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Strategies for Retinal Tissue Repair and Regeneration in Vertebrates: From Fish to Human,
2007: 97-112 ISBN: 978-81-308-0200-8 Editor: Chikafumi Chiba
6
Retina regeneration in the
embryonic chick
Jason R. Spence1, Christian Gutierrez2 and Katia Del Rio-Tsonis2
Cincinnati Children’s Hospital Research Foundation, Division of
Developmental Biology, 3333 Burnet Ave, Cincinnati, OH 45229, USA
2
Miami University, Department of Zoology, Oxford, OH 45056, USA
1
Abstract
Several animal models exist that are able to
regenerate or repair retina. In this review we explore
the chick as a model to dissect the molecular
mechanisms of retina regeneration. The embryonic
chick has the capability of regenerating a retina,
complete with all of the different cell types during
restricted stages of its development. The retina is
regenerated in two distinct ways, and mitogens must
be added in order to induce regeneration. One type of
regeneration, called transdifferentiation, takes place
when a source of fibroblast growth factor (FGF)
stimulates the retina pigmented epithelium (RPE) to
Correspondence/Reprint request: Dr. Katia Del Rio-Tsonis, Miami University, Department of Zoology, Oxford
OH 45056, USA. E-mail: delriok@muohio.edu
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dedifferentiate, proliferate and then re-differentiate into retina. The other type
of regeneration occurs when stem/progenitor cells in the ciliary margin of the
eye are stimulated to proliferate and give rise to a new retina. In this review,
we attempt to give a comprehensive historical overview of both types of
regeneration while focusing on the recent mechanistic advances made in the
field. Finally, the future research directions for both types of retina
regeneration are briefly discussed.
Introduction
Blindness is a debilitating condition that affects millions throughout the
world. According to the American Foundation for the Blind
(http://www.afb.org/Section.asp?sectionid=15), in the US alone, there are over
Figure 1. Retina regeneration from different eye sources. In the embryonic chick, there
are several pools of cells that can be induced to regenerate the retina. One of these
includes stem/progenitor cells of the Ciliary body/Ciliary marginal zone (CB/CMZ)
located in the anterior portion of the eye. The other source includes the cells of the retinal
pigmented epithelium (RPE) which undergo transdifferentiation to replace the retina that
was damaged. In the post-hatch chick, and in mammals, only Müller glia cells have been
shown to participate in retina repair. In fish both retinal stem/progenitor cells as well as
Müller glia participate in retina repair. In amphibians RPE transdifferentiation is the major
source of retina regeneration while activation of cells in the CMZ also plays a role.
(Modified from Haynes and Del Rio-Tsonis, 2003; Ref # 4).
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ten million cases of blindness. A large percentage of blindness is due to the
degeneration of retina. Age-related macular degeneration is a prime example in
which the central portion of the retina degenerates leading to varying degrees of
vision loss. Currently, therapies for retina degeneration are limited, however the
possibility of using adult or embryonic stem cells to replace lost or damaged
tissue is being investigated (1,2). The ideal mechanism for replacing lost or
damaged retinal tissue would be to activate stem/progenitor cells that reside
within the eye. In the adult human, a population of quiescent stem cells has been
identified in the ciliary body (CB) (2; see Figure 1 for location). While stem cells
have been identified in the human eye, they remain quiescent in their native
environment and do not respond to injury. However, when these cells are
isolated and cultured, they have the ability to self-renew as well as differentiate
into many different retina cell types. In addition, they have the ability to integrate
and differentiate when transplanted into the chick and mouse retina under certain
conditions demonstrating they have the potential to be induced for retinal repair
(2). Therefore, studying animal models that are capable of regenerating lost or
damaged retinal tissue becomes extremely important for deciphering the
mechanism for induction of regeneration. Many animals, including amphibians,
fish and birds can repair or regenerate damaged retina to different degrees (For
review, see 3, 4, 5; Figure 1). In this review we will explore the different ways
that the embryonic chick responds to retinal injury for repair/regeneration.
