468?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 121 Number 2 February 2011
Epithelial-mesenchymal transition (EMT)
in kidney fibrosis: fact or fantasy?
Wilhelm Kriz,1 Brigitte Kaissling,2 and Michel Le Hir2
1Department of Anatomy and Developmental Biology, Medical Faculty Mannheim, University of Heidelberg, Mannheim, Germany.
2Institute of Anatomy, University of Zurich, Zurich, Switzerland.
Decline in renal function in chronic renal failure is caused by a pro-
gressive loss of viable nephrons. Kidney diseases that progress to
chronic renal failure most often start with problems in the glomer-
ulus. From there, the disease process spreads to the tubulointersti-
tium, leading to the loss of the entire nephron and its replacement
by a fibrotic scar. This is a focal process — meaning that individual
nephrons are lost and replaced by fibrotic tissue on a local basis (1).
There are many important questions open in this sequence: how
glomerular disease transfers to the tubule has been controversial,
as has the process by which injury from the tubule transfers to the
interstitium, culminating in fibrosis (2). Among the mechanisms
responsible for this final step, epithelial-mesenchymal transition
(EMT) has become a popular hypothesis.
EMT is a process well known in ontogeny. In an early embryo,
the epithelial layer (epiblast) gives rise to most, if not all, mesoder-
mal structures of the body. Also, later in ontogeny, epithelial cells
(e.g., cells of the somites) transform into mesenchymal cells. Thus,
in ontogeny, mesenchymal cells emerge from epithelial cells. In
recent reviews, this type of EMT has been classified as type 1 (3).
In pathological processes, EMT has been described to occur in
carcinogenesis. In that realm, the dedifferentiation of cells with loss
of epithelial and acquisition of mesenchymal features is also termed
EMT (4) and has been recently classified as type 3 EMT (3).
Another type of pathological EMT (classified as type 2) has been
proposed to occur in a parenchymal organ (liver, lung, or kidney)
when specific cells are lost in the course of a disease and are replaced
by fibrotic tissue. This hypothesis is highly controversial. It pos-
tulates that parenchymal cells (hepatocytes, alveolar cells, or renal
tubular cells undergo transition into myofibroblasts, which then
produce collagenous matrix contributing to organ fibrosis (3, 5–7).
We will discuss only type 2 EMT in the context of kidney fibro-
sis here, listing the pros and cons of the hypothesis that renal
tubular cells, in the setting of parenchymal injury, undergo tran-
sition into mesenchymal cells, which participate in the develop-
ment of fibrosis.
Let us first consider this hypothesis in more detail. Renal tubu-
lar epithelia consist of a single layer of cells that are connected to
each other by a girdle of intercellular junctions on the apical side
and are attached to a tubular basement membrane (TBM) on the
basal side. They have apical and basolateral cell membrane com-
partments, each with its own inventory of transporters and other
membrane proteins. Mesenchymal cells (including fibroblasts and
myofibroblasts) do not have this kind of polarity; instead they dis-
play cytoplasmic processes that make contacts — generally in a
point-like manner — to other cells, capillaries, and tubules.
The transition of a tubular epithelial cell into a myofibroblast
is thought to be initiated by cell injury. At an early stage of tubu-
lar degeneration, some cells will respond to stimuli of autocrine
and/or paracrine origin by downregulating their epithelial and
upregulating a mesenchymal genetic program. The new cells
are thought to produce proteases that dissolve the TBM locally,
allowing cells to move through the TBM and to settle in the peri-
tubular interstitium. Here they promote fibrosis by producing
type I collagenous matrix (3, 8).
Structural details of this process have so far not been elucidat-
ed either by transmission electron microscopy (TEM) or by light
microscopy. The hypothesis has been derived exclusively from the
observation that cells of injured tubules lose epithelial markers
while they acquire mesenchymal marker proteins. Therefore, we
first discuss the reliability and relevance of the markers that were
generally used in this context.
