The study and teaching of gastrointestinal (GI) physiology necessitates an understanding of the cellular basis of contractile and electrical coupling behaviors in the muscle layers that comprise the gut wall. Our knowledge of the cellular origin of GI motility has drastically changed over the last 100 yr. While the pacing and coordination of GI contraction was once thought to be solely attributable to smooth muscle cells, it is now widely accepted that the motility patterns observed in the GI tract exist as a result of a multicellular system, consisting of not only smooth muscle cells but also enteric neurons and distinct populations of specialized interstitial cells that all work in concert to ensure proper GI functions. In this historical perspective, we focus on the emerging role of interstitial cells in GI motility and examine the key discoveries and experiments that led to a major shift in a paradigm of GI physiology regarding the role of interstitial cells in modulating GI contractile patterns. A review of these now classic experiments and papers will enable students and educators to fully appreciate the complex, multicellular nature of GI muscles as well as impart lessons on how shifting paradigms in physiology are fueled by new technologies that lead to new emerging discoveries.
- interstitial cells
- slow wave
- gastrointestinal motility
the major muscle tissues of the body can be divided into three types: cardiac muscle, skeletal muscle, and smooth muscle. Cardiac muscle is the muscle of the heart; it is striated in appearance and due to the presence of an electrical pacemaker, cardiac cells contract in a coordinated rhythmic manner to generate heart beats. Skeletal muscle is typically attached to the skeleton via tendons, hence its name. Skeletal muscle is also commonly referred to as striated muscle due to its striated morphology when viewed under a microscope. Smooth muscle is found throughout the body. It can be found to line the blood vessels, gastrointestinal (GI) tract, bladder, urethra, vas deferens, uterus, penile, and clitoral cavernosum tissues (66). Smooth muscle lacks the striated appearance of skeletal and cardiac muscle. It is fatigue resistant and is under autonomic or involuntary neural control; this is opposed to skeletal muscle, which requires somatic or voluntary nerve signaling to contract (80). The contractile mechanism of all three muscle types requires a rise in intracellular Ca2+ concentration (6, 81). Thus, cellular Ca2+ hemostasis is an important regulator of muscle force and cellular activity.
GI Smooth Muscle
The musculature of the GI tract is composed of many layers of distinct cellular identity. Throughout the GI tract, there are two smooth muscle layers: an outer longitudinally orientated layer and an inner circularly orientated layer. The circular layer then borders the submucosa, which gives way to the mucosa and lumen. In between the two smooth muscle layers is a network of nerve bodies of the enteric nervous system termed the myenteric plexus, which innervates the GI tract (7, 67).
As well as smooth muscle cells and enteric neurons, there are various populations of noncontractile interstitial cells located throughout the muscle layers of the GI tract. One of these cell types, interstitial cells of Cajal (ICCs), can be found at the myenteric plexus (ICC-MY) throughout the GI tract, which lies in between the circular and longitudinal smooth muscle layers, at the border of the circular smooth muscle and submucosa of the colon. Intramuscular ICCs (ICC-IM) are found in between muscle bundles within the circular and longitudinal smooth muscle layers of the stomach and colon and also within the deep muscular plexus (DMP) of the small intestine (ICC-IM/DMP) (64). A diagram illustrating this structural arrangement for ICC-MY and ICC-IM in relation to smooth muscle is shown in Fig. 1. Ultrastructural characterization of these cells has revealed that they have a dark condensed cytoplasm, dark ovoid nuclei, free ribosomes, numerous mitochondria, numerous caveolae, and an incomplete basal lamina (5). ICCs can be of various shapes from bipolar to stellate, and in the GI tract, their branched processes form gap junctions with neighboring smooth muscle cells (36, 68). The initial discovery of these cells and the elucidation of their physiological function in relation to smooth muscle contractility will be the topic of this article, which is aimed at upper-level biology and physiology students as well as postgraduate and medical students undertaking research in the field of GI motility and visceral smooth muscle.
GI Smooth Muscle Activity
In the GI tract, smooth muscle exhibits phasic contractions to move bulk material along the length of the organs (esophagus, stomach, small intestine, and colon) in a peristaltic motion. These contractions in the GI tract are associated with underlying electrical events. The underlying electrical activity that initiates and coordinates smooth muscle contraction in the GI muscle manifests as spontaneous cellular membrane depolarizations that are termed “slow waves” (SWs); an example of these events is shown in Fig. 2A. SWs occur throughout the GI tract and are rhythmic in nature, occurring at regular frequencies with consistent amplitudes, durations, and kinetics (although these parameters differ from organ to organ and between species).
