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Seminars in Cell & Developmental Biology 20 (2009) 691–696

Contents lists available at ScienceDirect

Seminars in Cell & Developmental Biology

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ot lost in translationensing the loss and filling the gap during regeneration

ndrás Simon ∗, Daniel Berg, Matthew Kirkhamarolinska Institute, Department of Cell and Molecular Biology, von Eulers väg 3, 17177 Stockholm, Sweden

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a b s t r a c t

Every organism responds to injuries by reparative processes in order to adapt to the altered conditions.The quality of the adjustment in terms of morphological and functional recapitulation of the originalstatus varies among species. One task is to understand the concepts by which animals with outstandingregenerative capabilities sense what and how much is missing, and how they translate that information

egenerationopaminearkinson’s diseaseeurogenesis

to the appropriate responses. These concepts may integrate various kinds of regenerative phenomenaalthough the specific molecular and cellular mechanisms that execute these processes are divergent anddepend on the type of the injury. The use of a variety of lesion paradigms could uncover common principlesthat link injury to successful regeneration. In addition they could indicate means how to further translatethis knowledge to the practice of regenerative medicine. We exemplify this possibility by outlining somecritical features of dopaminergic neurogenesis in the midbrain of adult salamanders, and the implications

for Parkinson’s disease.

© 2009 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6912. Epimorphic regeneration as a qualitative term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6923. Converging driving forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6924. Lesion models and regenerative medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6935. Modelling Parkinson’s disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6936. Cellular source of DA regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693

7. Starting and terminating DA regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694

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. . . . .

olecular characterization and manipulations. During the pastecade considerable progress has taken place and several typesf molecular links between injury and regeneration have beendentified [4]. Despite that regeneration is not just the mere

∗ Corresponding author. Tel.: +46 8 52487020; fax: +46 8 308374.E-mail address: Andras.Simon@ki.se (A. Simon).

084-9521/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.semcdb.2009.04.015

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recapitulation of embryonic development in terms of cell andtissue dynamics, no distinctive, regeneration-specific molecularsignalling pathway has been revealed. Instead, the same majorpathways, such as Notch, Bmp, Wnt, Hh, and Fgf signalling areinvolved similarly to embryonic development also during regen-eration [5–9].

Employing various lesion paradigms in regeneration modelorganisms could reveal distinctive regeneration-specific concepts.Such concepts may be generalized, irrespective of lesion sites andlesion types, although they are represented by temporally and spa-tially differently regulated signalling pathways. Specifically, we willoutline some critical aspects of how the highly regenerative newt

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Research on classical regeneration models such as hydra, pla-naria, and amphibians provides not just pointers for regenerativemedicine but has significantly formed our thinking about devel-opmental biology in an evolutionary context (for recent reviewssee [1–3]. These animals have long been relatively refractory to

replaces lost dopaminergic neurons in the midbrain that could alsopotentially indicate new directions for therapeutic strategies forParkinson’s disease.

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need to be active in concert or be co-wired in order to achievemorphologically and functionally perfect regeneration as exem-plified by the newt limb. In this context, it is noteworthy thatlimb regeneration and the growth of the blastema is nerve depen-dent, possibly ensuring that the regenerate will become functional

92 A. Simon et al. / Seminars in Cell & D

. Epimorphic regeneration as a qualitative term

Some anamniote vertebrates, such as salamanders (newts andxolotls) and zebrafish are remarkably regenerative as adults. Thesenimals can rebuild complex body parts throughout their livesy a process traditionally referred to as epimorphic regeneration10–13]. But amniotes, like mammals, are not devoid of regenerativebilities either. Although mammals cannot re-grow an appendage,hey can regenerate tissues, such as epidermis, gut epithelium orkeletal muscle. Partly because of this reason, a subdivision ofegenerative phenomena into epimorphic and tissue regenerationas emerged [e.g. [14]].

