A cat's upper eyelid carries about 25 to 30 meibomian glands in a row, the lower lid 20 to 25, and every blink lays down a film of lipid 40 to 100 nm thick over the cornea. That film is 1/1000 the thickness of the tear layer beneath it and it is the difference between a wet eye and an evaporating one. Below: the 2 gland types on the lid margin, the counts and calibres you can actually measure, the 3 jobs the lipid does, and the arithmetic that turns a nanometre of film into nanolitres of oil.
Two gland types on one lid margin
Both are sebaceous and both release their product by holocrine secretion, meaning the whole cell becomes the secretion. The meibomian glands are the large ones: 3 to 6 mm long, arranged vertically in the tarsal plate in a single row, each draining through a central duct to an orifice on the lid margin behind the eyelashes. The glands of Zeis are small sebaceous glands attached to the lash follicles, 1 to 2 per follicle, with ducts 10 to 20 µm wide that open into the follicle rather than onto the margin. Zeis glands contribute a small share of the lipid; the meibomian row contributes nearly all of it, which is why a blocked orifice matters more than a lash problem.
Counts and calibres
| Structure | Count per eye | Diameter (µm) | Length (mm) |
|---|---|---|---|
| Upper lid meibomian glands | 25–30 | 30–60 (acinus) | 4–6 |
| Lower lid meibomian glands | 20–25 | 30–60 (acinus) | 3–5 |
| Central duct | 1 per gland | 20–50 | 3–6 |
| Margin orifice | 45–55 | 50–100 | 0.2–0.5 |
| Glands of Zeis | 1–2 per lash | 10–20 (duct) | 0.5–1.5 |
Three jobs the lipid does
- It slows evaporation. The tear film is 7 to 10 µm deep, of which the aqueous layer is 6 to 8 µm and the mucin layer 0.2 to 1 µm. A 40 to 100 nm lipid cap on top raises the resistance to water loss by roughly 10 to 50 times, which is what lets a film that thin last 10 to 15 seconds between blinks.
- It seals the lid margin. When the lids close, lipid drawn from the orifices fills the 20 to 50 µm gap between the margins, so tears do not spill onto the cheek at every blink. A cat with a deficient lipid layer commonly shows a wet streak below the eye and dark tear staining within 2 to 4 weeks.
- It smooths the optical surface. The cornea needs a tear film whose surface varies by less than about 0.1 µm to stay clear; an uneven lipid layer scatters light and shows up as a dull, slightly cloudy eye under a 10× loupe even when the cornea itself is normal.
The lipid is solid at room temperature and liquid on the eye
Meibum melts at 32 to 35 °C. The ocular surface of a cat runs at 34 to 37 °C and core temperature at 38 to 39 °C, so the secretion sits only 1 to 5 °C above its melting point — a narrow margin that explains a lot of clinical behaviour. A room at 18 to 20 °C stiffens the lipid in the duct during a 5 to 10 minute examination; a cat with a body temperature of 37 °C instead of 38.5 °C has measurably thicker meibum. Composition matters too: the more saturated and longer the fatty acid chains, the higher the melting point, and the harder the gland is to express.
Observable scale
An orifice of 50 to 100 µm is half the width of a human hair, and the orifices sit 0.5 to 1 mm apart, so a 5 mm stretch of upper lid margin carries 5 to 10 of them. Under a slit lamp at 10× to 16× they appear as a row of dark dots; at 40× the expressed lipid is visible as a bead 0.1 to 0.3 mm across. An acinus of 40 µm is about 7 cat red blood cells across at 5.7 µm each, and a gland 5 mm long by 40 µm wide holds 20 to 40 acini along its length. Digital compression of one gland yields 0.01 to 0.05 µL — enough to see, not enough to measure with a syringe.
Worked conversion: nanolitres per blink
Take an ocular surface of 2 cm² (cornea 1.2 to 1.5 cm² plus the exposed conjunctiva) and a lipid layer of 60 nm. One cm² is 100 mm², and 60 nm is 0.00006 mm, so the film over 1 cm² occupies 100 × 0.00006 = 0.006 mm³. Since 1 mm³ is 1 µL, that is 0.006 µL, or 6 nL per cm², and 12 nL over 2 cm². Now the gland side: 45 glands × 30 acini × the volume of a 40 µm sphere (4/3 × 3.14 × 20³ = 33,500 µm³, or 3.35 × 10⁻⁵ µL) gives 0.045 µL, or 45 nL in the pool. One film therefore draws about a quarter of the stored lipid, which is why the glands must refill between blinks and why losing a third of them still leaves a film for a while.
Reference values and how much they move
Different individuals and laboratories differ slightly: meibomian gland counts of 20 to 30 per lid are quoted for cats, with the upper lid at the top of that range, and a total of 45 to 55 orifices per eye. Tear film breakup time is commonly given as 10 to 15 seconds, with under 5 seconds treated as abnormal. Expressed lipid is graded 0 to 4 by how many glands yield fluid under standard pressure, and a grade of 0 to 1 across half the lid is the usual threshold for meibomian gland dysfunction. All of these shift with room temperature, restraint, and the force used to compress the lid, so record the conditions with the number.
What changes the count and the output
Age is the largest factor: gland orifices become less visible and the expressed lipid thicker from about 8 years on. Conformational change matters more in cats than in dogs because the eye sits deep in a bony orbit with a short palpebral fissure, so any lid swelling of 1 to 2 mm closes the meibomian row against the globe. Entropion, trichiasis, and chronic conjunctivitis all push the orifices against the cornea and reduce output. Signs that need a veterinarian within 2 to 3 days: a thick yellow or green discharge, a cornea that looks cloudy or blue, blepharospasm that keeps the eye shut for more than a few hours, or a visible defect in the corneal surface.
Four practical steps
- Count the visible orifices on both lids and write the number down: 45 to 55 per eye is the reference, under 30 is worth investigating.
- Warm compresses at 40 to 42 °C for 5 to 10 minutes raise meibum above its 32 to 35 °C melting point; a compress that cools in 2 minutes is doing almost nothing.
- Massage from the lid margin toward the eye, not the reverse, and expect 0.01 to 0.05 µL from a gland that is flowing.
- Compare the eye with the tear film in mind: sweat gland density explains why cats rely on evaporation so little elsewhere, colour vision covers what the cornea is protecting, and the breed quiz lists the flat-faced shapes with the shortest fissures.
Terms mentioned in this article
Frequently asked
How many meibomian glands does a cat have on each eyelid?
The usual range is 20 to 30 per lid, with the upper lid near the top of that range at 25 to 30 and the lower lid at 20 to 25, giving 45 to 55 orifices per eye. Each gland is 3 to 6 mm long, drains through a 20 to 50 µm central duct, and opens through a 50 to 100 µm orifice spaced 0.5 to 1 mm from its neighbour.
Why does the melting point of meibum matter so much?
Meibum melts at 32 to 35 °C while the ocular surface runs at 34 to 37 °C, so the secretion sits only 1 to 5 °C above its melting point. In a room at 18 to 20 °C the lipid stiffens inside the ducts during a 5 to 10 minute examination, which is why a 40 to 42 °C compress for 5 to 10 minutes, rather than a brief warm wipe, is what makes expression possible.
What signs mean the lipid layer has failed and the cat needs a vet?
A wet streak or new dark staining below the eye, thick yellow or green discharge, a cornea that looks cloudy or blue, blepharospasm lasting more than a few hours, or any visible defect in the corneal surface call for an appointment within 2 to 3 days. Tear film breakup under 5 seconds, against a normal 10 to 15 seconds, is the measurement that usually confirms the lipid layer is the problem.