Markers of EMT
The seminal work on EMT in kidney fibrosis was published in 1995
by Strutz and coworkers (9), in which FSP1 was introduced as a mark-
er for cells undergoing EMT. FSP1 was described as a protein that, in
primary cultures of murine renal cells, was expressed by fibroblasts
but not by epithelial cells. Thus the authors coined the term “fibro-
blast-specific protein–1,” although the protein was already known
as S100A4 in the cancer literature. Strutz et al. (9) reported a low
incidence of FSP1-positive cells in healthy kidneys, while in a model
of renal fibrosis they observed a massive increase in their incidence
in the interstitium and, more importantly, in tubular epithelia. Two
conclusions were drawn from those observations. First, fibroblasts
are scarce in the healthy kidney. Second, under pathological condi-
tions, interstitial fibroblasts originate from tubular cells undergoing
EMT. This work has stimulated widespread use of FSP1 antibodies
for the identification of fibroblasts in order to detect EMT (10–14).
In light of our current knowledge, the use of FSP1/S100A4 anti-
bodies as a specific marker for fibroblasts is not tenable. The inter-
stitium of the healthy kidney contains an extensive network of
Conflict?of?interest: The authors have declared that no conflict of interest exists.
Citation?for?this?article: J Clin Invest. 2011;121(2):468–474. doi:10.1172/JCI44595.
?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 121 Number 2 February 2011
fibroblasts (Figure 1), which do not express FSP1/S100A4 (15, 16);
also, myofibroblasts do not express FSP1/S100A4 (17). The FSP1/
S100A4-positive cells in the renal interstitium belong rather to the
immune system (14, 15, 18–20). In addition, the endothelial cells
of some renal microvessels express FSP1/S100A4 (15, 19). These
findings are in agreement with the detection of S100A4 in inflam-
matory cells and in some endothelia in humans (21).
Similarly, another marker used to indicate EMT, vimentin, can-
not be taken as an unequivocal marker for mesenchymal cells. In
rat and human material, it was shown (22) that vimentin is very
commonly expressed in injured tubular cells not only during tubu-
lar degeneration in chronic diseases, but also during regeneration
in acute tubular injury. Later studies (15, 23, 24) showed that the
expression of vimentin in injured tubular cells displayed a clear
basolateral, thus an epithelial, pattern. In a model of transiently
injured tubular cells (23), epithelial repair took place by local pro-
liferation of surviving cells. Vimentin expression was maintained
at a high level until day 5 after ischemia; thereafter, it decreased
concurrently with recovery. This observation supports the conclu-
sion by Gröne and colleagues (22) that vimentin may be regarded
as an indicator of tubular regenerating activity.
α-SMA is the characteristic actin isoform found in vascular
smooth muscle cells, pericytes, and myofibroblasts (25, 26). Proxi-
mal tubular cells contain a cytoskeleton with prominent F-actin
microfilaments that do not stain with antibodies against α-SMA.
In some studies of renal disease models with fibrosis expression,
α-SMA has been found in tubular cells, and that finding was con-
sidered as evidence for EMT (27–29). However, in most other stud-
ies α-SMA was exclusively found in interstitial cells around injured
tubules, whereas the tubules themselves remained α-SMA–nega-
tive (15, 17, 19, 30–33). The same observation was made in biopsies
of patients with nephritic syndrome (34).
Since direct detection of collagen synthesis by immunolabeling has
not yet been achieved, proteins that participate in its synthesis, HSP 47
(a collagen-specific molecular chaperone) and prolyl-4-hydroxylase,
have been used as surrogate markers. Expression of HSP47 in injured
tubules has been taken as evidence for synthesis of collagen by tubu-
lar cells and thus for EMT (10, 11). However, HSP47 is not specific
for collagen type I (35) but is equally involved in collagen type IV
synthesis, and thus in the synthesis of TBM collagen.
The loss of proteins that are typically found only in epithelia
has also been used to indicate EMT. Most frequently, the loss of
keratin, β-catenin, E-cadherin, and the kidney-specific cadherin 16
(36) has been invoked in this respect (37). When tubules undergo
decomposition, the cells progressively lose their intercellular junc-
tional contacts, and also their contacts with the basement mem-
brane. Thus, it is not surprising that they lose some of the junc-
tional proteins. This, however, by no means constitutes evidence
of a mesenchymal phenotype.
In summary, a variety of markers were used to establish evi-
dence for EMT. Since none of the markers is unequivocally indic-
ative of EMT, it is our view that marker expression data are not
sufficient to prove EMT.