While there is an excellent body of work focusing on the response of the
post-hatch chick retina to injury and to exogenous growth factors (5-11), this
review will focus specifically on retina regeneration in the embryonic chick.
Over a century of study
The process of retina regeneration in the embryonic chick has intrigued
scientists for over a century (12-23). According to I. E. Alexander (14), one of
the first experiments to demonstrate the ability of the embryonic chick to
regenerate retinal tissue was carried out by Barfurth and Dragendorff in 1902
(12). Barfurth and Dragendorff used a hot needle to damage the retina of
embryonic day E2.5-E3 (Hamburger and Hamilton: HH 13-18) chicks, and
made the observation that the presumptive tapetal tissue [tapetal tissue refers to
the retina pigmented epithelium (RPE)] was able to regenerate some neural
tissue. However, the amount of damage done in the procedure was extensive,
and it was unclear the extent to which the presumptive RPE was able to
regenerate neural retina.
In 1929, Reverberi was the next scientist to tackle the mystery of retina
regeneration (13). It appears that Reverberi performed the first surgical
retinectomy, removing the majority of the retina from the optic cup between
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29-45 hours of development (HH 9-12), after which the RPE would convert to
other cell fates and become presumptive neural retina. Following Reverberi,
Alexander was able to show that RPE isolated from E2.0 to E3.5 (HH 13-19)
embryos was able to regenerate retina when transplanted into the body wall of
20-40 hour (HH 5-11) embryos (14). In 1938, Dorris also showed that 78 hour
(HH 21) RPE explants grown in culture were able to give rise to a “young
retina” (15).
As the first half of the 1900’s approached, the phenomenon of retina
regeneration from the RPE of the embryonic chick was well accepted as there
were several different studies documenting the plasticity of the early
presumptive RPE.
The groundwork for modern studies
In 1960, elegant experiments done by Orts-Llorca and Genis-Galvez showed
that the RPE was able to regenerate retina when the two layers of tissue were
separated from one another by a strand of thread (16). This work was followed
by Coulombre and Coulombre in 1965. They performed a complete retinectomy
in embryonic chick eyes and demonstrated that up until approximately E4.5 (HH
24-25) the RPE was capable of regenerating the retina (17). Coulombre and
Coulombre also showed that it was absolutely necessary to include a small piece
of the retina back into the optic cup once it was removed. They also
demonstrated that the ability to induce retina regeneration is specific to certain
tissue types and is not a general phenomenon. That is, the otocyst (otic vesicle)
was able to induce regeneration, but a piece of the optic tectum was not able to
induce regeneration.
From their studies, Coulombre and Coulombre were able to propose two
mechanisms by which retina regeneration was induced. The first was a
“seeding” mechanism, whereby the tissue placed into the optic cup to induce
regeneration was responsible for contributing to the regenerating tissue.
Although they could not conclusively rule out this mechanism at the time, they
speculated that this was not the way regeneration was occurring. The second
mechanism they proposed was that of an “inductive cue” where certain tissue
types have the ability to induce retina regeneration.
In 1970, Coulombre and Coulombre were able to abandon the possibility
that there was a seeding mechanism involved in retina regeneration (18). In
this study, they used embryonic mouse retina to induce regeneration. Based on
histological differences between the mouse and chick retina, they concluded
that the mouse retina did not contribute to the regenerated retina, but that retina
regeneration was due to inductive signals from the mouse retina. This study
also demonstrated that there were common cues across species that were able
to induce retina regeneration.
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In addition to proposing that retina regeneration was a result of inductive
cues, Coulombre and Coulombre were also the first to describe in detail two
different types of retina regeneration from the embryonic chick eye (Figure 1).
The first has already been discussed in this review and was the focus of much
of the very early work in this field. This is the process where RPE is converted
to neural retina, a process that was referred to as “transdifferentiation” by Park
and Hollenberg (20). During this process, a differentiated tissue such as the
RPE dedifferentiates and loses its characteristic phenotypes (such as cell shape
and pigmentation), proliferates, and then re-differentiates to give rise to a
different tissue type (neural retina). The second type of retina regeneration
described by Coulombre and Coulombre was retina regenerated from the
anterior margin of the eye, where the putative ciliary marginal zone (CMZ)
and ciliary body (CB) are located. In the chick, the retina of the eye forms a
laminated, multi-layered sensory structure in the posterior of the optic cup.