The distinction between epimorphic and tissue regeneration hasowever rarely been addressed experimentally. We suggest that itould be equally meaningful to focus at the integrating principlesnderlying tissue and whole appendage regeneration in animals,hich are capable for both. In particular, these experiments could

arget tissues and cell types that cannot be repaired or replaced inammals, and which normally do not display extensive homeo-

tatic cellular turn over.Carlson compared in a series of experiments the behaviour of

keletal muscle after limb amputations and minced muscle injuriesn salamanders, which led to the suggestion that the type of thenjury (amputation or mincing) determines a shift between thepimorphic and the tissue modes of regeneration within the samenimal, and that the epimorphic mode dominates over tissue modef regeneration [15–17].

Despite these studies, the possibly exclusive mechanisms of epi-orphic regeneration compared to alternative regenerative modes

re still largely unclear.Epimorphic regeneration is usually described as a process

equiring cellular proliferation and involving formation of alastema after wound healing (for a recent review see [18], andor further discussions concerning epimorphic versus morhallacticegeneration modes in invertebrates see [19,20]). According to thelassical definition, the regeneration blastema is a self-organizinglump of proliferating cells, which upon transplantation to ectopicites gives rise to structures corresponding to its origin [21,22].pplying this definition, blastema formation is indeed exclusive

or appendage regeneration. However, we do not find any decisivergument at present why lens regeneration in newts for example,hich proceeds without wound healing and blastema formation,

hould not be classified as epimorphic. Adult lens replacement cannly occur in newts (not even in all salamanders), and it depends onhe proliferation of transdifferentiating pigmented epithelial cells23–25]. Extrapolating further on these notes, it is not unconceiv-ble to envisage that perfect regeneration of any tissue or cell typen newts that cannot be replaced in mammals share fundamentalttributes with regeneration of complex body parts.

The question remains whether it is possible to identify com-on concepts that allow faithful replacement, should regenerative

ells arise from a blastema originating from a diverse cellularomposite such as the limb stump, from transdifferentiating pig-ented epithelial cells without forming a blastema, or from a single

nipotent progenitor for that matter. Clearly, these concepts wille manifested in miscellaneous molecular and cellular dynam-

cs and signalling events, which may be subdivided into distinctategories. Regeneration of an appendage, during which a multi-ude of cell types needs to be organized into several functionalnits will involve only partially overlapping mechanisms com-ared to the situation in which the missing organ’s composition

s much less diversified. Still, it is tempting to speculate that a crit-cal issue to understand the common strategies in various injuryaradigms of how the animal is sensing what and how much isissing, and how that information is relayed to the appropriate

esponses.

mental Biology 20 (2009) 691–696

3. Converging driving forces

During salamander limb regeneration loss is sensed partly as aphysical gap, which is represented by the discontinuity of positionalvalues of neighbouring cells along the limb axes after amputation.Limb progenitor cells in the regeneration blastema are stimulatedto fill the gap by proliferation and differentiation, a process calledintercalation. Blastema cells have correct and stably coded posi-tional identities and they only give rise to missing structures.Interestingly, the larger part of the limb is removed the greater isthe amount of cell division, and it has been postulated that overlap-ping cell interactions control both the growth and spatial pattern ofdifferentiation [26,27]. With other words, qualitative and quantita-tive outcomes are intertwined and reside at least to some extent inpositional information.

Loss could be perceived in other ways than sensing a physicalgap. Fig. 1 outlines three, not exclusive, and possibly cross-talkingways. Recent evidence suggests that signals originating from cellsdying as a result of injury evoke a proliferative response in regener-ative progenitors [28]. Furthermore, proliferation during larval tailregeneration in Xenopus requires caspase activity [29]. Althoughthe tail regenerate in pre-metamorphic Xenopus is not a perfectrecapitulation of the original [30], the findings indicate that apop-totic cells may be one driving force for regenerative progenitors (foran in depth review see also [31]. The amount of stimulatory signalsthat are derived from dying cells could stimulate increase in cellnumber thereby maintaining the size of a certain tissue. But howcould the correct cell types be produced?

