127 Salivary film thickness and MUC5B levels of the palate during swallowing; this can possibly explain why the salivary film thickness and the MUC5B level are not particularly low in individuals with larger palatal dimensions. Additionally, the palatal salivary film is not only formed by the palatal salivary glands, but it is also dependent on the salivary film of the tongue. The tongue also plays an important role in moistening and lubricating the palate. As the salivary film thickness at the tongue is already 2.3–6 times thicker than that of the palate, this will promote the transfer of additional saliva from the tongue to the palate. Finally, the retention of saliva by the anterior palate also plays a possible role. The structural orientation of the anterior palate, especially of the rugae with their irregular, asymmetric ridges [61], causes the retention of mucins and moisture despite the negative effect of gravity. A possible limitation of the current study is the use of Sialopapers for the collection of MUC5B. Although the elution efficiency of MUC5B from the Sialopapers is good (84 ± 15%), it has to be noted that the absorption of all MUC5B glycoproteins from the mucosal surfaces to the Sialopaper seems virtually impossible. Namely, the oral mucosal surfaces are more or less covered with a double layer: a lower surface-bound layer, which is the mucosal pellicle, and an upper salivary film, loosely attached to the mucosal pellicle [4]. It is plausible to assume that the efficiency of absorption of MUC5B from the loosely attached salivary layer to the Sialopaper is probably more effective than for MUC5B from the mucosal pellicle. In this light, it also has to be noted that oral epithelial cells express membrane-bound mucin (MUC1), which can interact with MUC5B to develop the mucosal pellicle [4]. Consequently, this interaction hinders the adsorption of MUC5B of the mucosal pellicle to the Sialopaper. Transmission Electron Microscopy and immunogold labelling could be applied to study these interactions and shed light on the absorption efficiency [4]. These techniques already have successfully been applied for buccal epithelial cells, but not for other intra-oral surfaces [62]. A recent study revealed that the intra-oral scanner was a suitable instrument to investigate the palatal soft tissue in terms of shape, colour and curvature [63]. In line with our experience, the shape of the palatal surface, especially the palatal rugae, was documented very precisely with the intra-oral scanner. Yet, it has to be noted that the intra-oral scanner lacks the resolution to analyse the full microstructure of the palatal surface, and this could lead to a slight underestimation of the total palatal surface area determined in the current study. 6
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