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BRESR-100704; No. of pages: 16; 4C:
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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / b r a i n r e s r e v
Review
Updating old ideas and recent advances regarding the
Interstitial Cells of Cajal
P. Garcia-Lopez1, V. Garcia-Marin,1, R. Martinez-Murillo, M. Freire
Cajal Institute, CSIC, Avda Doctor Arce 37, 28002 - Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
Since their discovery by Cajal in 1889, the Interstitial Cells of Cajal (ICC) have generated
Accepted 1 June 2009
much controversy in the scientific community. Indeed, the nervous, muscle or fibroblastic
nature of the ICC has remained under debate for more than a century, as has their possible
physiological function. Cajal and his colleagues considered them to be neurons, while
Keywords:
contemporary histologists like Kolliker and Dogiel categorized these cells as fibroblasts.
Cajal
More recently, the role of ICC in the origin of slow-wave peristaltism has been elucidated,
Interstitial Cells of Cajal
and several studies have shown that they participate in neurotransmission (intercalation
theory). The fact that ICC assemble in the circular muscular layer and that they originate
from cells which emerge from the ventral neural tube (VENT cells), a source of neurons, glia
and ICC precursors other than the neural crest, suggests a neural origin for this particular
subset of ICC. The discovery that ICC express the Kit protein, a type III tyrosine kinase
receptor encoded by the proto-oncogene c-kit, has helped better understand their
physiological role and implication in pathological conditions. Gleevec, a novel molecule
designed to inhibit the mutant activated version of c-Kit receptors, is the drug of choice to
treat the so-called gastrointestinal stromal tumours (GIST), the most common non-
epithelial neoplasm of the gastrointestinal tract. Here we review Cajal's original
contributions with the aid of unique images taken from Cajal's histological slides
(preserved at the Cajal Museum, Cajal Institute, CSIC). In addition, we present a historical
review of the concepts associated with this particular cell type, emphasizing current data
that has advanced our understanding of the role these intriguing cells fulfil.
(c) 2009 Elsevier B.V. All rights reserved.
Contents
1.
Introduction: the controversial origin of ICC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
2.
Location and morphology of ICC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
2.1.
Intravillous and periglandular plexi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
2.2.
Auerbach plexus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
2.3.
Deep muscular plexus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
2.4.
Intramuscular plexus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
2.5.
Pancreas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0
Corresponding author. Fax: +34 91 585 47 54.
E-mail address: vgmarin@cajal.csic.es (V. Garcia-Marin).
1 Both authors contributed equally to this article.
0165-0173/$ - see front matter (c) 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.brainresrev.2009.06.001
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
Cajal, Brain Res. Rev. (2009), doi:10.1016/j.brainresrev.2009.06.001

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2.6.
Other locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.
The physiological function of ICC from Cajal to the present day . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.1.
ICC in neurotransmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.2.
ICC as pacemakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
3.3.
Stretch sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
4.
Pathological ICC and GIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
5.
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
1.
Introduction: the controversial origin of ICC
analysed by light and electron microscopy, Taxi referred these
cells as neuronoids in an attempt to distinguish them from
Between 1889 and 1893, Cajal published some articles describing
neurons, Schwann cells, smooth muscle cells, fibroblasts and
a new type of cell that he classified as a primitive neuron. This
macrophages, although he recognised their tendency to co-
type of cell was located in the stroma of the villi (Figs. 1A, 2), in
stain with nerves (Taxi, 1952, 1965; for a review see Thuneberg,
the Auerbach's plexus (Figs. 1A, 3), deep muscular plexus (Figs.
(1999)). Following these findings, ultrastructural studies on the
1A, 4A), circular muscular layer of the intestine (Figs. 1A, 4B-D),
ICC suggested that they were primitive muscle cells (Imaizumi
and around the acini and blood vessels of the pancreas (Fig. 5).
and Hama, 1969; Faussone-Pellegrini et al., 1977) or fibroblast-
Following Cajal's first descriptions, these cells were identified
like cells (Richardson, 1958; Komuro, 1989).
using different names, including: celulas simpaticas intersticiales
The most important advance in this area came with the
(sympathetic interstitial cells, 1891); celulas simpaticas (sympa-
discovery that ICC express the tyrosine kinase receptor (c-Kit:
thetic cells, 1892); neuronas simpaticas intersticiales (sympathetic
Maeda et al., 1992), which permitted them to be chemically
interstitial neurons); or celulas intersticiales (interstitial cells,
differentiated from other cell types sharing similar morpholo-
1899-1904). However, Dogiel subsequently called them -Ca-
gical characteristics in the tunica muscularis (Thuneberg, 1982;
jal'sche zellen - and more than 100 years after their discovery the
Ward and Sanders, 2001a). The use of this specific marker
name of Interstitial Cells of Cajal (ICC) is still used.