One possibility is that dying cells release cell type specific sig-nals that could prime the activated, otherwise plastic progenitorstowards specific cell types. Alternatively, the correct regenerativeresponse is achieved by sensing the loss of a particular propertythat normally is derived from the cells that need to be replaced, orwhich is directly related to their function. Such a concept wouldnot just allow the appropriate sensing of what is missing but couldalso calibrate regeneration in quantitative terms and contribute tothe appropriate termination of the regenerative processes. It wouldimply the existence of feedback loops ensuring that products arenot made unless they are needed. Liver regeneration provides anexample for the functional loss being sensed by the concentrationof bile acid, which is a product of the liver [32]. Liver regeneration ishowever perfect only in terms of functional recovery and mass com-pensation, and the original morphology of the liver is not restoredafter partial hepatectomy.

Thus it seems likely that several types of sensing mechanisms

Fig. 1. Examples of driving forces for regeneration. Regenerative processes aim tofill a gap that is created after injury or cell loss. Loss may be sensed in different ways,such as a physical gap represented by discontinuity of positional values, paracrinesignals originating from dying cells or by sensing a functional loss. It is likely thatsuch mechanisms are co-wired to ensure morphologically and functionally faithfulregeneration.

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6. Cellular source of DA regeneration

New neurons could have several origins. Fig. 2 outlines a num-ber of potential ways. First, regeneration may or may not involve

Fig. 2. Possible sources of neurons during DA regeneration. A neural stem cell mayreenter the cell cycle producing a committed or uncommitted population, similarto transit amplifying cells in mammals [91], which subsequently differentiate into

A. Simon et al. / Seminars in Cell & D

1,33]. Recently, the plasma membrane anchored receptor Prod1as identified as a mediator of positional values. It is expressed

s a gradient along the proximo-distal axis and its expression isegulated by retinoic acid [34,35]. Its ligand (nAG), which in turns derived from Schwann cells surrounding the peripheral nerves,romotes blastema cell proliferation. In addition, it rescues regen-ration of denervated, regeneration-incompetent limbs [36]. Thisolecular interaction may couple positional information and func-

ionality at the level of an entire body structure, and analogousoncepts may act during regeneration in other contexts after dif-erent types of injuries. Analyzing regeneration in various types ofesion paradigms in the same animal could address this possibility.

. Lesion models and regenerative medicine

A great body of work in regeneration model organisms dealsith the replacement of an entire body structure, such as limbs

r tails. The salamander limb provides one of the most fascinatingxamples of how an adult vertebrate can re-grow a fully functionalnd complex structure that is indistinguishable from the original.owever, even according to the most optimistic scenario, we canxpect a long while before we have amassed means to achieve theormation of an autonomous blastema that can lead to completeimb regeneration in mammals. This remark should not be takens a discouragement. In opposite, salamander limb regeneration isrobably the best illustration of the potential of adult progenitorells given the right cues and the right context.

Fewer attempts have been made to mimic loss of individual cellypes within otherwise largely intact tissues, which also charac-erize a number of human degenerative diseases. Compared to theesection of entire body parts, simpler ablation models could alsoffer alternative perspectives to reveal general principles of howoss is linked to regeneration in highly regenerative species.

Recent papers using zebrafish and newts describe regenera-ion after cell type-selective genetic or chemical ablations [37–39]ithout significant disruption of the overall tissue architecture or

mputating a substantial part of an organ or body part. In thesexperimental settings, the degree of lesion is selective, adjustable,nd, at least in the case of genetic systems, the encoding sequencesould in principal be transmitted to the germ-line.

Below we will use the recently established dopaminergic (DA)egeneration model as a starting point to highlight key questionshat can be addressed in such experimental paradigms.

