113 Salivary film thickness and MUC5B levels INTRODUCTION The salivary glands produce saliva which contains a wide range of proteins and ions [1]. After secretion, and facilitated by swallowing, saliva is spread over the hard and soft tissues in the oral cavity as a thin film [2, 3]. A major compound of this salivary film is MUC5B, a large glycoprotein with a wide variety of hydrophilic carbohydrate side chains [4]. MUC5B plays a crucial role in saliva’s water retaining properties, such as moistening, visco-elasticity and lubrication [3, 5]. As a consequence, an impaired flow rate, i.e. hyposalivation, leads to lower availability of both water and salivary proteins and to the insufficient replenishing of the intra-oral salivary film [6]. Subsequently, this leads to impaired mucosal moistening and clinical problems, such as difficulties with speech and swallowing, pain and xerostomia [7, 8]. It was recently shown that the severity of xerostomia differed at different intra-oral locations [9, 10]. In particular, it was found that the perceived oral dryness was most profound for the (posterior) palate. Hypothetically, this could be related to an impaired salivary film and lower MUC5B content, especially at the palate. In the past, multiple studies have investigated the salivary film thickness including the total protein concentration at various mucosal surfaces [11–14]. These studies found that the total protein concentration displayed a wide variation depending on its location [12–14]. The protein concentration showed a negative correlation with the salivary film thickness, indicating that thinner salivary films were related to higher protein concentrations [12–14]. These findings reveal that the protein levels in the film are mainly influenced by the film volume, but did not provide detailed insights into the protein composition at various intra-oral locations. Determination of MUC5B levels in the salivary film could help to increase our understanding of salivary distribution in the oral cavity. To understand the distribution of saliva over various surfaces, it is also important to measure the intra-oral surface areas. The dimensions of the intra-oral surface areas have previously been analysed in order to determine the distribution and average thickness of the salivary film covering the teeth and oral mucosa [15–17]. Especially the palate plays a major role in xerostomia because the salivary film thickness at the anterior part of the palate is relatively thin compared to other intra-oral surfaces [2, 11–14, 18–21]. Besides, the central part of the anterior palate is devoid of minor salivary glands [22]. Therefore, in order to increase our understanding of the distribution of the salivary film, measurements of the palatal surface area were included in the current study. 6
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