allowed the mesenchyma to be identified as the source of ICC
At that time, there was considerable controversy about the
(Lecoin et al., 1996) and indeed, the detection of c-Kit expression
nature of these cells and while some researchers, including Cajal
in aneural explants confirmed that ICC are not of neural crest
and his colleagues (LaVilla, 1897), thought they were neurons,
origin in mammals (Young et al., 1996). Moreover, it was shown
others, such as Kolliker and Dogiel, classified them as fibro-
that both smooth muscle cells and ICC have a common
blasts. This debate was somewhat clouded by the ongoing
mesodermal origin (Torihashi et al., 1997; Kluppel et al., 1998).
discussion as to whether neurons were individual structures or
However, it is noteworthy that a subset of ICC might be of neural
if they simply formed a syncitium. According to the reticularist
origin, since the ICC of the circular muscular layer originate from
point of view, neurons were thought to form an interconnected
cells that emerge from the ventral neural tube (VENT cells: Sohal
continuous network built up of either axons and dendrites
et al., 2002), a source of neurons, glia and ICC precursors that
(Gerlach's), or exclusively of axons (Golgi's). Cajal's first paper on
differ from the neural crest cells. Hence, the neuronal origin of
the nervous system (Cajal, 1888) proposed that neurons end
ICC suggested by Cajal could be at least partially true. VENT cells
freely, and that they connect with each other by contiguity and
originate in the ventral part of the hindbrain neural tube and
not by continuity. This relevant observation was the first
they migrate through the site of attachment of the cranial
description of the neuronal doctrine: neurons are independent
nerves to colonize the gastrointestinal tract, particularly the
units from a morphological and even physiological point of view.
duodenum and stomach (Sohal et al., 1996; Bockman and Sohal,
However, Cajal's description of the ICC as neurons was in
1998). Significantly, not all ICC express c-Kit, such as the ICC-
contradiction with his neuron doctrine as he was unable to
DMP (deep muscular plexus, see Fig. 1A) in the human small
recognize the axonal process in these cells that characteristically
intestine (Torihashi et al., 1999; Wang et al., 2003). Moreover,
establishes the typical network. In this respect, he recognized:
there are many different cell types besides ICC that express c-
Kit, such as mast cells, melanocytes, neurons and glia (Zhang
"I am neither exclusive nor dogmatic. I am proud of
and Fedoroff, 1997). Thus, ultrastructural analysis has proved to
retaining a mental flexibility which is not afraid of
be essential to finally determine whether a particular cell
correction. Neuronal discontinuity, extremely evident in
belongs to the ICC family. The ultrastructural characteristics of
innumerable examples, could sustain some exceptions. I
ICC have been well defined (Faussone-Pellegrini and Thuneberg,
myself have mentioned some of them, for example those
1999; Rumessen and Vanderwinden, 2003; Komuro, 1999) and
probably existing in the glands, vessels and intestines (my
they have been summarized as a gold standard (Huizinga et al.,
interstitial neurons)" (Cajal, 1933).
1997) for ICC identification. ICC are generally characterized by a
number of morphological aspects including the presence of: i)
During the twentieth century, there has been a longstanding
numerous mitochondria and caveolae; ii) a basal lamina,
dogmatic division regarding the nature of these cells (Jabonero,
although discontinuous; iii) abundant intermediate filaments;
1960; Thuneberg, 1990; for a review see Thuneberg (1999)). When
iv) moderately developed Golgi apparatus, few ribosomes and a
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
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Fig. 1 - (A) Location of the different ICC subtypes according to Cajal and to the present nomenclature based on a semi-schematic
drawing of Cajal (Cajal, 1899-1904) of a longitudinal section of the guinea pig small intestine stained by the Golgi method. According
to Cajal: A, longitudinal muscle fibres; B, circular muscle fibres; C, submucose connective tissue in the Meissner plexus; D,
Lieberkuhn glandules; E, intestinal villi; a, Auerbach plexus; g, Auerbach ganglion; b, deep muscular plexus; c, fibres from the
Meissner plexus; e, periglandular plexus; f, intravillous plexus. According to the present nomenclature: ICC of the myenteric plexus
(ICC-MP or ICC-MY); ICC of the circular muscle (ICC-CM) and ICC of the longitudinal muscle (ICC-LM), both referred to collectively as
intramuscular ICC (ICC-IM); ICC of the deep muscular plexus (ICC-DMP); ICC of submucosa and submucosal plexus (ICC-SM and
SMP). (B) Multipolar ICC-MP (asterisk) from the guinea pig small intestine evident through c-Kit immunohistochemistry. The
cytoplasmic processes undergo repeated dichotomous branching and they make many contacts with those of the neighbours
(Hanani et al., 2005). (C) Multipolar ICC-DMP of the guinea pig small intestine stained by c-Kit immunohistochemistry, with their
secondary and tertiary slender processes mainly parallel to the axis of the circular muscle fibres (arrows: Hanani et al., 2005).