. Modelling Parkinson’s disease

Parkinson’s disease is a debilitating neurological disorder withurrently no cure. Although being quite heterogeneous in terms ofellular and clinical manifestations, loss of DA neurons in the mid-rain is one of the major hallmarks of the disease. Decreased DAelease by midbrain DA neurons to the striatum leads to anomaliesn the initiation and coordination of movements in patients [40,41].harmacology-based compensation strategies are to date the mainife quality improving treatments but they neither prevent nor dohey seem to delay the progression of the loss of DA neurons [42].n addition, the efficiency of pharmacological treatments decreases

arkedly with time. Considerable efforts have also been taken torevent the progression of the disease by characterizing how DAeurons die in order to find means of preventing cell death [43]. Celleplacement is another promising strategy. The two main principalays for cell replacement are the activation of endogenous pro-

enitor cells or the implantation of cells from various sources afterdequate in vitro manipulations [44–46]. However cell replacementrials have so far showed limited success [45,47].

The dominating models for Parkinson’s disease are based onhe use of the selective chemical toxins, 6-hydroxy-dopamine (6-

mental Biology 20 (2009) 691–696 693

OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP),both of which eliminate DA neurons within a relatively short timeframe [48,49]. Alternatively, genetic models have been developedthat also recapitulate certain aspects of the progressive nature ofthe disease [50,51].

Similarly to mammals, 6-OHDA-injection leads to a rapid deple-tion of DA neurons by cell death in newts, which is assessable bothby histological analyses and by locomotor performance assays [38].The DA neuronal population in salamanders is identifiable by theexpression, among other markers, of tyrosine hydroxylase (TH),nurr1, and engrailed1 [38]. Strictly anatomically regarded, midbrainDA neurons are evolutionarily relatively late inventions, appearingin amphibians. In functional terms however, the main features ofthe mesostriatal organization is largely conserved in all vertebrates[52,53].

The gross distinctive attribute of the newt 6-OHDA lesion modelcompared to other available Parkinsonian animals is that all lost DAneurons are replaced. DA regeneration in salamanders is evidencedboth in terms of complete histological restoration and locomo-tor recovery. Midbrain tissue composition entirely recovers, andnot just in terms of that the correct number of DA neurons arere-produced. There is no evidence either for the generation of addi-tional neural subtypes nor for any other cell types [38]. Formally itis not possible to exclude that other neurons are transiently formedbut even it does happen such cells are very rapidly eliminated.

While it is well established that neurogenesis occurs in adultmammals including the human brain [54–58], DA neurogenesis inthe midbrain has remained controversial. Available data suggestthat midbrain DA neurogenesis may occur in mammals [59–62] aswell, but the process is by far less efficient compared to newts. Thesestudies are important in the sense that they highlight the regenera-tive potential of the mammalian brain but currently the salamanderoffers the only model to determine critical features of efficient adultDA regeneration in the midbrain.

mature DA neurons (bold arrows). A stem cell may undergo asymmetric divisionproducing one stem cell and one postmitotic progenitor that differentiates into amature DA neuron as described during embryonic DA neurogenesis in mammals([92]; dashed bold arrows). A differentiated cell may undergo direct transdifferen-tiation without proliferation or dedifferentiation and subsequent proliferation of adifferentiated cell type as during hair cell or lens regeneration ([23,39]; arrows).

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ellular proliferation. Second, it could occur by cell cycle reentry,edifferentiation and subsequent transdifferentiation of a termi-ally differentiated cell, exemplified by pigmented epithelial celluring lens regeneration after lentectomy [24,63]. Alternatively,

ess differentiated progenitor or stem cells proliferate and subse-uently differentiate into DA neurons. An in vitro model of newtnner ear hair cell regeneration showed lack of prior mitotic activ-ty in newly formed hair cells [39]. In contrast, DA regeneration inhe newt midbrain depends on cellular proliferation, since elimi-ation of mitotic cells blocks both cellular and locomotor recovery38]. Retrospective analysis of regenerated DA neurons shows thathey derive from proliferating progenitors since many of them con-ain the nucleotide analogue 5-bromo-2-deoxyuridine (BrdU) thatncorporates into cells that had undergone DNA replication. Mean-

hile, there is no evidence for cell cycle reentry by postmitoticeurons, as single pulses of the nucleotide analogue never labelhese cells, nor do we detect expression of proliferating cell nuclearntigen (PCNA; Berg, manuscript in preparation).