GI Pacemaker Cells
The concept of spontaneous electrical activity in the heart being driven by a pacemaker is well established in the minds of life science students. However, the presence of a pacemaker in the GI tract, which drives SW electrical activity, is less well known among undergraduate physiology students today, despite a wealth of evidence from the last 25 yr pointing to exactly that. Among GI physiologists, there is now wide acceptance of the concept that the generation of spontaneous electrical activity in the GI tract is due to the presence of specialized pacemaker cells, the ICCs mentioned above. While ICC are themselves noncontractile, these pacemaker cells possess the necessary machinery to generate electrical activity in smooth muscle that drives motility through GI tissues. The story of the discovery of ICCs and the elucidation of their physiological function is one that provides essential lessons for educators teaching GI function. Indeed, there are eternal scientific lessons in the story that are applicable to all fields on how scientific paradigms shift over time, sometimes over generations, in light of rapid advances of new technologies and emerging new scientific ideas and discoveries.
Initial Observations on GI ICCs
Santiago Ramon y Cajal, the namesake of ICCs, was a late 19th/early 20th century Spanish neuroscientist and also, importantly, an artist. Fascinated by the nervous system, Cajal drew by hand the neural plexus and individual cells with such artistic quality that many of his drawings can still be found today in educational textbooks on neuroanatomy. Cajal's studies on the neural cells of the body and his contributions to the development of the neuron doctrine (the concept that the nervous system is a collection of individual cells) would reveal themselves to be milestone achievements in neural physiology. This was recognized in 1906, when Cajal, along with Camillo Golgi, was awarded the Nobel Prize in Physiology or Medicine “in recognition of their work on the structure of the nervous system.”
Curious about the nervous system in the entire body and not just the brain, Cajal also made detailed drawings of neural structures in the GI tract. As outlined above, typically, the smooth muscle of the gut is found in two distinct layers: the circular and longitudinal smooth muscle layers. In between these two layers is a nerve network of the enteric nervous system (the innervation center of the GI, which is functionally distinct from the central nervous system, sometimes referred to as the ‘little brain’) termed the myenteric plexus. While studying this neural network, Cajal observed and made drawings of a cell population, which he noted were located between the nerve endings of the enteric nervous system and bulk smooth muscle. At the time, Cajal believed that these cells were an extension of the enteric nerve network and simply named them “interstitial cells” and postulated that they may serve a function in relaying signals from enteric neurons to the surrounding smooth muscle cells (10, 11). While much later work would show that Cajal was on the right track, he did not proscribe any other function to the interstitial cells. Some years later, Sir Arthur Keith, a Scottish anatomist who is most well known for first describing the sinoatrial node of the cardiac pacemaker, was the first to suggest that these interstitial cells might act as pacemaker cells in the GI tract to coordinate phasic contractile activity. Keith developed this hypothesis based on the perceived structural similarities between the interstitial cells and sinoatrial pacemaker cells of the heart (31).
For much of the early to mid 20th century, the pursuit of elucidating the physiological role of ICCs was slow to progress. However, during this time, an insightful PhD student from the University of Utrecht also suggested that ICCs might indeed drive rhythmic contractions in the GI tract (46). This hypothesis was also supported in publications from Ambache (1) and Nelemans and Nauta (53), as they pointed out that those GI organs that contained ICCs also displayed spontaneous rhythmicity, suggesting that motility itself may originate from these cells. At the time, the only experimental resource available to physiologists to explore the roles of ICCs was using basic microscopy techniques to study their cellular structure and location within the GI tract. Advances in the microscopy field allowed researchers to study the morphology of ICCs in ways unthought of by Cajal or Keith. Using electron microscopy, the ultrastructural properties of ICCs were published by Imaizumi and Hama (30), Gabella (22), and Cook and Burnstock (13). While these studies hinted at a physiological role for ICCs, little could be discerned except for the anatomic location and arrangement of the cells. The authors of these studies did note that gap junctions existed between ICCs and smooth muscle cells, suggesting that coupling interactions may take place between the different cell types. However, the nature of its physiological role remained a mystery.