(D) Bipolar ICC-CM from guinea pig small intestine demonstrated by c-Kit immunohistochemistry that emit only a few processes
(Hanani et al., 2005). (E) Human exocrine pancreas. In the insterstitium, amongst the acini (a), note some spindle-shaped or
triangular cells (arrows) with very long cytoplasmic processes (several tens of m), indicated by dashed lines. The acini marked by
asterisks appear to be surrounded by periacinar pICC processes. Methylene blue staining (Popescu et al., 2005).
rough and smooth endoplasmic reticula; and v) close contacts
connective tissue) and the Auerbach plexus (located between
established with nerve varicosities and the formation of
the longitudinal and circular smooth muscle fibres), Cajal found
numerous gap junctions, both with each other and with smooth
them in three more plexi: the deep muscular plexus, the
muscle cells.
periglandular and the intravillous plexi (summarized in
Fig. 1A). At these sites, he found small fusiform and triangular
cells with little protoplasm that had a number of varicose
2.
Location and morphology of ICC
anastomosed processes, often ramifying at a right angle. These
general characteristics of ICC varied at their different locations
Cajal described ICC at different sites in the intestinal tube. Apart
(Figs. 1B-E) and thus, it is worthwhile describing the characte-
from the classical Meissner plexus (located in the submuscular
ristics of the ICC at the locations where Cajal observed them.
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
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Fig. 2 - (A) Drawing of the periglandular and intravillous plexi in the guinea pig intestine stained by the Golgi method (Cajal,
1899-1904): a, triangular cell; b, fusiform cell whose inner process result in fascicles of fibrils; c, triangular cell with a
similar pattern; d, fusiform cell of the periglandular plexus; e, f, fusiform cell of the villi; g, layer of subglandular nerve fibre
fascicles receiving processes of cells of the periglandular plexus. (B-D) Cajal's original histological slide from the intestine of
the guinea pig impregnated by the Golgi technique. (B) Periglandular and fusiform cell of the villous plexi. (C) Fusiform cells
of the villous plexus. (D) Triangular cell of the periglandular plexus. Scale bar: 100 m (B) and 50 m (C, D).
2.1.
Intravillous and periglandular plexi
site directions, up and down (Fig. 2C). By contrast, the cells were
round, triangular or stellate in the apical portion, close to the
Cajal described this new type of cell for the first time in the
lumen of the intestinal tract (Fig. 2A). The periglandular plexus
intestinal villi (Cajal, 1889), situated either at the basal or the
has triangular or stellate cells (Fig. 2D), and these cells had
apical portion of the villi and forming the periglandular plexus
numerous processes that ramified in a complex way. Cajal
(Figs. 2A-B). In the basal portion of the intestinal villi, the cells
realized that the numerous processes anastomosed and he
were long and fusiform, with two processes emanating in oppo-
could not differentiate any axonal process (Cajal, 1893). In
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
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Fig. 3 - (A) Drawing of ICC from the frog Auerbach plexus stained by Ehrlich's method (Cajal, 1892): A, long cells; B, star shape
cell; a, intercellular anastomosis; b, small terminal branches with varicosities; d, nucleiform protuberances; c, protoplasmatic
granules]. (B) Drawing of ICC from the frog Auerbach plexus stained by Ehrlich's method: a, nerve cells; b, nerve fibres;
c, bundle of nerve fibres; d, cellular processes ending in a bundle of nerve fibres; e, fusiform cell along a bundle of nerve fibres;
f, ganglion formed by three cells; g, unstained cells in a ganglion; h, nuclei. (C) Drawing of ICC in the adult rabbit Auerbach
plexus stained by Ehrlich's method (LaVilla, 1898): A, cells in the interganglionar mesh; B, anastomosis between two of these
cells; C, marginal or periganglionar cells. D, ICC of the Auerbach plexus from one of Cajal's original histological preparation
(Ehrlich's method). Scale bar: 50 m.
Cajal's original histological preparations preserved in the Cajal
blasts in the villi are connected through gap junctions
Museum, we have found these cells in the basal portion of the
(Komuro, 1990; Komuro and Hashimoto, 1990). In contrast to
villi and in the periglandular plexus but not in the apical portion.
typical ICC, these myofibroblasts do not bear c-Kit (Vannucchi
In this regard, Cajal said (1899-1904), that they ICC in the apical
et al., 2002) but they do express NK1r. Moreover, since they are
portion of the intestina villi are very difficult to stain.
in close contact with one another and with nerve fibres
Besides these first observations, Guldner also found a
(Vannucchi and Faussone-Pellegrini, 2000), they can still be
connected system of fibroblasts in the villi of the duodenum
considered as a class of ICC. Concerning the possible
(Guldner et al., 1972; for a review see Thuneberg (1999);
physiological role of these cells, it is thought that they may
Huizinga and Faussone-Pellegrini (2005)). These cells formed
serve as a barrier/sieve, a flexible mechanical frame, mechan-
a cellular network establishing close contact with axons,
osensors and signal transduction machinery in the intestinal
smooth muscle cells and partially embracing the capillaries
villi, regulated locally and dynamically by rapid changes in cell
and terminal arteriole. Later, it was observed that the fibro-
shape (Furuya et al., 2005; Furuya and Furuya, 2007).
