What are the layers of the tear film and their sources?
The basic structure of tears includes oil (shown in yellow) floating near the air interface, aqueous mixed with proteins and soluble mucins (blue) beneath the oil, and a dense membrane bound mucin layer (the glyocalyx) on the cornea surface with its numerous microvillous processes.
(Click on figure to the left to enlarge).
The aqueous layer of tears is produced by the lacrimal glands at the upper outer of the eyelid and within the eyelid. The lacrimal gland produce proteins and aqueous. To the left is a photomicrograph of the main lacrimal gland with its lumen (1) acinar structures lined by cells with red granules (2) and an interstitium (3) containing scattered lymphocytes and plasma cells.
The oil layer of the tear film is produced by the Meibomian glands in the upper and lower eyelids. These glands are specialized sebaceous glands, composed of lobules of clear cells that contain a variety of lipids. In the sagittal section to the left one can see quite nicely the Meibomian gland of the upper tarsus all along the conjunctiva surface (6). The duct that will empty the lipid is shown adjacent to the marginal conjunctiva (4). By the way the accessory lacrimal gland of Krauss (5) is shown in the fornix (5). The soluble mucins are largely produced by specialized Goblet cells that are scattered within the conjunctiva among epithelial cells. The are most numerous in the fornix (3 in the photo to the left).
For a detailed look at conjunctival anatomy with clinical, histologic and ultrastructural photos click on this LINK. All components act in concert to provide adequate lubrication and protection to the cornea.
From the Mebomian gland the oil is secreted from the small orifices that dot the inside edge of both eyelids (Figure left, Click to Enlarge). In the slit lamp photograph a human lower eyelid shown to the left one can see the Meibomian orifices located inside the lash line. At one of the orifices there is a droplet of oil emanting from the surface. Click on the photograph to enlarge.