On the Origin of SWs
In the GI tract, rhythmic contractions are driven by spontaneous electrical activity in the form of SWs. SWs have been observed in the human GI tract and in various species, including the rabbit, mouse, canine, and cat (23). Such activity has been observed in most organs of the GI tract, including the stomach (15), small intestine (32, 79), and colon (70). SW activity generates the electrical basis for driving a coordinated propagating phasic contraction to move a bolus toward the anus for egestion (27). This occurs due to the fact that SWs are depolarizing events, which activate voltage-sensitive ion channels in smooth muscle, which lead to a rush of Ca2+ entering the cells. As noted above, cellular contraction in smooth muscle is dependent on Ca2+ (Fig. 2). It is this contractile response that forms the basis of peristalsis and allows the GI tract to contract in a manner to move a bolus along its length. One of the first questions that comes to mind is “How are these SWs generated?.” The origin of SWs has been debated for many years. Initially, it was believed that their generation was an intrinsic property of the smooth muscle itself and there was no specialized pacemaker cells involved (18, 75).
However, in the late 1970s and early 1980s, the concept of a pacemaker in the GI tract gathered steam. In a study carried out by Durdle et al. (17), a pacemaker region was discovered in the canine colon. Using microelectrode impalement to measure electrical activity of the muscle, it was observed that upon removal of the submucosa from the circular smooth muscle layer, SWs were abolished. This was supported by Smith et al. (70), who showed in the canine colon that when a thin strip of muscle along the submucosal border was removed, SWs were abolished in the bulk circular smooth muscle and SWs could not be evoked by application of ACh (the main excitatory neurotransmitter of the GI tract). However, excised strips with the submucosa still attached could still generate spontaneous SWs. In 1992, Liu et al. (48) found that 66% of SW activity in the canine colon was abolished upon removal of the submucosal plexus.
While in the colon, the pacemaker region appears to be at the submucosal region; in the small intestine, pacemaking originates from cells in the myenteric plexus region, between the circular and longitudinal smooth muscle layers. The presence of a pacemaker at this region of the small intestine had been discussed by Connor (12); in this study, the author observed that when longitudinal and circular layers of the feline small intestine were separated, the longitudinal layer retained the ability to generate SWs, whereas in the circular layer SWs were abolished. This led the author to hypothesize that SWs originated from a pacemaker region in the GI tract, in this case, the longitudinal muscle layer or a location in between the two layers. This was supported by Suzuki et al. (73), who recorded SWs in the cat small intestine and demonstrated that they originated in between the two smooth muscle layers (myenteric region). Going further, Suzuki et al. (73) also suggested that ICCs located at this region were essential for the generation of SWs as isolated muscle strips in which ICCs were removed lacked SWs.
In the 1980s, a young scientist named Lars Thuneberg reopened the concept of ICCs acting as electrical pacemaker cells in the GI. Thuneberg was also the first to provide experimental evidence that ICC-MY were responsible for generating SWs (74). He noted that ICCs of the murine small intestine stained with methylene blue after muscle separation into longitudinal and circular portions. ICC staining was retained in the longitudinal muscle portion but not in the circular muscle. This highly suggested that ICC-MY were responsible for generating SWs. Thuneberg argued that a network of ICCs formed an electrical syncytium with smooth muscle cells in the GI tract, suggesting that the pacemaking of SWs originated from ICCs rather than the longitudinal smooth muscle.
Unraveling the Role of ICCs
Following Thuneberg’s exciting publications, many laboratories sought to further investigate ICCs and to elucidate their physiological role. Using Thuneberg’s methylene blue staining experiments coupled with electron microscopy, researchers were now poised to discern the roles of ICCs. Similar results to Thuneberg were observed in the canine colon by Sanders et al. (61), in which methylene blue staining led to a loss of SWs, and this was further supported by studies in the small intestine (47, 49). While methylene blue accumulates in the endoplasmic reticulum of ICCs, it is not a specific marker for this cell type (26). Methylene blue can have multiple nonspecific effects in smooth muscle, such as membrane depolarisation and interfering with electrogenic ion pumps (65).