The exact identity of DA progenitors is still an open question.owever available data suggest that cells lining the ventricle consti-

ute the main source of the newly formed DA neurons. Ventricularells are first to re-enter the cell cycle after 6-OHDA-administration,nd it is possible to trace their progeny migrating towards neuronalayers [38], as well as their conversion into TH-expressing cellsKirkham, unpublished). These ventricular cells have radial pro-esses and express the intermediate filament glial fibrillary acidicrotein (GFAP), both of which are hallmarks of radial glia cells64–66]. Similar cells are neurogenic precursor in the zebrafishrain as well [67–72]. To what extent these radial glia-like cellsonstitute a homogeneous population is not known at present. Nors it established if they correspond in terms of function and devel-pmental origin to radial glia or other cells found in ependymalosition in the mammalian brain. We refer to them as ependy-oglia cells in accordance with earlier classification in the spinal

ord [73,74]. It should be noted that a clear distinction betweenhe identity and function of the numerous cell types located in theeriventricular region in mammals also awaits further clarificationfor a recent extensive account on this topic see [75]).

Currently we prefer to describe newt ependymoglia cells asrogenitor rather than stem cells because we lack evidence for long-erm self-renewal by individual cells. In addition, BrdU pulse-chasetudies did not reveal any label retaining cells in the ependymogliaells during DA regeneration. Nevertheless, the ependymoglia layeremains intact and does not become depleted during regeneration.his is in contrast to what happens to ependymal neurogenic “reser-oir cells” in mouse after stroke [76]. These data indicate eitherhat salamander ependymoglia cells, despite the lack of evidence,o undergo self-renewal during regeneration or that ependymogliaells that have entered a neurogenic pathway are replaced from aet unidentified source, for example from remaining ependymogliaells.

Given that persistent radial glia is often considered being aemaining embryonic attribute in the adult, a simple explanationor midbrain DA regeneration in adult newt could be the mere exis-ence of ependymoglia cells. There are however several argumentsgainst this assumption. First, CNS regeneration in adult frogs isuch more limited compared to salamanders despite the pres-

nce of large number of radial glia-like cells [77]. Ventricular cellsith radial processes also exist in non-neurogenic regions of the

odent brain [78]. Second, the adult newt brain is remarkably qui-scent in most areas. Similarly to mammals, in the midbrain there

s almost no detectable cellular proliferation. Without injury, pro-iferating cells are only found in the telencephalon lining the lateralentricles and in the most rostral part of the diencephalon (Berg,anuscript in preparation). These attributes of the newt brain are

ery similar to mammals but distinguish it from the teleost fish

mental Biology 20 (2009) 691–696

brain, in which proliferative hot spots and neurogenic areas aremuch more widespread [70]. More specifically, DA neurogenesis inthe midbrain is a response to 6-OHDA-injection, and does not occurnormally. Third, adult DA regeneration is likely to require distinctmechanisms since activated progenitors are instructed to repopu-late an existing structure with one specific neural subtype. This isclearly different from embryonic development when neurogenesisis concomitant with the overall formation of a brain region.

7. Starting and terminating DA regeneration

How are ependymoglia cells instructed to produce the correctneural subtype in the correct number? How is the productionof alternative cell types prevented? Are individual ependymogliacells plastic or are they determined towards specific fates? Duringadult mammalian neurogenesis, neural stem cells are not homoge-nous but are a composite of diverse populations of progenitors[79]. This question has not yet been addressed in the salamanderbrain, but both limb and spinal cord salamander cells show con-siderable plasticity both in terms of cell cycle control [80–82] andalso developmental potential during regeneration [83–85]. If newtependymoglia are multipotent, they have to receive correct instruc-tions in order to produce DA neurons. Thus one important line forfurther investigations could be why and how the brain activatesspecifically DA-instructive signals after DA loss.

