There was one patient awaiting consultation in the ophthalmology department and whilst he was doing so, he also had to undergo a quantitative assessment of tear production. In other words, testing to see if his lacrimal glands produced enough lacrimal fluid. The action was taken because he was complaining of dryness in his eyes. The Schirmer method was used and the scores that result from taking the test actually represent the length of a paper strip that was crossed by the lacrimal fluid. Turns out, that was the reason his eyes felt dry- his tear production accounted for a low score.
Yet, that was not exactly the most interesting thing that came out of that consultation (and why I won't insist on that matter for now).
I learnt that while you can assess the quantity of tears secreted, you can also assess the lacrimal fluid in terms of
quality. My question, then, was: What are you looking for, what are you really assessing when having to do a qualitative test? I knew of the presence of lysozyme in tears, a antibacterial enzyme, but as a part of a complex group of substances which make up tears.
There are three basic layers which together form the tear film in humans:
- The first, outermost layer is the LIPID layer
- The middle (or second) layer is the AQUEOUS layer
- The third, inner layer is the MUCOUS layer
From this point on, things do get more complicated because, for instance, each layer is secreted by different glands (for those of you studying anatomy and physiology- you know you love it when you hear that)
1. Lipid Layer made out of lipid. That is pure logic. The glands responsible for the production of meibum (as it is called) that coats the tears and prevents them from spilling on your cheek are the Meibomian glands. You can think of this layer as a gel coat that gives tears their shape and helps them stay onto your eyes.
2.The Aqueous Layer is made out of water and other substances such as proteins (in this category we can include lysozyme, antibodies, lacritin etc.) and is the layer secreted by the Lacrimal glands. Its role is to promote the spreading of tears and it also has important immunity roles as it contains lysozyme and antibodies (you can read a little about them in another post of mine).
3.The last is the Mucous Layer, which is made by the goblet cells (special epithelial cells that specialize in the secretion of mucins, which are glycoproteins). These cells have very interesting features, in my opinion, so I might write a post about them later. The even distribution of the film is their role.
All in all, tears are very complex secretions of our bodies which are very important for our well-being as they participate in processes like hydration of the eyeball and immunity. Moreover, the secretion of lacrimal fluid can be checked in terms of quantity and quality. Who knows what we'll discover next...
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