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
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the nervous cells, Cajal also described his interstitial cells as
small fusiform or triangular shaped cells with little proto-
plasm, a long and thick nucleus, and with very long and
varicose processes (Fig. 3: Cajal, 1892; 1899-1904). The fusiform
shaped cells give off processes from opposing poles while the
multipolar ones had several cytoplasmic processes (Cajal,
1892; 1899-1904). With regard their connections, Cajal stated
that these cells could establish associations either with
Auerbach's plexus, with other ICC or with the muscle cells
(Cajal, 1892). Nowadays, these cells are referred to as
myenteric interstitial cells (ICC-MY). The distribution of ICC-
MY varies greatly between different parts of the gastrointes-
tinal tract, and they are fewer in number and their cellular
networks are relatively looser in the gastric corpus and colon
than in the small intestine (see Komuro, 2006).
2.3.
Deep muscular plexus
Cajal also described ICC in the deep muscular plexus, located
under the muscle tunic of circular fibres, in which the majority
of fascicles course parallel to the contractile fibres (Cajal, 1893;
1899-1904). Here, the somata of the ICC are small and fusiform,
with a triangular or stellate morphology. Indeed, they are
multipolar cells, with secondary or tertiary branches oriented
parallel to the axis of circular smooth muscle cells (Fig. 4A).
These cells maintain a close relationship with both the
muscles and the nerve fibres, and their processes often span
about 200-300 m. The network established by the processes
covers the whole area of the intestinal wall in a large mesh
predominantly composed of long parallel partially anastomo-
sised lines (see Hanani et al., 2005). These cells are currently
referred to as interstitial cells of the deep muscular plexus
(ICC-DMP).
2.4.
Intramuscular plexus
Cajal observed bipolar ICC with small ramifications in the
circular muscle layer (ICC-CM), parallel to the axis of muscle
fibres (Figs. 4B-D: Cajal, 1892, 1893).
"We have also noticed or believe to note some fusiform
Fig. 4 - (A) Drawing of ICC in deep muscle plexus from the
nervous corpuscle between the circular muscular fibres
guinea pig seen in a section parallel to the muscle layer and
directed in the same direction as the muscular fibres"
stained by the Golgi method, according to Cajal (1893):
(Cajal, 1892).
A, nerve cells. a, b, nerve cells of the interstitial plexus;
e, arborization of one interstitial cell process; f, interstitial cell
The morphology, distribution and density of ICC-CM differs
with long processes. (B) ICC under the layer of circular muscle
considerably from organ to organ in a given species. ICC-CM of
in the rabbit stained by Ehrlichs' method (Cajal, 1893)
the small intestine often show secondary cytoplasmic
(C-D) ICC-CM stained by Ehrlichs' method (C) and the Golgi
branches and they are sparsely distributed in association
method (D) from Cajal's original histological preparations.
with rather thicker nerve bundles, without forming their own
Scale bar: 25 m (C, D).
cellular network. By contrast, ICC-CM of the stomach and
colon have a simple elongated spindle shape and they are
densely distributed along nerve bundles (see Komuro, 2006).
2.2.
Auerbach plexus
Some other ICC are also found in the longitudinal muscle
layer ICC-LM. These cells are similar to ICC-CM in shape but
Cajal studied the Auerbach's plexus in the frog with the help of
there are usually fewer in nearly the whole gastrointestinal
methylene blue staining. He identified a network of stained
tract (i.e. in the stomach, small intestine and colon: Komuro,
thick, flexible and ramified fibres parallel to the circular mus-
2004). ICC-CM located in the circular muscle layer and those
cular fibres (Cajal, 1892). This plexus is endowed with gang-
located in the longitudinal muscle layer (ICC-LM) are referred
lions (2 to 12 cells) joined into anastomotic bundles. Besides
to collectively as intramuscular ICC (ICC-IM).
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Fig. 5 - (A) Terminal axonal plexus of the rabbit pancreas stained by the Golgi method (Cajal and Sala, 1891): A-F: ICC.
(B) Terminal axonal plexus of the rabbit pancreas stained by the Golgi method. A, perivascular nerve cell; B, C, ICC; a, b, terminal
branches between epithelial cells; D, neural plexus of an arteriole (Cajal and Sala, 1891).