At the time, there was still no specific marker for ICCs, and this issue was the greatest stumbling block to the advancement of the field. To firmly establish a pacemaker role for ICCs, researchers would need to move beyond morphological studies and delve into functional experiments. The first of these experiments was performed in 1989, by Langton et al. (42), who dispersed cells from the canine colon and used phase-contrast microscopy to differentiate ICCs from smooth muscle cells based on their morphology as had been previously done in situ. For the very first time, electrical activity was recorded from ICCs directly using the patch-clamp technique. Using this technique, the authors demonstrated that ICCs were indeed excitable and exhibited spontaneous electrical activity in the form of membrane depolarizations, which were similar to SWs observed in intact muscles. Further evidence for the ICC pacemaker hypothesis was put forward when it was demonstrated that while SWs actively propagated in the colon in pacemaker regions containing ICCs, SWs decayed when these pacemaker regions were removed (63). This indicated that not only did SWs originate in ICCs but also that they were responsible for the active regenerative propagation of SWs in the gut, as shown in Fig. 3.
c-Kit, the ICC Marker
The study of ICCs in the GI tract took a great leap forward when it was discovered that ICCs express the protein c-Kit. c-Kit (CD117) is a proto-oncogene that encodes a receptor tyrosine kinase. c-Kit expression in ICCs is robust compared with smooth muscle cells or enteric nerves (19, 50). Signaling via this receptor has been shown to be vital for normal ICC development and GI motility (63).
In 1992, Maeda et al. (50) outlined a series of experiments designed to determine how ablation of c-Kit affected neonatal development in mice. A surprising finding of this study showed that injection of the monoclonal antibody ACK2 (an inhibitor of c-Kit function) into neonatal mice resulted in the loss of normal phasic contractions in the GI tract. The result was incredibly significant. We must recall that in the early 1990s, the biggest obstacle to tackling the ICC functional role question was the lack of a specific marker for these cells. Now, Maeda et al. had reported a specific inhibitor for a specific receptor that led to a loss of rhymiticty and normal GI motility. If a loss of motility was correlated with inhibition of c-Kit, was it the case that a loss of c-Kit was associated with a loss of ICCs? In other words, was c-Kit a specific marker for ICCs?
In a critical paper, using the lessons from Maeda et al. (50), Ward et al. (79) studied W locus mutant mice to examine the role of c-Kit in GI motility (the W locus is allelic with c-Kit). In this study, electrical SWs were absent from the intestines of mutant mice lacking c-Kit (W/Wv mice). It was also observed that ICC-MY development was greatly impaired in these W/Wv mice, as confirmed by immunohistochemistry. This was supported by Torihashi et al. (76), who noted that the distribution of c-Kit-positive cells in the mouse small intestine was the same as that of ICCs and that these cells formed a network similar to cells that stained positive for methylene blue. The authors noted that injection of ACK2 resulted in a loss of SWs, and immunohistochemical studies showed that c-Kit-expressing cells were markedly reduced in response to ACK2. Further studies using c-Kit mutants in the mouse small intestine (9, 27) and guinea pig stomach (68) also supported the hypothesis that ICCs are required for the generation of SWs in the GI tract. These experiments provided a powerful tool for identifying ICCs in the GI tract by identifying c-Kit as specific marker for ICCs, and use of the genetic knockdown approach of the receptor provided valuable evidence that supported the ICC pacemaker hypothesis.
An elegant demonstration of the pacemaking ability of ICCs in the GI syncytium came from the University of Melbourne in 1999 by Dickens et al. (15). In this study, three different cell types were identified in the guinea pig stomach on the basis of their electrical activity. The first group of smooth muscle cells in the circular layer exhibited SWs; the second group of cells exhibited large and rapidly rising driving potentials. These “driver” cells were shown to have numerous processes and formed a network when visualized after Lucifer yellow injections, and subsequent labeling with c-Kit antibodies revealed they were ICCs. The third type of smooth muscle cell in the longitudinal layer exhibited activity similar to SWs similar to the circular region but were smaller in amplitude. Dual recordings of smooth muscle cells and ICCs showed that the driver potentials and SWs were initiated at the same time. However, closer examination revealed that the initial upstroke depolarisation of the driver potentials preceded the initial upstroke component of the SW. This strongly indicated that ICCs generated the initial component required for SWs.