Experimental studies supporting EMT
The crucial study supporting EMT in kidney fibrosis is a cell fate
tracing study published by Iwano and coworkers in 2002 (10). The
authors used the unilateral ureteral obstruction (UUO) model to
induce kidney fibrosis in mice. They permanently labeled tubular
cells with LacZ using the γGT promoter and showed that within
a degenerating tubule, epithelial cells may acquire FSP1 positivity
and that cells positive for both LacZ and FSP1 appeared to have
settled in the fibrotic tissue surrounding the tubules. Moreover,
colocalization of HSP47 in LacZ-positive cells located in the peri-
tubular interstitium was taken as evidence that these cells were
derived from epithelial cells and that they participated in the pro-
duction of fibrous collagens.
It is our opinion that this study contains serious flaws. Later
studies (33, 38) showed that the immunofluorescent labeling of
LacZ is prone to artefacts, which may account for some of the
following inconsistencies in the crucial Figure 2 of the study by
Iwano and coworkers (10), reproduced here as Figure 2. First, γGT
is a brush border protein, expressed specifically in the proximal
tubules. However, all tubules in the control (Figure 2A) appear
equally labeled, although two profiles (both at 7:00 from the glom-
erulus, one of them partially covered by “fibroblasts”) are definite-
ly not proximal. Second, the LacZ labeling is homogeneous in all
tubular cells. However, insertion of a transgene using Cre recombi-
nase usually yields variable levels of expression. Third, the merged
pictures showing the colocalization of LacZ and FSP1 or HSP47
In the kidney, FSP1/S100A4 does not stain fibroblasts or myofi-
broblast. (A) Double immunofluorescence for S100A4 (red chan-
nel) and ecto-5′nucleotidase (5′NT, green channel) and (B) for
S100A4 (red channel) and α-SMA (green channel); nuclei in B
are stained by DAPI. (A) 3-mm-thick cryostat section of renal
cortex (rat); the thick arrow points to a 5′NT-positive interstitial
fibroblast; S100A4-positive cells (thin arrows) are seen inside
capillaries. Scale bar: ~10 mm. Reproduced with permission
from Histochemistry and Cell Biology (15). (B) Renal cortex
after 4 days of ureteral obstruction (rat); no cellular colocalization
of S100A4 and α-SMA. G, glomerulus. Inset: S100A4-positive
cells, showing strong cytoplasmic and weak nuclear staining; the
shape of the cells is reminiscent of migrating lymphocytes. Scale
bars: ~100 mm; inset, ~10 mm. Reproduced with permission from
Histochemistry and Cell Biology (17).
470?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 121 Number 2 February 2011
after UUO exhibit a complete overlap in location and strength,
quite obvious in panels E, F, and G, but also in panels B and D.
This is possible only under the very unlikely condition that the
levels of expression of the two proteins are in a constant ratio at
all sites where coexpression takes place. Fourth, the FSP1 antibody
is not suitable to identify fibroblasts, as is illustrated in panel A,
where there are three green-labeled (FSP1+) structures, two of them
designated as fibroblasts. However, on the basis of structural stud-
ies (39), the area shown in panel A contains more fibroblasts. In
addition, the labeled cell at 8:00 from the glomerulus is a round
cell hardly compatible with a fibroblast. Last, in panel C, seven
arrows point to LacZ+FSP1+ structures, i.e., cells that have under-
gone EMT, suggesting a high frequency of such an event. This is
hardly compatible with the fact that cells in the process of EMT
have so far not been found in structural studies.
The study by Iwano and coworkers (10) stimulated widespread
activity in the field. Numerous studies were undertaken aiming at
elucidating details of this new mechanism. Most of them were cell
culture studies (12, 40–45) that clearly showed that renal tubular
epithelial cells when exposed to profibrotic cytokines (especially
TGF-β1) lose cell polarity, downregulate epithelial markers (E-cad-
herin, β-catenin, zonula occludens protein 1), acquire mesenchymal
features (become spindle-shaped and mobile, produce MMP-2),
and express mesenchymal markers (vimentin, α-SMA). This pat-
tern of changes was taken to be a common signature for EMT and
was proposed to be valid as indirect evidence of EMT in vivo (5, 46).
