Pathology of gastroparesis: ICC, enteric neurons and fibrosis

Introduction

Gastroparesis is a disease that affects the stomach’s motility and digestion, preventing proper stomach emptying. As a complex organ, the stomach empties both solid food and liquids at different rates, with liquids normally emptying faster than solids. In the absence of obvious mechanical outlet obstruction, gastroparesis is classified as delayed emptying of food from the stomach into the small intestine .

Gastroparesis is a distressingly painful disorder with a nationwide prevalence of 0.16% in the United States with a rising trend, particularly among children and minorities, mainly due to an increase in diabetes .

Characterized by a constellation of upper gastrointestinal (GI) symptoms in association with delayed gastric emptying (GE), major symptoms of gastroparesis can include early satiety or postprandial fullness, postprandial nausea, vomiting, abdominal bloating, heartburn, gastroesophageal reflux, lack of appetite, weight loss, malnutrition and abdominal or epigastric pain. Often gastroparesis is prominent in both type 1 and type 2 diabetes .

The pathogenesis of gastroparesis is poorly understood, in part because of a lack of comprehensive studies of gastric pathology and histology in these patients. Recent studies aimed to investigate the underlying mechanisms of gastroparesis through looking at the pathology of the stomach . This chapter summarizes pathological findings of gastroparesis by looking at different elements of the gastric movement including enteric neurons, interstitial cells of Cajal (ICC), and fibrosis. A better understanding of the physiology of gastric emptying and accommodation would help us to better appreciate the underlying pathological changes in gastroparesis.

Physiology of gastric motility

As a smooth muscle organ, the stomach connects the esophagus to the small intestine. Stomach movements are intricate involving both neurogenic and myogenic components. The neurogenic elements arise from the parasympathetic or vagus nerve and the enteric nervous system. The vagus nerve synapses with enteric interneurons that in turn synapse with inhibitory motor neurons releasing nitric oxide (NO) and ATP as a co-neurotransmitter to relax the gastric stomach smooth muscles. Myogenic factors stem from the pacemaker networks near the myenteric plexus that generate electrical slow waves propagating into the circular and longitudinal muscle layers that underly gastric contractions. Electrical slow waves are generated in a network of c-KIT positive cells lining the myenteric plexus in the entire stomach called the myenteric interstitial cells of Cajal (ICC-MY) . Electrical slow waves propagate through the network of ICC both around and down the stomach to produce ring-like peristaltic contractions of the wall every three minutes. The dominant slow waves originate in the mid-stomach or c-Kit positive ICC-MY cells in the corpus , as the dominant pacemaker region for the stomach (see Fig. 8.1 ) .

Other polarized ICC within the muscle layers are intramuscular ICC (ICC-IM) that act as neuroeffectors between enteric inhibitory and excitatory motor nerve terminals and smooth muscle cells (SMC) .

As food propagates into the antrum, the antrum contracts and retropulses solid matter back into the stomach, producing mixing and grinding of food into smaller particles. Only small particles with a diameter of 1–2 mm of viscous liquid food called chyme are normally propelled through the pylorus and into the duodenum (see Fig. 8.2 ).

Gastric accommodation

Gastric filling requires adaptive relaxation of the stomach (see Fig. 8.2 ). Adaptive relaxation is a reflex in which the fundus and upper body of the stomach dilates in response to food entering the stomach. Another reflex which is involved in gastric accommodation is receptive relaxation, which by definition, is a reflex in which the gastric fundus dilates when food passes down the pharynx and the esophagus or during a swallow leading to a decrease in gastric tone. Filling is due to activation of vagal neurons that have cell bodies in the dorsal motor nucleus (DMN) synapsing with enteric interneurons, that in turn synapse with inhibitory motor neurons (IMN) which largely release nitric oxide (NO) from neuronal nitric oxide synthesis positive (nNOS) motor neurons within cell bodies in the enteric nervous system. NO is a major inhibitory co-neurotransmitter, which along with ATP accommodates food by relaxing the gastric smooth muscle .

Interstitial cells and the smooth muscle cell/ICC/PDGFRα ⁺(SIP) syncytium

Coordinated contractions of the stomach are integrated motor responses. In the last 20 years, dissection of the myogenic elements using genetic, molecular, morphologic, and physiologic approaches have identified several cell types that contribute to these components of gastrointestinal motility. Interstitial cells of Cajal (ICC-MY) in the myenteric regions are recognized as major contributors to gastric and gastrointestinal (GI) motor activity . Dense ICC within the corpus region of the stomach generates dominant electrical slow waves (see Fig. 8.3 ) . These cells are gastric pacemakers, whereas other ICC within the muscle layers, called intramuscular ICC (ICC-IM) act as neuroeffectors between enteric motor nerve terminals and smooth muscle cells (SMC) and are essential for inhibitory and excitatory neurotransmission, whereas others act as gastric stretch receptors and have been found in close proximity to enteric nerves and contribute to neurally-mediated GI motor responses .