During early stages of eye development, the retina is continuous with the CB,
which is comprised of a nonpigmented epithelium as well as a pigmented
epithelium (6, 7, 21, 23). The CB and the CMZ are indistinguishable at early
stages of development, but later they clearly differentiate as seen in the
illustration depicted in figure 1.
Coulombre and Coulombre demonstrated that the anterior margin of the eye
was able to regenerate new retina after the retina had been completely removed.
Like transdifferentiation of the RPE, regeneration from the anterior margin
required an inductive cue, such as a piece of chick or mouse retina (17, 18).
Modern studies in retina regeneration
For almost 20 years after the seminal work in retina regeneration performed
by Coulombre and Coulombre, the advancement of the field was very limited. It
was not until 1989 that more work on retina regeneration in the embryonic chick
was published. In this study, Park and Hollenberg revealed that the inductive cue
that is required for retina regeneration to take place was basic FGF (bFGF, also
know as FGF2) (19). Their paper did not discuss both types of retina
regeneration, but focused only on transdifferentiation of the RPE. However,
upon careful inspection of their paper, it is clear that retina regeneration was also
taking place from the ciliary body when stimulated with FGF2. Park and
Hollenberg went on to show that acidic FGF (aFGF, also known as FGF1) was
also able to induce retina regeneration, whereas several other growth factors
were not, including transforming growth factor beta (TGF-beta), Insulin, Insulin
like growth factor 1 and 2 (IGF-1, IGF-2) and nerve growth factor-beta (NGFbeta) (20). Over a decade passed since Park and Hollenberg showed that
members of the FGF family of growth factors are responsible for inducing retina
regeneration in the embryonic chick before a new study on the subject was
published (21). In 2004, we contributed a detailed cellular and molecular study
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on both transdifferentiation of the RPE and regeneration from the ciliary margin.
We have followed up with two recent studies, one focusing solely on
transdifferentiation and one focusing on regeneration from the ciliary margin.
For the purposes of this review, we will discuss each type of regeneration
separately in the following sections.
Retina regeneration via transdifferentiation of the RPE
Prior to our work (21), the characterization of transdifferentiation of the
RPE was based on histological observations only. It had been demonstrated
that when the RPE transdifferentiates, the resulting regenerated retina appeared
in the reverse orientation from that of normal retina (17-21; Figure 2 G & H ).
Figure 2. Retina regeneration in the embryonic chick. The retina is removed at E4
and an FGF bead (*) is placed in the optic cup (A). By three days, a new
neuroepithelium forms from both the CB/CMZ and the RPE (E, F), while no
regeneration took place when FGF was not added (B). By seven days after retinectomy,
the neuroepithelium differentiated into the different retina layers and cell types of the
retina (C, H, I). These regenerated retinas are similar in appearance (in terms of retina
layers) to an E11 developing retina (G). RPE: retina pigmented epithelium, CB/CMZ:
ciliary body/ciliary marginal zone, cr: ciliary regeneration, td: transdifferentiation, l:
lens, ne: neuroepithelium, GCL: ganglion cell layer, IPL: inner plexiform layer, INL:
inner nuclear layer, OPL: outer plexiform layer, and ONL: outer nuclear layer. Scale
bars: 100 µm. (A, B); 500 µm. (C); 100 µm. (D-I). (Modified from Spence et al., 2004;
Ref # 21 and reproduced with permission of the Company of Biologists).