2.5.
Pancreas
3.
The physiological function of ICC from Cajal
Cajal studied the distribution of ICC in the pancreas of different
to the present day
species with the aid of the Golgi method and he found that they
were independent elements throughout the organ. These cells
The history of gastrointestinal motility extends deep into the
were fusiform, triangular or stellate, with 3 or more divergent
past. From the observations of gastric contractions (Beau-
processes (Fig. 5: Cajal and Sala, 1891), and processes from these
mont, 1833) through to the discovery of spontaneous colon
cells formed a strong plexus around the acini or the blood
contractions in the cat tract using X-rays (Cannon, 1902), there
vessels (Fig. 5B). Again, the ICC processes anastomized such that
have been many advances in this field. The implication of the
he could not discriminate a typical axon emanating from them.
Auerbach and Meissner plexi in the motility of the gastro-
Recently, the presence of ICC in pancreas (pICC) was confirmed
intestinal tract was fully appreciated from the beginning of
using non-conventional light microscopy, immunohistoche-
last century:
mistry and transmission electron microscopy (Popescu et al.,
2005). These studies suggested that pICC may play a role in
"The presence of these centres (referring to the Meissner
neurotransmission/modulation through two possible strate-
and Auerbach plexi) explains the automatism of intestinal
gies: a) by co-operating with acinar cells and with small vessels;
movements" (Cajal, 1899-1904).
and b) to engage mutual contact with pancreatic stellate cells or
with Pacinian receptors. In addition, ICC could fulfil another
However, the role of the ICC was less clear. Nevertheless,
hypothetical role in controlling normal mechanoreceptor phy-
Cajal proposed these cells to be mediators of enteric transmis-
siology, since they were closely apposed to the Pacinian
sion (Fig. 6: Cajal, 1899-1904):
corpuscle. Interestingly, they also considered that stromal
pancreatic tumours (SPT) might originate from a subset of
"It is not a risky conjecture, however, that the interstitial
pICC by analogy with GIST (see below).
cells are subordinated to fibres of the autonomic ganglia.
The impulse brought by these fibres would elicit a sup-
2.6.
Other locations
plementary discharge in the interstitial cells, able to add
strength to the contraction or increase its duration". (Cajal,
ICC are also located outside the gastrointestinal tract and by
1899-1904).
applying the Golgi technique, some histologists even found
them in the serose glands of the tongue (Krause, Fusari and
Later in this text and in reference to the motor pathway of
Panasci) or in the myocardium (Berkley, 1893; review in Cajal,
the autonomic system, Cajal stated:
1899-1904). With modern techniques, ICC have also been
found in many other locations such as in the upper urinary
"Here, in all probability, the motor chain has two
tract (Lang and Klemm, 2005), urethra (Sergeant et al., 2006),
additional neurons: that of myenteric and mucosal plexi,
myometrium (Ciontea et al., 2005), myocardium (Hinescu and
and the interstitial or terminal cell". (Cajal, 1899-1904).
Popescu, 2005; Popescu et al., 2006), uterus, fallopian tube
(Popescu et al., 2007), human placenta (Suciu et al., 2007) and
For Cajal, the ICC would function as mediators of neuronal
the ciliary muscle in monkeys (Paula et al., 2009). Thus, these
transmission and subsequent studies supported this theory
findings confirm Cajal's conclusions that:
(Imaizumi and Hama, 1969; Yamamoto, 1977; Oki and Daniel,
1973; Daniel, 1977). Currently there is no doubt that inter-
"All or almost all glands have terminal neural plexi, similar
stitial cells serve as mediators of enteric transmission and
to those of the intestine, made of Remak fibres originated
indeed, interstitial cells were proposed as pacemakers by
in special autonomic ganglia, as well as fusiform or stellate
Keith in 1915 (Keith, 1915; for a review see Thuneberg (1999))
cells of the previously described interstitial type" (Cajal,
and this role in gastrointestinal pacemaking activity has
1899-1904).
since been further demonstrated. These different functions
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3.1.
ICC in neurotransmission
Several studies have revealed a close and functional relation-
ship between ICC and enteric nerve fibres, confirming that ICC
participate in neurotransmission (Cajal, 1899-1904; Daniel and
Posey-Daniel, 1984; see also Ward, 2000; Ward and Sanders,
2001b). The knock-out animals available for c-kit have been
particularly useful in defining the role of ICC in neurotransmis-
sion (Burns et al., 1996), especially W/Wv mutant animals. The
absence of ICC-IM in the murine fundus not only led to reduced
NO (nitric oxide) dependent post-junctional responses (Burns
et al., 1996; Ward et al., 1998) but also, it resulted in the loss of
cholinergic excitatory responses (Ward et al., 2000).
