126 Chapter 6 were found; they found the highest MUC5B levels at the anterior hard palate and the lowest levels at the lower labial mucosa and the anterior tongue [14]. In contrast, in our study, the anterior tongue had the highest levels, and the anterior palate had the lowest levels of MUC5B. This is difficult to explain as to a large extend MUC5B is secreted by the submandibular and sublingual salivary glands with their sublingual caruncle lying on the floor of the mouth [55, 56], in which the tongue is embedded. As mentioned before, the anterior hard palate lacks the presence of salivary glands [22], and MUC5B found on the anterior palate is translocated there mainly by tongue movements. Surprisingly, the floor of the mouth contained approximately 18 times less MUC5B levels than the anterior tongue, despite the fact that the caruncles of both submandibular and sublingual glands are located on the floor of the mouth. Gravity forces help the floor of the mouth to create a reservoir for all the saliva that does not adhere to the various surfaces. So, the saliva on the floor of the mouth is a mix of various salivary glands. Especially after swallowing episodes, not all the saliva is swallowed; the salivary clearance is approximately 28% [57, 58], indicating that the majority of saliva remains in the mouth. Additionally, the structure of the tongue helps to adhere to all the mucins, as the dorsal (superior) surface has a rough structure of stratified squamous epithelium with numerous circumvallate, filiform and fungiform papillae. Potentially, this rough or plicated surface offers the MUC5B glycoprotein a surface to which it can reside more effectively during oro-facial movements, such as swallowing, than to the smooth structure of the floor of the mouth. An interesting finding in the current study was the significant correlation between the salivary film thickness and the MUC5B levels. MUC5B forms hydrophilic polymer brushes causing water retention [59]. For this reason, MUC5B is considered as the key lubricant in saliva. So, it could be expected that higher MUC5B levels will influence the increment of the salivary film thickness. Another interesting finding was the lack of correlation between the palatal surface area and the palatal salivary film thickness and/or the palatal MUC5B levels. We hypothesised that individuals with a larger palatal surface area would have a thinner salivary film at the palate and also less availability of MUC5B glycoproteins. However, we found that all individuals showed comparable salivary film thickness and MUC5B levels. This last result could be explained by the palatal saliva that contained relatively high levels of MUC5B [60]. Palatal saliva is excreted by the orifices of the palatal glands, which are all located near the right and left maxillary second and third molars [22]. The palatal saliva including MUC5B is propelled towards the anterior part
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