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It was hypothesized that the reverse orientation was a result of cell polarity
established early during eye development before invagination of the optic cup
when the RPE and retina are a continuous epithelial sheet (17, 18). We used
retina cell type specific markers to compare retina regeneration with normal
retina development both temporally and spatially (21). When the retina was
removed at E4 – E4.5 and an FGF2 soaked heparin bead was placed into the
optic cup, a well defined neuroepithelium co-expressing the markers Pax6 and
Chx10 was present by E7. Pax6/Chx10 co-expression in a cell indicates that the
cell is a retinal progenitor (6, 21, 24). The neuroepithelium present at E7 after 3
days of regeneration is approximately equivalent to an E4 developing retina
(Figure 2E). In addition to the regenerated Pax6/Chx10 positive neuroepithelium,
by 3 days of regeneration, differentiation of the neuroepithelium is also
underway, and some photoreceptors and ganglion cells are identifiable using
immunohistochemistry. By 7 days of regeneration, at E11, all major cell types
can be observed using different cell markers. Indeed, the regenerated retina has
“caught up” to the equivalent stage of a developing retina both histologically
(Figure 2H) and when analyzed for the presence of specific cell types (21).
In addition, the birth of the different retinal cell types seems to occur in the
same order as it does during normal development. While the appearance of
different retinal cell types is overlapping, the general trend during development
is for the ganglion, horizontal, cone and amacrine cells to differentiate early
and the bipolar, rods and Müller glia to differentiate late (reviewed in 25, 26).
Importantly, once the RPE transdifferentiates in the embryonic chick, it is lost
for good, as it does not self-renew after giving rise to the new retina (21;
Figure 2 E, H).
The usual suspects: FGF, Pax6, Mitf
While it was thought that regeneration via transdifferentiation of the RPE
was only able to happen during a limited window of developmental time (until
E4.5), it was demonstrated by Azuma et al. that ectopic expression of the basic
helix-loop-helix transcription factor Pax6 was enough to induce
transdifferentiation of the RPE in the developing chick eye up until E14 (27).
Because of its inductive properties, Pax6 has been called the “master regulator”
of RPE transdifferentiation (22). We have shown that Pax6 upregulation is
required for FGF2 stimulated transdifferentiation. Work from both Azuma et al.
(27) and our lab (22) has demonstrated that Pax6 is downstream of FGF
signaling. Thus, at E4.5, during transdifferentiation of the RPE, a rapid
upregulation of Pax6 is observed soon after FGF2 is added to the optic cup
(Figure 3).
Furthermore, we demonstrated that the FGF signaling that caused an
upregluation of Pax6 was mediated by FGFR/MEK/Erk signaling (Figure 4).
We also showed that the RPE specific transcription factor, Microphthalmia (Mitf)
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Figure 3. FGF induced transdifferentiation. When the retina is removed from E4 chick
eyes and a heparin bead coated with FGF2 is added to the eye cup, within 6 hours, Pax6
protein appears in the RPE (A, top). The amount of Pax6 in the RPE increases with time
with a significant amount present by 24 hours (C, top). Heparin beads with no FGF2 have
no effect on Pax6 regulation (D). Microphthalmia (Mitf) which is normally expressed in
the RPE disappears upon retina removal independent of FGF2 treatment (A-E, bottom).
Scale bars represent 50 µm. A was taken at a closer magnification than that of B-E; see
scale bars. (Modified from Spence et al., 2007; Ref # 22).
Figure 4. FGF regulates transdifferentiation through MAPK signaling. When
FGF2 beads are added after retina removal in E4 chick eyes, levels of phosphorylated
Erk increase (A) compared to control eyes that received heparin beads only (B). If FGF
receptors (C) or MEK (D) are inhibited after adding FGF2, using specific inhibitors (for
the FGFR inhibitor PD173074 and for MEK inhibitor PD98059), phosphorylation of
Erk is inhibited, suggesting that FGF2 is signaling through the MAPK pathway. Topimmunohistochemistry using an antibody against the phosphorylated form of Erk;
bottom- DIC images corresponding to the immunohistochemistry represented on the top
panels. Scale bars represent 50 µm. (Modified from Spence et al., 2007; Ref # 22).