Immunohistochemical studies have revealed that isolated
ICC are responsive to a variety of enteric transmitters, both
excitatory and inhibitory. Nerve fibres stained with the
primary transmitters of excitatory motor neurons, such as
vesicular ACh transporter (VAChT) and substance P, are
closely associated with cell bodies and processes of ICC-IM
in the stomach (Beckett et al., 2002; Horiguchi et al., 2003; Song
et al., 2005; Wang et al., 1999; Ward et al., 2000), and with ICC-
DMP in the small intestine (Faussone-Pellegrini, 2006; Iino
et al., 2004; Lavin et al., 1998; Wang et al., 1999). Many
inhibitory motor neurons that contain nitric oxide synthase
(NOS), vasoactive intestinal polypeptide (VIP) or adenosine
triphosphate (ATP) are closely associated with both the cell
bodies and processes of ICC-IM (Beckett et al., 2002; Horiguchi
et al., 2003; Song et al., 2005; Wang et al., 1999; Ward et al.,
2000) and ICC-DMP (Toma et al., 1999; Wang et al., 1999). In
addition, different neurotransmitter receptors are expressed
by the ICC (See Box 1). These data demonstrate that ICC are
densely innervated by excitatory and inhibitory enteric motor
neurons.
It should be noted that the close apposition of enteric nerve
fibres (both excitatory and inhibitory) and ICC, as well as the
Fig. 6 - Diagram of sensory and motor pathways of the
presence of neurotransmitter receptors on these cells, does
autonomic system according to (Cajal, 1899-1904).
not imply that functional synaptic contacts are made between
A, sympathetic ganglion; B, ventral horn of the spinal cord;
these structures. With the aid of electron microscopy,
C, dorsal root ganglion; D, small intestine; E, pancreas;
abundant close contacts (<25 nm) between varicose nerve
F, visceral ganglion; J, glandular interstitial cell;
terminals and ICC were demonstrated that support the
K, perivascular nerve cell; V, blood vessels; A, motor
existence of a specialized kind of transmission of neural
spino-gangliar fibres of preganglionic Langley fibres;
information between enteric nerves and ICC (Daniel and
b, another spino-sympathetic fibre terminating exclusively
Posey-Daniel 1984). These contacts have been confirmed
in a single ganglion; c, sympathetic axon coursing through
(Wang et al., 1999; Ward et al., 2000) and the existence of
two ganglia; d, sympathetic axon incorporated into a spinal
these close interactions led to the recovery of the intercalation
nerve through the gray ramus communicans; e, autonomic
theory which favours the existence of nerve transmission
axon ending on a blood vessel (postganglionic fibre of
through ICC (Cajal, 1899-1904; Daniel and Posey-Daniel, 1984;
Langley); f, autonomic axon ending in the myenteric plexus
see also Ward, 2000; Ward and Sanders, 2001b). These synaptic
of the intestine; g, motor cell of a myenteric ganglion;
specializations are functional, since the soluble N-ethylma-
h, sensory spinal fibre ending in the intestinal mucosa;
leimide-sensitive attachment protein receptors (SNARE)
m, ganglion of the myenteric plexus; I, motor fibre ending in
implicated in neurotransmitter release is located in varicos-
a visceral ganglion.
ities associated with ICC-IM (Nirasawa et al., 1997; Aguado et
al., 1999; Beckett et al., 2005; for a review see Ward and
Sanders, (2006)). The postsynaptic densities are also func-
tional since they contain typical postsynaptic proteins. There
(nervous transmission and pacemaking activity) are to some
is also a decrease in the expression of postsynaptic density
extent carried out by different types of ICC. While nervous
(PSD-93 and PSD-95) in kit mutant mice (W/Wv), whereas
transmission is mediated by ICC-IM, the pacemaking activity
immunohistochemistry for the PDZ domain of the PSD-95
is preferentially mediated by ICC-MY even though ICC-IM
family and for Kit revealed some degree of co-localization
may also regulate the frequency of the pacemaker.
(Beckett et al., 2005; for a review see Sanders and Ward (2006)).
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
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9
Box 1
ICC receptors.