Taken together, at the dawning of the new millennium, advances in the field, and novel genetic manipulation and innovative cell identification techniques led to the wide acceptance of the concept among GI physiologists that GI motility was dependent on rhythmic, coordinated electrical SWs. Through novel genetic experiments and innovative cell identification techniques, it was consistently shown that ICC-MY expressed the specific marker c-Kit and that ICCs were putative pacemaker cells (Fig. 2B) and coordinated SWs to drive rhythmic GI contractions from the stomach to the distal colon.
ICCs as Enteric Neuromodulators
Cajal was the first scientist to note that a population of interstitial cells might transduce signals from the enteric nervous system to the smooth muscle of the GI tract due to their intermediary location within the GI synctium. Nearly 90 yr after his initial findings, researchers would make the same observation and come to the same conclusion.
It is now known that as well as ICC-MY there are other ICC types throughout the GI tract. In the stomach and colon, ICC-IM run parallel with the longitudinal smooth muscle (64). In the small intestine, ICC-DMP are similar in structure and location to ICC-IM (8, 77). These populations of ICCs are distinct from ICC-MY not only in their location but also in their function. In the small intestine and stomach, respectively, ICC-DMP and ICC-IM lack the ability to generate SWs but appear to be in prime locations to interact with the enteric nervous system and transduce neural signals from enteric neurons to smooth muscle cells (6, 33, 64).
After Cajal originally put this hypothesis forward in 1911, there was little to no interest for quite some time. As we have seen, as the physiological function of ICCs was explored in the early 1970s and 1980s, the focus remained almost entirely on testing the pacemaker hypothesis. However, with the widespread use of electron microscopy beginning in the 1960s, investigators were able to observe the ultrastructural features of ICC-IM/DMP, which offered tantalizing glimpses into their function. In 1977, separate publications from Yamamoto (82) and Faussone Pellegrini et al. (21) observed that ICCs of the stomach and small intestine were located in close proximity to nerve terminals. Also in 1984, Daniel and Posey-Daniel (14) observed that ICCs in the esophagus of the opossum were in close proximity to nerve varicosities (~23 nm) and that ICCs were also coupled to smooth muscle cells via gap junctions. These morphological studies highly suggested that ICC-IM/DMP may relay neural signals to smooth muscle. However, much like the development of the pacemaker hypothesis, the appropriate experimental tools were not yet available to fully elucidate the role of ICCs in neurotransmission.
Thanks to the identification of the c-Kit marker in ICCs and the development of mutant animals lacking ICC (i.e., W/Wv mice), the mid 1990s saw a giant stride forward in making the ICC neuromodulator hypothesis viable. In 1996, Burns et al. (9) demonstrated that ICC-IM and ICC-MY were functionally distinct classes of ICCs in the murine stomach. ICC-IM were found to be in close apposition to nerve terminals, and, furthermore, using the W/Wv mouse, in which ICC-IM were absent in the stomach fundus, it was shown that inhibitory nerve responses were also ablated in tissue that lacked ICC-IM (nerve distribution was found to remain intact in such tissues). This study provided evidence of the functional role of ICC-IM as neuromodulators in the gut.
Subsequent studies demonstrated that ICCs were closely associated with nitric oxide synthase and ACh nerve terminals and that mice with reduced ICC-IM in the stomach had reduced neurally evoked cholinergic responses (4, 72, 78). Recent studies have also shown that ICC-DMP and ICC-IM are present in the stomach, small intestine, and colon of primates, suggesting that ICCs are also involved in neurotransmission in higher mammals (8). As stated above, in mutant animal models lacking ICCs (i.e., W/Wv mice), the enteric nervous system remained intact and electrical and contractile responses to direct neural stimulation were greatly reduced in animals where ICC-IM were lacking (4, 79). However, it should also be noted that in these animals, GI smooth muscle cells might undergo remodeling to partially compensate for the absence of ICCs (62). Neurotransmitters may overflow to smooth muscle cells in the absence of ICCs. Smooth muscle cells retain the expression of receptors and effector mechanisms that can respond to neurotransmitters in W/Wv mice; for example, it is known that smooth muscle cells are able to elicit electrical and contractile responses to exogenous applications of neurotransmitters in a similar manner to control animals (4, 79). It should be noted that possible remodeling of postjunctional cells in mutant animals may recruit additional mechanisms in smooth muscle cells that are capable of sustaining excitatory neuronal responses, which could be manifested in pathological conditions where ICC-IM/DMP are absent or reduced (62).