is spontaneously down regulated after retinectomy in the chick RPE at E4.5
(Figure 3), but that overexpression of Mitf in the RPE is sufficient to inhibit
FGF stimulated transdifferentiation (22). This inhibition is likely due to an
interaction between Mift and Pax6, as they have been shown to negatively
regulate each other (28). However, direct interactions between Mitf and Pax6
have not been shown in the embryonic chick transdifferentation model. The
observation that downregulation of Mitf after retinectomy did not lead to
transdifferentiation of the RPE, contrasts with developmental studies where
Mitf downregulation leads to spontaneous transdifferentiation of the RPE (29,
30, 31). In these developmental studies, the retina is still present in the eye and
it could provide stimuli, such as FGFs, that would induce transdifferentiation.
In addition to FGF, Pax6 and Mitf, Sonic Hedgehog (Shh) has also been
implicated in both RPE development and transdifferentiation/regeneration (21, 32).
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Figure 5. Shh modulates chick retina regeneration. (A) FGF2 can induce retina
regeneration from transdifferentiation (td) of the RPE and via activation of
stem/progenitor cells (cr) when present in E4 retinectomized chick eyes. The regenerated
retina in A was analyzed 7 days post-retinectomy. (B) Inhibition of Shh in the presence of
FGF increases the domain of transdifferentiation. (C) When Shh is over-expressed in the
chick eye in the presence of FGF2, transdifferentiation is inhibited while regeneration
from the CB/CMZ is not. Eyes in B and C were analyzed 3 days post-retinectomy. Scale
bar= 500 µm and applies to A-C. td= transdifferention; cr= retina regenerated from the
CB/CMZ; KAAD= Shh inhibitor. (Modified from Spence et al., 2004; Ref # 21 and
reproduced with permission of the Company of Biologists).
In the developing chick eye, inhibition of Hh signaling induced RPE
transdifferentiation to retina whereas ectopic Shh induced pigmentation of the
retina (32). In our regenerating model, Hh inhibition increased the amount of
RPE that underwent transdifferentiation when stimulated with FGF2 (Figure
5B), and conversely, overexpression of Shh in the RPE inhibited FGF2
stimulated transdifferentiation (21; Figure 5C).
Transdifferentiation of the RPE: Perspectives
While the body of scientific work investigating RPE transdifferentiation
has grown slowly over more than a century, new advances in technology and
the use of molecular tools and pharmacological inhibitors has allowed more
recent and detailed studies of the process of retina regeneration from
transdifferentiation of the RPE. However, there are still more questions than
answers at this point in time. First and foremost is the issue of the temporal
competence of the embryonic chick RPE to transdifferentiate. Simply stated,
why can the RPE transdifferentiate at E4 and not E5? Based on what is known,
several different explanations can be explored. For example, the reason may be
as simple as differential regulation of FGF receptors between E4 and E5.
However, FGF signaling is very complex, so another possibility is that FGF
signaling is differentially regulated between E4 and E5 by modulators of FGF
signaling, such as the Sprouty or Spred proteins, as has been seen in different
developmental contexts (33). Additionally, how Hh signaling inhibits
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transdifferentiation is still unknown. It will be interesting to see if Hh signaling
is responsible for the difference in RPE competency to transdifferentiate
between E4 and E5, as well as the mechanism by which it inhibits
transdifferentiation at E4. It is possible that Shh, Mitf and Pax6 participate in a
complex web of molecular interactions where Shh could drive Mitf expression
which in turn could inhibit Pax6 and repress transdifferentiation of the RPE. In
the developing frog RPE, it was demonstrated that Shh downregulation can
lead to Mitf downregulation as well (34). Alternatively, it is possible that Shh
inhibits Pax6 independently of Mitf expression. This is also a viable
possibility, since it has been demonstrated that Hh signaling is required in the
midline during early brain and eye development to downregulate Pax6 inorder
to separate the developing eye fields (35). It will be interesting to see where
this exciting field of regenerative biology goes in the future.