Receptors
ICC location
References
5-HT, 5-HT3, 5-HT4 subtypes
ICC-MY
Glatzle et al. (2002); Liu et al. (2005); Poole et al. (2006);
ICC-DMP
Bombesin, subtype-3
Almost in all ICC Porcher et al. (2005)
Cholecystokinin A
ICC
Patterson et al. (2001)
G protein-coupled, Protein Kinase A (PKA),
ICC
Southwell (2003); Poole et al. (2004)
Protein Kinase C (PKC)
Muscarinic acetylcholine, M2, M3 subtypes
ICC-IM, ICC-MY
Epperson et al. (2000); Iino and Nojyo (2006)
ICC-DMP
Neurokinin, NK1, NK3 subtypes
ICC-IM, ICC-MY
Sternini et al. (1995); Grady et al. (1996); Portbury et al. (1996); Lavin et al. (1998);
ICC-DMP
Epperson et al. (2000); Iino et al. (2004)
Purinergic, P2Y1, P2Y4 subtypes P2X2, P2X5 ICC-IM, ICC-MY,
Burnstock and Lavin (2002); Van Nassauw et al. (2006); Chen et al. (2007)
subtypes
ICC-DMP
Somatostatin 2A
ICC-DMP
Sternini et al. (1997)
VIP
ICC-IM, ICC-MY
Epperson et al. (2000)
VPAC1 subtype
However, there is still little data regarding how the nervous
Early evidence of the implication of ICC in the generation of
impulse is transmitted from the ICC-IM to the smooth muscle
slow waves came from photochemical ablation of ICC, whereby
cells. Although gap junctions were proposed to be involved in
methylene blue followed by illumination blocks slow-wave
this process, recent research using gap junction blockers sug-
activity (Thuneberg et al., 1983; Liu et al., 1993, 1994). Alter-
gests that gap junctions are not necessary for pacing or nerve
natively, others used rhodamine 123, a cytotoxic fluorescent dye
transmission to the circular muscle of the mouse intestine
that specifically accumulates in the ICC, although it may also
(Daniel, 2004; Daniel et al., 2007).
accumulate in enteric neurons (Ward et al., 1990).
Other studies have been critical to confirm the role of ICC in
3.2.
ICC as pacemakers
generating slow-wave activity. Intracellular electrode record-
ing and methylene blue staining were used to confirm that
The first suggestion that ICC may act as physiological pace-
slow waves are only observed in smooth muscle cells when
makers was made by Arthur Keith, the discoverer of the
the ICC are attached to the muscle layer registered (Suzuki
cardiac sino-atrial pacemaker organization (Keith, 1915; for a
et al., 1986). At the same time, intracellular recording of the
review see Thuneberg (1999)).
longitudinal, inner and outer circular muscle layers of the dog,
cat, rabbit, opossum and human small intestine demonstrated
"A series of sections through the ileo-caecal junction of the
that the myenteric region is the dominant source of slow
rat's bowel revealed a collar of peculiar tissue...there was
waves in the small intestine (Hara et al., 1986). By contrast, the
also present a third element -numerous branching cells, not
pacemaker in the colon resides at the submucosal surface of
connective tissue in nature with processes which united
the circular muscle layer, since removing ICC-SMP from the
with muscle cells, on the one hand, and with the processes
submucosal border blocks the generation of slow waves
from true ganglionic cells on the other. I regarded these inter-
(Smith et al., 1987). In mice with c-kit mutations (W/Wv), the
mediate cells as a possible representation of the nodal tissue
networks of ICC-MY are grossly underdeveloped in the small
of the heart". (Keith, 1915); taken from (Thuneberg, 1999)
intestine where pacemaker activity was lacking (Ward et al.,
1994; Huizinga et al., 1995), even though ICC are present in the
Ambache was the first to show that electrical slow waves
DMP. In the latter experimental model, ICC-MY are evident in
control intestinal contractions and to relate these slow waves
the stomach where slow-wave activity can be recorded (Burns
to the ICC (Ambache, 1947; reviewed in Thuneberg (1999)).
et al., 1996; see also Huizinga, 2001). These data suggest that
Several subsequent studies (using different approaches that
ICC-MY but not ICC-DMP are crucial for generating slow-wave
included chemical lesions and dissection experiments, intra-
activity (Ward et al., 1994, 1995; Huizinga et al., 1995; Huizinga
cellular electrophysiological recording or mutant animals)
et al., 2001). Similar experiments in rat mutants (Horiguchi
indicated that ICC generated these slow waves. Slow waves
and Komuro, 1998) or in steel-Dickie mutant mice (Sl/Sld), in
are cyclic depolarizations of the membrane potential of
which the gene encoding the c-Kit ligand (stem cell factor, SCF)
smooth muscle cells. The cellular mechanisms involved in
is defective (Ward et al., 1995) confirmed these data. However,
the generation of pacemaker potentials are not completely
the generation of slow waves is not only mediated by ICC-MY.
understood, although it seems clear that the release of Ca2+
In the gastric corpus of the guinea pig where ICC-MY are
from internal IP3-dependent stores plays a key part in their
absent, the dominant pacemaker activity that entrains activity
generation (Suzuki, 2000; for a review see Nakayama et al.
in other regions of the stomach is provided by the ICC-IM
(2007)). When rhythmic electrical oscillations reach the open-
(Hashitani et al., 2005). Furthermore, ICC-IM contribute to the
ing threshold of calcium channels, calcium enters the cell and
amplification of pacemaker signals from ICC-MY by genera-
triggers the contraction of smooth muscle cells.
ting rhythmic oscillations known as unitary potentials,
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Fig. 7 - (A) Adult mouse duodenum physiologically distended by the intestinal contents according (Thuneberg and Peters, 2001).