An essential element of teaching undergraduate and indeed postgraduate smooth muscle physiology is emphasizing that unlike what has previous been taught in recent decades, the smooth muscle layers of the GI tract (and indeed many other visceral organs) is not a homogenous cell population. The discovery of ICCs as pacemakers and also as neural mediators to the bulk smooth muscle should allow students to realize that the GI tract is a complex multicellular network, made even more complex by the fact that smooth muscle cells, nerves, and ICCs are electrically coupled and act as a functional unit.
ICC Pathophysiology and Beyond the Gut
One aspect of the ICC field that makes them especially vital for a rounded knowledge of GI physiology is their potential translational value as clinical targets to alleviate GI disorders. Loss of functional ICCs is associated with a plethora of GI disorders, including pseudoobstruction, achalasia, gastroparesis, and chronic constipation (66), and hyperplasia of ICCs can lead to the formation of GI stromal tumors (66). The study of the proper cellular generation of pacemaker and neuromodulation activity in the GI tract is thus of critical importance to medical and physiology students, as in the future ICCs may be seen as valuable therapeutic targets for alleviating the above disorders. The potential of ICCs to act as targets for GI motility disorders has sparked an even increased interest in interstitial cells in the last 10 yr, both in and outside of the GI tract.
Since the function of ICCs in the GI tract was elucidated, many groups have reported the discovery of ICC-like cells in various tissues of many animal models, including the urethra (69), portal vein (59), prostate (20), pancreas (57), corpus cavernosum (24), mesenteric artery (60), myometrium (58), myocardium (25), ureter (55), and bladder (51). ICCs have been proposed to act as pacemakers similar to the GI tract in some of these tissues (urethra, portal vein, ureter, etc.); however, the role of ICCs in other tissues is currently unknown (24). For example, in the bladder, the close association of ICCs and nerve networks suggests that they may act as intermediatory cells to transduct nerve signals to the detrusor (16, 37, 52).
A New Player: PDGF Receptor α+ ICCs
The picture of how to properly teach the theory behind the cellular basis of GI motility has become even more complicated in recent years due to the discovery of a second population of interstitial cells in the GI tract that are distinct from ICCs. As early as 1992, Zhou and Komuro (83) made reference to a possible non-ICC interstitial cell in the small intestine of guinea pigs. Much like the early work on ICCs in the late 1980s, significant advances in probing the identity of these new interstitial cells was hindered due to the lack of a specific cell marker. In 1999, Komuro et al. (35) termed these non-ICC cells in the GI tract “fibroblast-like cells”; this term was later adopted by Pieri et al. (56), who observed structurally similar interstitial cells (stellate-shaped opposed to spindle-shaped smooth muscle cells) in the human gut. It was known at the time that these interstitial cells were not ICCs due to their lack of expression of c-Kit; however, the problem of identifying a specific cell marker remained.
A decade after Komuro et al. coined the term “fibroblast-like cells,” a specific marker was finally found. In two publications in the mouse, which came out in rapid succession, Iino et al. (28) and Iino and Nojyo (29) discovered that the fibroblast-like cells expressed PDGF receptor (PDGFR)α, and, thus, these interstitial cells were dubbed PDGFRα+ cells. These PDGFRα+ interstitial cells were later found to also reside throughout the GI tract of mice (2, 38, 41), primates (8), and humans (39, 40).
The discovery of PDGFRα+ interstitial cells is rapidly expanding our knowledge of how normal GI motility is maintained. Similar to ICC-IM and ICC-DMP, PDGFRα+ interstitial cells are noted to be in close apposition to enteric nerve terminals in the gastric fundus (2) and colon (39, 41). Therefore, it was hypothesized that PDGFRα+ interstitial cells may also act as neuromodulators in the GI to transduce nerve signals to the bulk smooth muscle from enteric neurons. As we have seen, since the early 2000s, it has been established that ICC-IM/DMP are responsible for transducing cholinergic and nitrergic inputs into GI smooth muscle. However, recent intriguing data with electrophysiological recordings, Ca2+ imaging, and genetic studies have pointed to PDGFRα+ interstitial cells playing a role in transducing purinergic signaling in the gastric fundus (2) and colon (3, 41). Purinergic neurotransmission is an essential component of inducing GI smooth muscle relaxation and thus regular motility, and it was previously thought that smooth muscle was directly innervated by purinergic nerve terminals. However, recent evidence strongly suggests that it is in fact PDGFRα+ interstitial cells and not smooth muscle that mediate the purinergic relaxation of the GI tract (3, 38). Much like the elucidation of the ICC pacemaker system in the 1990s, the discovery of PDGFRα+ interstitial cells may soon completely shift a previously firmly established paradigm in GI physiology. Thus, ICCs together with PDGFRα+ interstitial cells, enteric neurons, and smooth muscle cells can form a functional unit termed the smooth muscle cell-ICC-PDGFRα+ interstitial cell (SIP) syncytium, translating electrical pacemaking conductances and neural signals into phasic contractions (Fig. 4).
