in contrast to the newt, it is not restricted to only the dorsal iris, and this capacity is not present at later stages or as an adult.
It is interesting to also note here that the competent stages for lens regeneration coincide with early development of forelimbs and hindlimbs. Stage 44 is morphologically marked by the development of the forelimb bud [7, 8]. The capacity for regeneration is terminated around stage 54 which is marked by the beginning of hindlimb bud differenrtiation. While a direct correlation between the two events cannot be made at present, one idea could be that crucial factors for both events act during that time window.
Taken together our results underscore the importance of newts in the study of lens regeneration. However, the fact that there is a time window in which the axolotl can actually regenerate the lens provides an excellent comparative addition to the study of lens regeneration. Recently, the transcriptome of newt lens regeneration has been completed (unpublished) as well as similar studies during axolotl limb regeneration . Thus, this technology could be of great value to regeneration studies. It will be of great interest to compare, in the future, the transcriptome of lens regeneration in axolotl (permissive and non-permissive stages) with the newt transcriptome of the dorsal and ventral iris. Such analysis can pinpoint crucial regulation underlying induction of lens regeneration.
In this study, we have shown that lens regeneration is possible in the axolotl for a limited period of time. Lens can be regenerated between stage 44 (hatching) and stage 52 (2 weeks post hatching). Our results show that contrary to the newt that regenerates the lens only from the dorsal iris, the lens can be regenerated from either the dorsal or the ventral iris or even from both sites. Thus, the axolotl lens regeneration provides evidence for a new developmental plasticity.
fibroblast growth factor
- H & E:
hematoxylin and eosin.
This study was supported by NIH grant EY10540 to PAT and EY 17319 to KDRT.
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