Author:

Abstract:

The urinary bladder is a hollow structure, consisting mainly of smooth muscle cell bundles under neural control. It has two important functions: storage and emptying. During storage it must maintain low pressure. During emptying all smooth muscle bundles must contract synchronously. The neural control functions as an on/off switch. During the exertion of these functions, the bladder itself and the body must be protected from urine. Therefore the urothelium functions as a barrier. Although everything in the above stated paragraph is true, the truth is more complex. In recent years it has become clear that the bladder is a complex organ with important intrinsic bladder wall regulatory mechanisms. The urothelium does not function as a passive barrier, but is capable of active transport and even of bi-directional signaling. Afferent and efferent nerves are closely associated underneath the urothelium and are present in between the detrusor smooth muscle cells. Through intramural ganglia, nerves might form intramural circuits. Smooth muscle cells not only are the effector cells of the bladder, creating a contraction when needed. They are spontaneously active and create micromotions during filling. This autonomous activity probably induces afferent signaling. Underneath the urothelium and in between the smooth muscle cells, specialized cells similar to interstitial cells of Cajal in the gut, are present. Whereas different authors noted such cells in the suburothelium or in the detrusor of different species, a thorough morphological study in human tissue was lacking. In this essay, we first report our morphological study of these cells in human and rat tissue (chapter 3). We used criteria developed to identify interstitial cells of Cajal in the gut and applied them on interstitial cells of the urinary bladder. Using c-kit and vimentin immunohistochemistry and electron microscopy all previously mentioned criteria are fulfilled. Therefore interstitial cells in the bladder are "Cajal-like" cells. We identified two different networks of cells. A first network is located in the lamina propria of the urinary bladder, concentrated underneath the urothelium. A second network is located in between the smooth muscle cell bundles of the detrusor. Similar cells were shown to be present in rat urinary bladder. Since we have experience with urodynamic investigations in a rat model, the presence of these cells in the rat model is of specific interest for our further investigation. Immunohistochemical phenotyping of these cells in human tissue shows that they express a number of functionally interesting proteins. Interstitial cells in both networks of human urinary bladder express nNOS, connexin 43, synaptophysin, TRPV1, TRPV2 and nestin. In the detrusor they are surrounded by CB1 immunoreactivity (chapter 3). Interstitial cells in the detrusor seem to be involved in nitrergic signaling. In the urethra and in the terminal part of the bowel nitrergic signaling is mainly inhibitory. In the bladder NO probably has some excitatory effects as well. The expression of connexin 43 indicates the presence of gap junctions. Both the suburothelial and the detrusor network are strategically located to mediate/modulate neurotransmission. The detrusor network probably functions as a conduction network rather than a pacemaking network. Synaptophysin is a protein involved in exocytosis. TRPV1 and TRPV2 are receptors involved in signal transduction of nociceptive stimuli. TRPV1 is the site of action of vanilloids, which are used with varying success rates to treat (neurogenic) detrusor overactivity. Nestin is an intermediate filament present in interstitial cells of Cajal in the gut and in GISTs. Cannabinoids are known to have a relaxing effect on the urinary bladder. The close relationship between interstitial cells and the CB1 receptor suggests a role for interstitial cells in cannabinoid signaling and for endocannabinoids in bladder physiology. Immunohistochemistry in a rat model shows an identical expression pattern of interstitial cells as compared to human tissue (chapter 3). In chapters 4 to 6, we extended our previously developed immunohistochemical experience to the entire urinary tract and the prostate. We were able to demonstrate the presence of similar interstitial cells from the renal pelvis downward to the urethra in human tissue. There are some important regional differences. In the renal pelvis and pelvi-calyceal region of the human urinary tract c-kit expressing interstitial cells are seen in a suburothelial network. Some cells are seen in the muscular layer as well. In the entire ureter both a suburothelial network and an intramuscular network is seen. The number of interstitial cells is higher in the vesico-ureteral junctions and towards the bladder neck. In the urethra they are clearly less abundant. All interstitial cells express connexin 43, c-kit and TRPV2. The physiological implication of these observations is discussed together with a review of the recent literature. In the upper urinary tract interstitial cells might be involved in conducting pacemaker signals downward. In the ureter a specific connection to the afferent innervation might exist. In the urethra the tonic contractions could be frequency modulated, suggesting a role for interstitial cells. In the prostate interstitial cells may regulate smooth muscle cell tone, thus interfering with normal voiding. Interstitial cells located around the acini of the prostate gland might be involved in the secretory function. We describe the occurrence of a GIST of the prostate in one of the patients treated at our clinic. GISTs in the gut are believed to arise from interstitial cells of Cajal. The presence of a GIST in the prostate together with the description of "Cajal-like" cells in the prostate is an intriguing observation. In chapter 7 we examined differences in immunohistochemical expression pattern of interstitial cells in normal and neuropathic human bladder. We noted an up regulation of interstitial cells both in the suburothelium as in the detrusor of patients with neurogenic bladder disease. In the suburothelium interstitial cells probably function as a ‘sensing’ network in close contact with the urothelium and with afferent and efferent nerves. An up regulation can therefore decrease the micturition threshold. In the detrusor they probably form a conducting network and are involved in nitrergic signaling. An up regulation can increase excitability of detrusor smooth muscle cells. In chapter 8 and 9 we first evaluate synaptophysin as an immunohistochemical marker to quantify interstitial cells in rat urinary bladder. Confocal laser scanning microscopy confirms colocalization of synaptophysin, c-kit and connexin 43 on interstitial cells. Quantification of interstitial cells in normal and neuropathic rat bladder (detrusor) shows a downregulation of interstitial cells. Strangely, this is not in accordance with previous observations in human tissue. Species related differences might account for this observation. After intravesical instillation of RTX an up regulation of interstitial cells is noted. RTX clearly decreases detrusor contractility. In rats interstitial cells might have an inhibitory influence, for example by producing a relaxing factor. The development of a useful rat model to investigate interstitial cells is a key achievement of our experiments.