In a recent review supporting the EMT hypothesis (3), it was
conceded that the phenotypic alterations in vitro characterize a
“partial EMT,” lacking the final transition to a “fully fibroblastic
phenotype.” These studies do not contribute any relevant evidence
for a role of EMT in renal fibrosis, which clearly requires a “fully
Among the experimental in vivo studies supporting EMT, there
were a few that used routine techniques of light and electron
microscopy and claimed to show morphological alterations in
proof of EMT (11, 36, 40); however, the quality of the morpho-
logical data was not sufficient for that purpose. Neither a cell with
characteristics of a fibroblast within the TBM, nor a cell crossing
the TBM, nor gaps in the TBM were properly documented.
In a second category of pro-EMT articles, EMT was assumed as
an established fact, and the study was designed to uncover details
of this mechanism in the context of renal fibrosis. Data that
were compatible with EMT were considered as evidence for EMT.
This type of circular reasoning has been applied in several stud-
ies manipulating the activity of TGF-β in models of renal fibrosis
(41, 47, 48). According to these studies, the reduction of fibrosis in
response to suppression of the TGF-β pathway reflected a block-
ing effect on EMT. Other possible effects were not considered.
Similar reasoning was found in the study that used a tamoxi-
fen-inducible Snail1 transgenic mouse model (36) that expressed
significant levels of the protein in the kidney. The Snail family of
transcription factors induces EMT in vitro, shown in a mouse mam-
mary gland–derived epithelial cell line (NMuMG) (36). Induction
of Snail1 in the above in vivo model resulted in an increase in inter-
stitial fibrous collagen. This was interpreted only in the context of
EMT, although the authors did not provide any specific evidence
of EMT. In a recent study (49), activation of the Notch pathway
(involving the Snail transcription factors) was shown to induce
upregulation of mesenchymal markers in vitro but not in vivo.
Venkov and coworkers (12) showed in a proximal tubule epi-
thelial cell line that the transcription factor CBF-A led to down-
regulation of epithelial-specific proteins and upregulation of
mesenchymal-specific proteins including FSP1. In an attempt
to investigate a possible role of CFB-A in vivo, the authors exam-
ined the localization of CFB-A and the expression of FSP1 in
the UUO model in FSP1/GFP mice. Their data are confusing;
we cannot retrace how the authors came to the conclusion that
these pictures show FSP1-positive cells that were derived from
CBF-A–stimulated tubular cells and settled in the interstitium
(Figure 3). In the text the authors simply state, “This series of
images suggests that the nuclei of many tubular cells in the UUO
kidney are enriched for CBF-A and potentially positioned for an
EMT event.” Thus, the authors do not really claim to show EMT
in the UUO model.
In summary, none of these studies contain robust evidence
for a contribution of EMT to kidney fibrosis. None of them
met the crucial requirement of showing that in vivo, tubular
cells/tubule-derived cells were capable of producing type I col-
lagenous matrix, and none of them presented evidence for a
migration of cells across the TBM. The cell culture studies did
uncover a certain pattern of de-differentiation of tubular epithe-
lial cells, including the upregulation of mesenchymal proteins
that would be relevant to the EMT hypothesis. However, this
pattern does not include evidence for the decisive final steps to
Reproduction of Figure 2 from the article by Iwano et al. (10).
?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 121 Number 2 February 2011
a complete EMT, i.e., phenotype switch and ability to produce
fibrous matrix; so we are left with only a “partial EMT” (3, 8)
whose pathological relevance remains unclear.
Studies that did not find evidence for a contribution
of EMT to renal fibrosis
Cell fate tracing studies. As mentioned above, the cell fate tracing
study in the UUO model by Iwano and coworkers (10) has been
widely accepted as definitive evidence of EMT in the kidney; how-
ever, the results of this article have so far not been confirmed by
any other study. In contrast, two different groups using fate-trac-
ing techniques in the UUO model did not find any evidence for a
contribution of EMT to renal fibrosis (19, 33).
Humphreys and coworkers used three Cre/Lox transgenic lines in
which specific populations of renal cells were genetically labeled (33).