Regeneration from the ciliary margin
Retina regeneration from the ciliary margin was only briefly described by
Coulombre and Coulombre, and Park and Hollenberg (17, 20). In fact, until
our studies, there was not even a histological description of this type of retina
regeneration (21). We have subsequently determined that there is a population
of stem/progenitor cells that reside in this area of the eye, and that these cells
are able to proliferate in response to an FGF signal and give rise to new retina
after it has been completely removed (21). In fact, these stem/progenitor cells
persist in the ciliary margin well into post-hatch life (6). It is important to point
out that the ciliary margin is continuous with the retina and it is critical that it
is not removed during retinectomy in order for this type of regeneration to take
place as removing the source of stem/progenitor cells will ultimately inhibit
this type of regeneration.
Like transdifferentiation, all of the major different cell types are regenerated
from the ciliary margin. Using immunohistochemical markers, we showed that
ganglion, amacrine, bipolar, horizontal and photoreceptor cells are all
regenerated from the ciliary margin. The regenerated tissue also proliferates and
differentiates at an accelerated rate and catches up to the equivalent stage of a
developing retina (Figure 2 I), and like transdifferentiation, appearance of the
different cell types happens in an order that is stereotypical of development (21).
In contrast to its negative regulation during transdifferentiation,
overexpressing Shh in the absence of FGF2 stimulates retina regeneration from
the ciliary margin (21; Figure 6 B). The fact that Shh is able to stimulate
regeneration from the ciliary margin of the embryonic chick is consistent with
its ability to regulate proliferation in the margin of the post-hatch chick (11).
We went on to further characterize the role of both FGF and Shh in stimulating
retina regeneration from the ciliary margin (22). We showed that both signaling
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Figure 6. Cooperation between Hh and FGF signaling during retina regeneration
from the CB/CMZ. The Retina was removed at E4 and retina regeneration was
evaluated histologically 3 days post-retinectomy. (A) FGF2 induces regeneration from
both transdifferentiation (td) and by the activation of the CB/CMZ (arrowhead). (B)
Over-expressing Shh using a retrovirus (RCAS) also was enough to induce retina
regeneration from the CB/CMZ (arrowhead). (C) Inhibiting the Hh pathway with beads
soaked in KAAD (a Hh inhibitor) decreases retina regeneration from the CB/CMZ
(arrowheads) when stimulated with FGF2. Asterisks represent KAAD-soaked beads.
The arrowhead near the lens points to regeneration from the CB/CMZ that is closely
associated with the lens; td is transdifferentiating retina. Likewise if retina regeneration
is stimulated with Shh and either FGF receptors (D) or MEK are inhibited, retina
regeneration from the ciliary region dramatically decreases (D,E), indicating that both
the Hh and the FGF pathways are needed to be functional in order for retina
regeneration to take place properly from the CB/CMZ. FGF receptors were inhibited
using PD173074 and MEK was inhibited using PD98059. CB/CMZ: ciliary
body/ciliary marginal zone, R: retina, L: lens, RPE: retina pigmented epithelium, and
td: transdifferentiation. DMSO: dimethyl sulfoxide- control medium for inhibitors.
Scale bars in all panels are 100 µm. (Modified from Spence et al., 2007; Ref # 23).
pathways must be active in order for retina regeneration to take place. That is, if
regeneration was stimulated with FGF and the Hh pathway was inhibited,
regeneration from the CB/CMZ was significantly decreased (Figure 6C).
Similarly, if regeneration was stimulated with Shh and FGF signaling was inhibited,
regeneration was also decreased or inhibited (Figure 6 D). We also showed that
Shh was able to increase levels of phosphorylated Erk (pERK) and that this
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increase in pERK was dependant on new protein synthesis and FGF signaling
(Figure 7 A, B). If regeneration was stimulated with Shh and either protein
synthesis or FGF signaling was inhibited, Erk activation was also inhibited.
Furthermore, we showed that Shh was able to upregulate FGF ligands FGF1,
FGF2 and FGF3, as well as the FGFR1 in the ciliary margin. This increase is
likely responsible for increased FGF signaling and increased phosphorylated Erk.
However, we have not provided evidence on whether the Shh mediated
upregulation of FGF ligands and receptor is direct or indirect (22).