The electron micrograph shows the longitudinal muscle layer with five peg and socket junctions (*). Note the uniform,
narrow space between peg and socket membranes, in contrast to simple folds of the cell surface. (B) Diagram of the distribution
of the peg and socket junctions and the gap junctions in the muscularis of adult mouse small intestine (Thuneberg, 1999).
regenerating the depolarizing currents from ICC-MY or those
coupling and the internal sensitivity of a muscular layer to
that follow exposure to acetylcholine (Dickens et al., 2001;
stretch (Faussone-Pellegrini and Thuneberg, 1999; Thuneberg
Hirst et al., 2002a,b; Edwards et al., 1999).
and Peters, 2001).
Other studies have been carried out on isolated and/or
However, the most solid physiological evidence supporting
cultured ICC. Single ICC are spontaneously active, generating
the functioning of ICC as stretch sensors was obtained by
electrical depolarization similar to slow waves recorded in
applying length ramps to the murine antral muscles, while
intact smooth muscle cells, as demonstrated in patch clamp
recording intracellular electrical activity and isometric force
experiments on the canine colon (Langton et al., 1989). Indeed,
(Won et al., 2005). Increasing the length caused membrane
single ICC generate a rhythmic inward current insensitive to L-
depolarization and an increase in the slow-wave frequency.
type calcium channel blockers that is crucial for the genera-
The response was mediated by ICC-IM because no response
tion of slow waves in smooth muscle cells (Thomsen et al.,
was observed in antral muscles of c-kit mutants, W/Wv mice,
1998).
which lack ICC-IM. The stretch sensor mechanism associated
But how the slow waves spread from the ICC-MY to the
with the ICC is mediated by the cyclooxygenase enzyme
smooth muscle cells is still unclear. Although ICC-MY are
COX-II, since stretch-dependent responses were inhibited by
coupled through gap junctions, there are no gap junctions
the COX-II inhibitor indomethacin and they were absent in
between ICC-MY and smooth muscle cells of the longitudinal
COX-II deficient mice (Won et al., 2005). COX-II is constitu-
or circular muscle layer (Fig. 7). Thus, other mechanisms such
tively expressed by ICC-IM (Porcher et al., 2002) and it is
as peg and socket connections, have been proposed that might
implicated in the prostaglandin cascade. Therefore, products
provide electrical coupling through the accumulation of
of arachidonic acid metabolism, such as prostaglandin E2
potassium in the narrow cleft between the peg and the socket
(PGE2), are likely to mediate stretch-dependent responses
under appropriate conditions (Vigmond et al., 2000). These peg
(Won et al., 2005). Thus, ICC-IM seem to coordinate different
and socket connections have also been proposed as stretch
neural and mechanical inputs to regulate gastric motility.
sensors, warranting a description of these structures and their
possible implications.
4.
Pathological ICC and GIST
3.3.
Stretch sensors
There is evidence of a correlation between alterations to ICC and
The hypothesis that ICC could act as stretch sensors has been
some digestive pathologies. Indeed, ICC are lacking, reduced or
proposed repeatedly (Daniel, 1977; Thuneberg, 1989; Faus-
damaged in some of the following digestive pathologies:
sone-Pellegrini and Thuneberg, 1999), based on the existence
idiopathic gastric perforation, hypertrophic pyloric stenosis,
of special structures called peg-and-socket junctions that may
transient neonatal pseudo-obstruction, neonatal meoconium
represent the part of muscle cells most vulnerable to such
ileus, Hirshprung's disease, total colonic aganglionosis, intest-
tension (Fig. 7). These peg-and-socket junctions are thought to
inal neuronal dysplasia, hypoganglionosis, internal sphincter
consist of a peg of 0.5 to several-micrometers long, which
achalasia or congenital uretopelvic junction obstruction in
extends from one smooth muscle cell into a narrow pocket or
children and achalasia of oesophagus, gastroparesis, chronic
invagination of the plasma membrane of a neighbouring
idiopathic intestinal pseudo-obstruction, diabetic gastroentero-
smooth muscle cell or ICC, excluding the connective tissue
pathy, paraneoplastic dismotility, afferent loop syndrome,
components from the space between the tightly apposed
Chagas disease, or inflammatory bowel diseases such as ulce-
membranes. The morphology and fixed orientation of these
rative colitis or Crohn's disease in adults (for a review see
structures suggests that they could serve as mechanical
Streutker et al. (2007)). While in the paediatric population some
stretch sensors, regulating smooth muscle/interstitial cell
of these alterations may be caused by developmental delay, in
Please cite this article as: Garcia-Lopez, P., et al., Updating old ideas and recent advances regarding the Interstitial Cells of
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