First, using the Six2 promoter, all tubular cells derived from the cap
mesenchyme (nephron anlage); and second, using the Hoxb7 promot-
er, all tubular cells derived from the ureteral bud were labeled. Third,
as a positive control, all renal mesenchyme-derived cells were labeled
(using the transcription factor FoxD1). In all three transgenics, two
different labels were used, LacZ and Z/Red. After UUO, in the first
two models no cells were found in the peritubular interstitium that
were positive for the epithelial markers; thus, no cells were found that
settled in the peritubular interstitium. In the third transgenic line,
labeled myofibroblasts were easily detected in the peritubular inter-
stitium, proving that resident mesenchymal cells may transform into
myofibroblasts. Moreover, treatment with TGF-β1 of cultured proxi-
mal tubule cells derived from the Six2GC; Z/Red mice consistently
led to the induction of mesenchymal markers. Identical results were
obtained in an ischemia-reperfusion injury model. Taken together,
the findings of this rigorous study clearly show that tubular cells may
acquire mesenchymal markers in vitro. However, in vivo, tubular cells
did not transform into any cells contributing to fibrosis.
In another recent study, the authors used the Ksp-cadherin
promoter to label all renal tubular cells with enhanced yellow
fluorescence protein (EYFP) as a permanent marker (19). After
UUO, cells positive for EYFP were confined within the TBM, were
not found in the interstitium, and did not express markers of
fibroblasts or myofibroblasts.
A third cell fate tracing study addressed this question in a trans-
genic mouse model, in which renal fibrosis was induced by TGF-β1
overexpression in renal tubules (50). In this study, Pax8-rtTA mice
(51) were used to drive the simultaneous expression of TGF-β1 and
Cre recombinase in renal tubular epithelial cells. Whereas TGF-β1
expression triggered tubular damage and renal fibrosis, Cre recom-
binase expression led to recombination at the Rosa26-lacZ locus
(the fourth transgene employed in this model), thereby irreversibly
labeling the tubular epithelial cells with β-galactosidase activity.
TGF-β1 is considered as the principal cytokine that induces EMT
(52). However, even under these seemingly favorable conditions
for EMT, labeled epithelial cells were never found in an interstitial
position within fibrotic areas.
Furthermore, in two animal models of renal fibrosis, UUO (19)
and habu venom injury (32), tubular epithelia were labeled with
Texas red–dextran (TR-dextran) and tracked. In both studies,
TR-dextran–retaining cells were found in cells of atrophic tubules
but remained excluded from the peritubular interstitium.
Taken together, five cell fate and/or cell-labeling studies from
different groups did not find any evidence that renal tubular cells
transform into mesenchymal cells, more specifically into fibro-
blasts or myofibroblasts.
Structural studies. One of the most attractive features of the EMT
hypothesis is the idea that injured cells themselves contribute to
fibrosis and thus to repair through scarring. In contrast to the
liver, in the kidney these two processes — cell injury and fibrosis
development — are spatially separated from each other: cell injury
occurs within, fibrosis development outside the tubular compart-
ment. Therefore, the transition of tubular epithelial cells into
fibroblasts/myofibroblasts by EMT means both a fundamental
change in cell shape, which is expected to happen inside the TBM,
and a change in cell location, i.e., these cells have to cross the TBM
and to settle in the interstitium. Under this scenario, we would
expect to find cells with a mesenchymal morphology both inside
the TBM and in transit to the outside crossing the TBM. Such
events, if not extremely rare and thus of questionable significance,
should be easily documented in structural studies by TEM. So far,
no pictures are found in the literature showing such cells.
As has been shown in previous studies (1, 53–55), the process of
tubular degeneration occurs in a certain sequence. First, cells sim-
plify their structural elaborations, losing their surface differentia-
Empty profile of a degenerating tubule. TEM of tubular remnant from
a transgenic mouse with tubular overexpression of TGF-β1 (50). This
profile is devoid of any cells but is maintained as an entity by the pres-
ence of a continuous TBM (highlighted in yellow). Note that there is
massive deposition of fibrous collagen (asterisks) around this tubular
remnant, but there is not a single fiber inside. Scale bar: 5 μm.
Renal fibroblast. TEM of a fibroblast in the renal cortical interstitium of
a healthy rat. From the cell body, numerous processes emerge that
make contacts with a capillary (C, arrows), a tubule (double arrow),
and another fibroblast (arrowhead). Scale bar: 1 μm. Reproduced with
permission from Histochemistry and Cell Biology (16).