In addition to suggesting a mechanism whereby Shh and FGF are able to
stimulate retina regeneration from the ciliary margin, we also showed that both
Shh and FGF are able to stimulate proliferation and cell survival in this region.
Both molecules are also required for proliferation, as inhibition of either
pathway led to a decrease in basal proliferation in the ciliary margin. Finally,
we showed that Shh was required for the maintenance of progenitor cell
identity in the ciliary margin, and inhibition of Shh resulted in a decrease in the
number of Pax6/Chx10 positive cells in this region (22).
Figure 7. (A) E4 untreated CB/CMZ explants were used as control (lanes 1–3). E4
explants were treated with 10 ug/ml FGF2 (lanes 4-6) or 10 ug/ml Shh (lanes 7- 9) for 4
hr in vitro. Anti-FGFR or anti-FGF2 blocking antibodies were added to the culture
medium. Protein was extracted and a Western blot was performed using antibodies
against the phosphorylated form of Erk and actin as a control. Both anti-FGF2 and antiFGFR inhibited Erk phosphorylation stimulated by FGF2 (lanes 4–6) as well as Erk
phosphorylation stimulated by Shh (lanes 7–9). (B) Densitometry showing the ratio of
pErk/actin. (Modified from Spence et al., 2007).
Retina regeneration from the ciliary margin: Perspectives
Our group has laid the groundwork to study retina regeneration in the
embryonic chick, by first characterizing the process of retina regeneration at a
cellular level (21), and then by attempting to dissect some of the molecular
mechanisms that control regeneration from the ciliary margin (22). However,
there is still a lot of work that needs to be done to better understand this
process. For example, there are several candidate molecules that have been
implicated in control of stem/progenitor cells, or their niche, that need to be
investigated in this system. For example Wnt and Notch signaling have been
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extensively implicated in regulating stem/progenitor cells in the developing
retina (36-45). Furthermore, we have recently discovered that BMP signaling
regulates retina regeneration from the ciliary margin (46).
While there is still much to be understood regarding the mechanisms by
which regeneration from the ciliary margin is controlled in the embryonic
chick, the lessons that we learn from current and future studies will lead to a
better understanding of stem cell regulation in this zone of the eye. It will be
interesting to determine if the lessons we learn from development and from
regeneration of the chick retina can be applied to stem cell biology.
Specifically, since retinal stem cells have been identified in the ciliary body of
adult humans, it will be exciting to see how these cells can be manipulated and
regulated for the potential use in therapy to treat degenerative diseases of the
retina (2).
Conclusion
Tissue regeneration is a phenomenon in biology that has perplexed
scientists for centuries. The fact that a tissue or even an entire organ can be
damaged or removed and then regenerate is a fascinating process. In the
context of disease, discovering a way to promote self renewal of damaged or
injured tissue would obviously be one of the best ways to treat or cure the
disease. However, understanding the mechanism to stimulate self renewal and
regeneration of damaged tissue still remains elusive. Therefore, it is imperative
that biologists learn as many lessons about regeneration from the handful of
animals that possess this remarkable capability.
For over a century, the embryonic chick has been used as a model to study
retina regeneration. This has proven to be an excellent model as it provides two
types of retina regeneration to study in a single model organism (21). Further
molecular and functional analysis on regeneration via transdifferentation of the
RPE and regeneration from activation of stem/progenitor cells present in the
ciliary margin will give us insight into mechanisms by which the embryonic
chick regenerates a retina after it has been damaged or removed. It will also give
us insights on how to induce retina repair/regeneration in other animals incapable
of such repair, such as humans. While studies on this phenomenon have
progressed from the descriptive (12-18) to the mechanistic (19-23), there is still a
lot of exciting work to be done to continue moving this field of regenerative
biology forward.
Acknowledgements
This work was supported by NIA grant AG 24397-01 to KDRT and by the
Research Incentive Program, Miami University, Oxford Ohio to KDRT. We
thank Tracy Haynes and Natalia Vergara for critical review of the manuscript.
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