Also, much like ICCs, investigators are now identifying PDGFRα+ interstitial cells in other visceral smooth muscle organs, including the female reproductive tract (54) and bladder (34). Exciting new data from the bladder suggest that similar to the colon, PDGFRα+ interstitial cells are key players in modulating bladder excitability by mediating purinergic neurotransmission (43–45).
While a breath of indepth experimental work has been performed on interstitial cells, students of biology and physiology and, in particular, those engaged in smooth muscle or GI research should learn the following essential physiological principles from this work. First, the contractions of GI smooth muscle, which underlie motility behaviors such as peristalsis, segmentation, mixing, and storage, require pacemakers to coordinate and properly time contractions into rhythmic patterns of behavior for normal physiological function. Second, the pacemakers in the GI tract are networks of ICCs that generate and propagate electrical pacemaker events termed SWs, which, due to the electrically coupled nature of ICCs and smooth muscle cells, leads to passive transduction of SWs to the muscle walls of the gut, which allows for excitation-contraction coupling to occur. Third, the enteric nervous system does not innervate smooth muscle exclusively; instead, enteric nerves innervate populations of ICCs and PDGFRα+ interstitial cells to modulate their activity, which can affect the contractile state of smooth muscle, again due to the electrically coupled nature of the GI syncytium. Finally, the importance of interstitial cells in regulating the contractile state of smooth muscle organs makes them potentially valuable targets for alleviating pathophysiological conditions where smooth muscle function is defective both in and outside of the GI tract (gastroparesis, constipation, irritable bowel syndrome, and overactive bladder).
These learning outcomes may perhaps be best assessed through problem-based exam questions. To analyze the importance of the GI syncytium, students could be asked to outline the outcome of removing a particular cell type from the GI tract under an experimental setup. For example, would smooth muscle contractions become more or less coordinated in areas lacking ICCs? How might this lead to a pathophysiological condition such as constipation? If ICC-IM are removed from the stomach, what effect might this have on nitric oxide-induced relaxation in smooth muscle? What might postjunctional purinergic and nitrergic responses in the colon look like after ablation of PDGFRα+ interstitial cells and ICCs, respectively? How might this affect the contractile state of the tissue?
The story has gotten breathtakingly complicated, and we can now appreciate that the GI tract is an electrically coupled syncytium composed of at least three cell types: smooth muscle cells, ICCs, and PDGFRα+ interstitial cells (SIP cells). These cells comprise a functionally integrated, electrical SIP syncytium that generates GI motility patterns (Fig. 4). The educational importance of such concepts cannot be overemphasized, as current research has only demonstrated that the GI tract is even more complex than Cajal, Keith, or Thuneberg initially realized. Teaching a model of smooth muscle-only influence on the GI in terms of neural innervation and pacemaking is simply no longer viable based on the evidence amassed since Cajal first drew his interstitial cells in 1911 (a timeline of the key discoveries in the field since that time is shown in Fig. 5). Thus, any future significant advances in the fields of GI motility and physiology will rely on imparting the knowledge of current research to undergraduates and postgraduates to continue to investigate the complex and multicellular nature of smooth muscle rhythmicity and contractility.
B. T. Drumm and S. A. Baker received salary support from National Institute of Diabetes and Digestive and Kidney Diseases Grant R01-DK-091336.
No conflicts of interest, financial or otherwise, are declared by the author(s).
B.T.D. prepared figures; B.T.D. drafted manuscript; B.T.D. and S.A.B. edited and revised manuscript; B.T.D. and S.A.B. approved final version of manuscript.
- Copyright © 2017 the American Physiological Society