A second type of interstitial cells are identified as Platelet-derived growth factor cells (PDGFRα ⁺ ) which form a syncytium . PDGFRα ⁺ cells, like ICC, have also been found in close proximity to enteric nerves and also appear to contribute to neurally-mediated GI neural motor responses. Smooth muscle cells are electrically coupled to ICC and PDGFRα ⁺ cells forming an integrated unit termed the SIP syncytium (see Fig. 8.3 ) . SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affects the excitability and response of the entire syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity .

Pathology of gastroparesis

Histologic abnormalities were found in 83% of diabetic patients. The most common defects were loss of ICC with remaining ICC showing injury, an abnormal immune infiltrate containing macrophages and decreased nerve fibers .

The pathogenesis of gastroparesis is poorly understood, in part because of a lack of prospective comprehensive studies of gastric pathology and histology in these patients. Further, the disease may be multifactorial involving autonomic dysfunction as well as the gut-brain pathways which could not be detected with pathological studies .

Understanding the cellular mechanisms of gastric emptying helps us to better categorize the pathological findings of gastroparesis. Any defect in the pacemaker cells of the stomach, i.e. ICC, enteric neurons and smooth muscle cells – are the endpoint of gastric movement and would interfere with normal emptying of the stomach in gastroparesis. Moreover, as different parts of the stomach play a different role in the process of gastric emptying, it is necessary to study the pathology of the stomach in different regions including stomach body, antrum and pylorus. In the next part of this chapter, we will describe studies which have looked at the structure of the stomach in gastroparetic patients.

Gastric biopsy techniques in gastroparesis

Studying the pathology of gastroparesis requires access to full-thickness gastric samples which are mainly obtained during the implantation of gastric electrical stimulator (GES) in patients with severe gastroparesis. Most of the studies have looked at the pathology of the stomach body and antrum. However, surgeons are reluctant to biopsy the pylorus due to the fear of perforation. Pyloroplasty along with GES implantation has provided a new opportunity to study the full-thickness pyloric tissue obtained during the procedure . As gastric surgery is performed in severe gastroparetic patients, studies on patients with mild and moderate symptoms are limited.

EUS-guided FNA biopsies of the muscularis propria of the antrum in 11 patients with gastroparesis was able to obtain sufficient tissue for the histologic assessment of ICCs in 9 patients (81%) and for the myenteric plexus in 6 patients (54%) with no perforations, serosal tears or major adverse effects .

Percutaneous, endoscopically-assisted transenteric full-thickness gastric biopsy is an alternative method instead of through invasive surgery with a reported success rate of 90% in 9 of 10 patients without any reported complications .

A future potential method of full-thickness muscular biopsy of the stomach would be sampling during gastric peroral endoscopic myotomy which has not been well-established yet.

Gastric ICC in gastroparesis

To stain ICC, pathological studies in gastroparesis have more commonly considered using c-Kit ( Fig. 8.4 ) and sometimes anoctamin 1 (ANO1)/TMEM16A . ICC which emerge from mesenchymal stem cells express the receptor tyrosine kinase c-Kit, which is involved in the maintenance and functional roles of these interstitial cells. A calcium regulated chloride channel that is specifically expressed in ICC is encoded by ANO1; this channel is essential for slow wave generation in the gastrointestinal tract .

Interstitial Cells of Cajal–opathy or ICC depletion has been well documented in gastroparesis. This phenomenon is not limited to diabetic gastroparesis, but is also observed in idiopathic gastroparesis as well as gastroparesis-like syndromes or chronic unexplained nausea and vomiting (CUNV) . Tables 8.1 and 8.2 summarize studies on ICC counts in the gastric tissue of animal models of gastroparesis and gastroparetic patients, respectively.

Table 8.1

Interstitial cells of Cajal (ICC) in the stomach of animal models with gastroparesis.

	Chen et al.	Mogami et al.	Jin et al.	Wu et al.	Zhang et al.	Wang et al.	Long et al.	Mitsui	Ordög
Animal model	Diabetic rat (STZ)	Diabetic rat (STZ)	Diabetic rat (STZ)	Diabetic mouse (STZ)	Ws/Ws rat	Diabetic rat (STZ)	Diabetic rat (STZ)	Ws/Ws eat	NOD mouse
Gastric region	Antrum	Whole stomach, antrum and body	Proximal stomach	Antrum and body	Antrum	Antrum and fundus	Antrum	Antrum	Antrum
ICC count	ICC-IM (↓) ICC-MP (↓) ICC-SM (↓)	ICC-IM (↓) ICC-MP (→) ICC-SM (↓)	ICC (↓)	ICC-IM (↓) ICC-MP(↓)	ICC-IM (↓) ICC-MP (→)	ICC-IM (↓) ICC-MP (→) ICC-SM (↓) ICC (→)*	ICC (↓)	ICC-MP(↓)ICC-SM (↓)	ICC-IM (↓)ICC-MP (↓)