When we think about tears, we usually think about water. But healthy tears also contain lipids — oily substances that are an important part of the tear film.
Most of these lipids come from tiny glands hidden inside your eyelids: the meibomian glands. These glands produce a complex oily substance called meibum, which is released at the edges of the eyelids and becomes part of the tear film.
Understanding how this system normally works makes it much easier to understand what can happen when the glands stop functioning properly.
Meibomian glands are long, narrow glands located inside the upper and lower eyelids. They are arranged vertically, side by side, rather like rows of tiny tubes running through the eyelid.
Each gland contains multiple secretory structures called acini. These produce the material that eventually becomes meibum. The acini connect to a central duct, which carries the meibum toward a small opening — or orifice — at the eyelid margin.
So, in very simplified terms:
acini produce meibummeibum enters the ductit travels toward the lid marginit is released through the gland opening
The upper eyelid generally contains more meibomian glands than the lower eyelid, but both contribute to the tear-film lipid layer.
Meibum is the oily material produced by the meibomian glands. It is not a single type of oil. It is a complex mixture containing many different lipids.
In healthy glands, meibum moves through the ducts toward openings along the eyelid margin. Once released, it becomes the major source of lipids at the surface of the tear film. As we discussed in The Tear Film: What It Is and How It Works, this lipid-rich component sits at the air-facing surface of the tear film.
The meibomian glands do not simply pour oil continuously across the eye. Meibum is delivered to the eyelid margin, where a reservoir of lipid material is available to the tear film. Blinking helps distribute these lipids across its surface.
This is one reason the meibomian glands, eyelids, blinking and tear film should not be thought of as completely separate systems. They work together.
This is a little more complicated than it is often presented online. You may have heard:
“The meibomian glands make oil, and the oil stops your tears from evaporating.”
That is a useful starting point, but it is an oversimplification.
The lipid component contributes to the physical behavior and stability of the tear film and plays a role in regulating evaporation. But this depends on more than simply how much lipid is present. Research has not established a simple rule that:
“more oil = less evaporation” — or “a thicker lipid layer = slower evaporation”
The composition, organization and distribution of the lipids matter too, and researchers are still working to understand exactly how these properties affect tear-film behavior.
So the lipid layer is important — but it is not simply a passive “oil seal” sitting on top of watery tears.
The lipid layer is not a visible coating of oil sitting on the eye. It is extremely thin.
Clinicians can evaluate some of its characteristics using a technique called interferometry, in which patterns created by reflected light provide information about the lipid layer.
Lipid-Layer Interferometry: what are we actually looking at?
Blinking does more than spread watery tears across the eye. Each blink redistributes the tear film and helps spread lipids across its surface. Eyelid movement also interacts mechanically with the meibomian glands and their secretions.
For now, the important idea is simple: meibomian gland function and blinking are closely connected.
They are the major source, although the complete lipid composition of the tear film is more complex than simply “meibomian oil.” Other structures around the ocular surface can contribute lipid-related material as well.
For understanding the basic system, however, the key point is that meibomian gland secretions provide most of the lipids at the surface of the tear film.
When the meibomian glands do not function normally, this is known as meibomian gland dysfunction (MGD) — a common condition associated with dry eye. We explore what MGD is, what can happen inside the glands, and how it can affect the tear film separately:
Meibomian Gland Dysfunction (MGD): what actually happens inside the glands — coming soon
TFOS DEWS III: Digest.
An updated review of research relevant to dry eye disease, including tear-film biology and meibomian gland dysfunction.
TFOS DEWS III: Diagnostic Methodology.
The current evidence-based framework for dry-eye diagnosis and etiological subclassification, including lipid deficiency and meibomian gland assessment.
TFOS DEWS II: Tear Film Report.
A detailed review of tear-film composition, lipids, lipid-layer structure and tear-film dynamics.