472?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 121 Number 2 February 2011
tion, but maintaining cell polarity even after collapse of the lumen.
The process of decomposition generally starts with the accu-
mulation of lysosomal elements indicating autophagy (19, 50).
In this stage, the degenerating cells generally lose their fixation
to the TBM, frequently forming strands of dying cells. There is
no noticeable invasion of the tubule by inflammatory cells; thus,
autophagy and fratricide are the most likely mechanisms account-
ing for the disappearance of the tubular cells.
It is important to remember that during all these stages of
degeneration, the integrity of the TBM — even if it becomes col-
lapsed and highly folded — is preserved (Figure 3). This survival of
the TBM represents the crucial prerequisite for the high regenera-
tion capacity of the tubular epithelium in acute renal failure.
Over the years, we have studied tubular degeneration associated
with fibrosis in various rat and mouse models. Since at the time of
most of these studies EMT was not yet in discussion, we did not
look for it. However, we described the specific pattern of tubular
degeneration eventually leading to almost cell-free remnants of
intact, frequently thickened TBM (54, 56, 57).
In two recent studies (50, 58), we deliberately searched in a large
number of electron micrographs for cells crossing the TBM from
inside the tubular compartment to outside. We did not find such cells.
Similar efforts were undertaken in an model of acute renal failure (59);
also, in this study cells crossing the TBM were not encountered.
These data only show that any cells in transit were not found;
they do not prove that such cells do not exist. Yet after 15 years
of EMT research, cells in transit from inside an injured tubule
through the TBM into the peritubular space have only been shown
in schematics. Not a single picture of a cell in transit has ever been
published, much less any regular structural evidence that might
give significance to this mechanism.
In the model of TGF-β–induced renal fibrosis (50), we also
addressed the question of whether tubular cells underwent EMT
and produced collagen still within the tubular compartment.
From this study, Figure 3 shows almost cell-free tubular remnants
with massive deposition of fibrous collagen in their surroundings,
but without a single fiber inside. Cells with a mesenchymal mor-
phology were never encountered inside the TBM.
In summary, though EMT describes a process with manifold
structural implications, no structural evidence has so far been pre-
sented that supports this hypothesis.
Studies showing that the bulk of myofibroblasts
originate from fibroblasts
Until the mid-1990s, nobody questioned that organ fibrosis
develops from proliferation of local fibroblasts and their con-
version into myofibroblasts, which proliferate and produce the
fibrous intercellular matrix (60–64). Since that time, however,
two other sources of myofibroblasts came into discussion, bone
marrow–derived fibrocytes and tubular cells undergoing EMT.
The contribution of bone marrow–derived fibrocytes is generally
believed to be small (65–67).
Nevertheless, there is a widespread resistance to the idea that local
resident interstitial cells are the major, if not the only, source for the
fibrosis-producing myofibroblasts. These interstitial cells are tradi-
tionally termed fibroblasts. They are the ubiquitous cells that per-
form a scaffolding function interconnecting the various organ-spe-
cific substructures — in the kidney, these are the nephrons and the
supplying capillaries. They perform this function by focal contacts
of their cytoplasmic processes with neighboring fibroblasts as well as
with capillaries and tubules (refs. 39, 68, and Figure 4). In addition,
they produce the fibrous matrix, which serves together with these
cells as a common scaffold throughout the renal parenchyma.
Interstitial fibroblasts in the kidney cortex can be unequivocally
detected by electron microscopy and fluorescence microscopy by
their strong expression of ecto-5′-nucleotidase (CD73) (16, 33, 39,
68, 69). Some unique ultrastructural features, such as the abundance
of rough ER (RER) and F-actin filaments and an association with col-
lagen fibrils, make identification in the electron microscope easy.