Non-specific, mechanical brain injury results in a substantialproliferative response by midbrain ependymoglia cells but with-out detectable number of new DA neurons [38]. This finding doesnot mean that one should exclude possible mechanisms throughwhich activation of ependymoglia could be concomitant with cellfate determination. What this observation however shows is thatDA neurons are produced from ependymoglia only if needed. Italso indicates that DA regeneration is probably controlled at sev-eral steps, which could be by cell cycle reentry, neurogenic cell fateswitching, and DA specification.

How does the loss of DA neurons impinge on these steps? Inaddition to understand how DA regeneration starts, it is equallyessential to address why and how it stops once the correct num-ber of new neurons has been reached. As trivial as this questionsounds, there is very little information available on the termina-tion of regenerative (and developmental) processes. Finding out theunderlying mechanism could not only explain the fine tuning thenumber of regenerated cells but also the lack of superficial additionof cells without injury in a regeneration competent system. Fig. 3outlines two principle ways based on the assumption that stimula-tory and inhibitory check points exist, either alone or, perhaps morelikely, in combination with each other.

It is possible that DA neurogenesis is actively prevented by thepresence of DA neurons. This could be achieved for example directlyor indirectly by secreted factors from neurons. Once neurons disap-pear, the inhibition is removed and cells could enter a regenerativeprogram. A block could act at multiple stages; it could prevent cellcycle reentry of ependymoglia cells, block neurogenic switch, or DAspecification of already activated progenitors. When regenerationis complete, inhibition becomes re-established, ensuring that theregenerative process ceases. Obvious candidates for such actionsare neurotransmitters, several of which have recently been shownto regulate stem and progenitor cell proliferation in the CNS andelsewhere [86–90].

A dual role of neurotransmitters by signalling not only to otherdifferentiated neurons but also to neural progenitors would be a

plausible mean to couple regeneration to functionality in the CNS.

Alternatively, loss of DA neurons could lead to signals that eitherstimulate cell cycle reentry, neurogenic switch or DA specifica-tion. As mentioned above, paracrine signals from dying cells couldact as one driving force. Hypothetically, these could be cell type

A. Simon et al. / Seminars in Cell & Developmental Biology 20 (2009) 691–696 695

Fig. 3. Initiation and termination of DA regeneration. (A) DA regeneration could be actively prevented by sensing either directly or indirectly the presence of appropriaten enterc mogliar timuli

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umber of DA neurons. Once neurons die, the inhibition is removed and cells canell cycle reentry of quiescent progenitors, blocking neurogenic switch of ependye-established. (B) Loss of DA neurons directly or indirectly generates signals that snitial stimulatory signals determines the degree of regenerative response.

pecific, i.e. dying DA neurons could give cell type specific acti-ation cues priming ependymoglia cells concomitantly with theirell cycle reentry. While a stimulatory model could determine theegree of the initial regenerative response it appears unlikely that

t on its own could explain the duration and termination of the celleplacement process, which lasts for several weeks.

These alternatives are experimentally testable hypotheses.nswers to them could reveal important aspects of how tissueomeostasis is restored after cell loss in a regeneration competentystem. It will be equally important to find out how the mechanismsf progenitor cell proliferation and differentiation are intertwinedith cellular migration and functional integration processes.

. Conclusions

Understanding the principles that allow regeneration to occuraturally will advance the field of regenerative medicine. Lower-

ng the complexity of concomitantly active signalling pathways bysing cell type selective lesion paradigms could facilitate revealingome of these principles. Targeting cell types, which normally doot undergo homeostatic cell turn over and are not replaced after

oss in mammals, may contribute to new therapeutic strategies forarious degenerative diseases.

cknowledgements

This work was supported by grants from the Swedish Researchouncil and from Parkinsonfonden to AS.

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