In the UUO model, Picard and coworkers (17) investigated the
alterations of fibroblasts upon conversion into myofibroblasts. Rel-
evant changes were seen already on the first day following ligature
consisting of the appearance of bundles of actin microfilaments
and of abundant cisterns of RER. Double immunofluorescence
revealed the gradual expression of α-SMA exclusively in ecto-5′-
nucleotidase–positive cells. Thus, the conversion of a fibroblast
Fibroblasts are the origin of myofibroblasts. Rat renal cortex in
sham-operated (A) and in ureter-ligated kidneys of rats (B and
C) (3-μm cryostat sections; red: 5′NT, green: α-SMA, blue: cell
nuclei). In controls (A), the interstitium and the brush border of
proximal tubules are strongly labeled by 5′NT; α-SMA labels
exclusively arterial vessels (A). After 2 days of ureter ligature (B),
interstitial 5′NT staining decreases, whereas α-SMA appears and
becomes increasingly prominent throughout the cortex after 3 days
(C). (D–F) Interstitial fibroblast in ureter-ligated kidney after 2 days.
The weakly expressed 5′NT is distributed in a granular manner
over the plasma membrane and the cytoplasm; α-SMA is appar-
ent along the plasma membrane and in the cellular processes.
Scale bars: 100 μm (A–C), 10 μm (D–F). Reproduced with per-
mission from Histochemistry and Cell Biology (17).
?The?Journal?of?Clinical?Investigation http://www.jci.org Volume 121 Number 2 February 2011
into a myofibroblast was documented in a continuous sequence
of steps (Figure 5). A corresponding continuous sequence was also
documented after uranyl acetate–induced acute renal failure by
transmission and scanning electron microscopy (59).
There are several other recent in vivo studies showing that the
myofibroblasts emerge from proliferation of local interstitial cells.
Lin and coworkers (65) used a coll1a1-GFP transgenic mouse line
to identify pericytes/fibroblasts as the major contributors to the
myofibroblast population in UUO. In a cell fate study, using trans-
genic mice in which LacZ was expressed specifically in pericytes/
fibroblasts, Humphreys and coworkers (33) found that the total-
ity of α-SMA–expressing cells in the interstitium originated from
pericytes/fibroblasts both in UUO and after ischemia-reperfusion
injury. A further study (32) showed similar results in an acceler-
ated model of angiotensin II–induced renal fibrosis.
These studies certainly do not rule out a source of myofibroblasts
from other cell populations, but they show that the transition of
fibroblasts into myofibroblasts occurs on a large scale and may
occur very early after injury, long before tubular degeneration.
A final point remains to be discussed: the controversy over
whether these resident interstitial cells should be termed fibro-
blasts or pericytes. The fact that renal interstitial fibroblasts have
frequent contacts with capillaries (Figure 4) led some authors to
identify these cells as pericytes (33, 65). However, the same cells
have frequent contacts with tubules and other interstitial cells. In
total, they are identified in the renal cortex by the expression of
ecto-5′-nucleotidase (CD73) (16, 33) as well as of PDGFRβ (65). If
we term these interstitial cells pericytes, the peritubular intersti-
tium of the kidney would be devoid of any fibroblasts.
Fibroblasts are an exceptionally versatile cell type with strong
organ-specific elaborations, but the common function in every
organ is to provide the scaffold that lends mechanical support to
tissue architecture. From this point of view, it makes sense that
they are responsible for fibrosis.
In summary, we feel there are no solid data supporting EMT as
an in vivo process in kidney fibrosis. We are not aware of any
study showing transdifferentiation of a tubular epithelial cell
into a cell capable of synthesizing fibrous matrix, nor one pre-
senting a tubular cell in the process of transit across the TBM.
The only cell fate tracing study that supports EMT awaits con-
firmation, while other fate tracing studies cast strong doubts
on the existence of EMT. It is hard to understand why EMT
has become so deeply ingrained into fibrosis research. Similar
doubts about a role of EMT in organ fibrosis have recently been
raised concerning the liver (38, 70).
On the other side, there is ample evidence from various animal
models and human pathology that kidney fibrosis results from the
proliferation of resident fibroblasts transforming into myofibro-
blasts. There are strong reasons to focus on this cell population in
studies of renal fibrosis.
We would like to thank Benjamin Humphreys for helpful dis-
Address correspondence to: Wilhelm Kriz, Medizinische Fakultät
Mannheim der Universität Heidelberg, Abteilung: Anatomie und
Entwicklungsbiologie, Ludolf-Krehl-Str. 13-17, Tridomus C, Ebene 6,
D68167 Mannheim, Germany. Phone: 49.0.621.383.9941; Fax:
49.0.621.383.9949; E-mail: email@example.com.
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