Tear Film, Lacrimal and Meibomian
Glands
Ladan Espandar, MD, MS
Assistant Professor of Ophthalmology
Department of Ophthalmology
University of Pittsburgh School of Medicine
DIAGRAM OF A HUMAN EYE
Sagittal section through the ocular surface
Anterior
Chamber
The ocular surface epithelium is continuous (pink) with regional
specializations in the cornea, conjunctiva, lacrimal, and accessory
lacrimal glands, and meibomian gland. Each specialized region of this
ocular surface epithelium contributes components of the tear film (blue).
Frontal view of the Ocular Surface System
The ocular surface includes the surface and glandular epithelia of the
cornea, conjunctiva, lacrimal gland, accessory lacrimal glands, and
meibomian gland. The functions of the system’s components are integrated
or linked by innervation, and the endocrine, vascular, and immune systems
Tear Film
• The tear film is a complex composite with
multiple sources, which include the lacrimal
gland, meibomian glands, goblet cells, and
accessory lacrimal glands (Krause, and Wolfring).
• The function of the tear film includes lubrication,
protection, nutrition of the cornea, and a critical
role in the optical properties of the eye
• Normal tear volume:6 μL
• Production rate: 1.2 μL/minute
• turnover rate: 16% per minute
Lipid Layer of Tear Film
• The meibomian glands are the main source of
lipids for the human TF.
• The meibomian gland secretions: extremely
complex mixture of various polar and
nonpolar lipids containing cholesteryl esters
(CEs), triacylglycerol, free cholesterol, free
fatty acids (FFAs), phospholipids, wax esters
(WEs), and diesters.
Lipid Synthesis in Meibomian Glands
• Oleic acid is the most
abundant fatty acid, 18C,
monounsaturated (18:1(cis#9)
• Key enzymes: Fatty acid and
cholesterol synthesis enzymes.
• Regulatory mechanism: sex
steroids, corticosteroids,
hypothalamic and pituitary
hormones, insulin, retinoids,
thyroxine, melanocortins,
neurotransmitters, growth
factors, and peroxisome
proliferator activated receptor
ligands.
Wax ester
Anatomy of the Meibomian Glands
Embryologic Development of the
Meibomian Gland
• Third to the seventh month of
gestation, during the sealed lid
phase of eyelid development.
• Loose connective tissue of the
mesoderm in the lid folds
differentiates into the tarsal
plate and muscles (orbicularis
and Riolan’s muscle), the
blood vessels, and the loose
connective tissue underlying
the outer lid skin and the
conjunctiva.
• Meibomian gland anlage grow
from the ectodermal sheet
Histologic Appearance of
the Meibomian Gland
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(A) The holocrine acini filled with
the secretory cells (meibocytes)
and surrounded by a basement
membrane (bm).
(B) In the area of the disintegration
zone, located at the transition of
the acinus to the ductule
(C) Numerous acini of spherical to
elongated shape are radially
arranged around the central duct
(cd) of a gland, seen here in a
longitudinal section
Physiology of the
Meibomian Glands
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The driving forces (1) the
continuous secretion of meibum
by the secretory acini
(2) the mechanical muscular action
by muscle fibers of the pretarsal
orbicularis muscle (M. orbicularis),
and of the marginal muscle of
Riolan (M. Riolan), which encircles
the terminal part of the
meibomian gland.
During a blink, these muscles may
exertca compression of the
meibomian gland that drives the
oil out of the orifice into the
marginal lipid reservoir, where it
eventually constitutes the tear film
lipid layer (TFLL), as observed
clinically
MEIBOMIAN GLAND STEM CELLS AND
CELL DYNAMICS
• After a process of maturation including lipid synthesis and
accumulation, centripetal cell movement, and eventual cell
degeneration and membrane disintegration, the lipids and other
cell components are shed into the lumen of the ductal system.
• This holocrine secretion process hence results in the dynamic
consequence that secretory cells are continuously lost and
replaced.
• The continuous loss of acinar cells requires a consequent
continuous production of new cells and therefore a continuous cell
turnover and differentiation within the acinus.
• Meibomian Gland Stem Cells concluded from their observations
that the stem cells of the meibomian glands lie at the
circumference of each acinus
Innervation
• The meibomian glands of the human have a dense
meshwork of unmyelinated nerve fibers (nerve plexus)
around the acini outside the basement
• Mainly represent cholinergic parasympathetic nervous
system, from the pterygopalatine ganglion,
sympathetic nerves from the superior cervical ganglion
and sensory fibers from the trigeminal ganglion.
• Other neuropeptides: calcitonin gene-related peptide
(CGRP) and substance P, vasoactive intestinal
polypeptide.
Aqueous part of tear film
ANATOMY OF THE
HUMAN LACRIMAL GLAND
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Main lacrimal gland, accessory lacrimal gland.
The main lacrimal gland: palpebral and orbital lobes,
separated by aponeurosis of levator palpebrae superioris
Excretory ducts coming from the palpebral and orbital lobes
open into the superior conjunctival fornix.
The accessory lacrimal gland is divided into 2 anatomic
groups: the glands of Krause and glands of Wolfring. The
glands of Krause are located in the lamina propria of fornix
and the glands of Wolfring are in the edge of the tarsus. The
ducts of both glands open on the conjunctival surface.
HISTOLOGY OF THE
HUMAN MAIN LACRIMAL
GLAND
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The lacrimal gland is an
exocrine gland. The main
lacrimal gland comprises many
lobules separated from one
another by loose connective
tissue.
Each lobule has many acini
and intralobular ducts.
The connective tissue contains
interlobular ducts, vessels,
nerve fibers, fibroblasts, many
plasma cells, and a few
lymphocytes.
Interlobular ducts finally
become approximately 12
excretory ducts, which open
into the fornix of conjunctiva.
Histology
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The acinus comprises pyramidshaped acinar cells with central
lumen. Acinar cells have many
periodic acid-Schiff (PAS)-positive
secretory granules and basally
located nuclei.
Myoepithelial cells are distributed
surrounding the acini and
intercalated ducts. Myoepithelial
cells have stellate, multiprocessed
morphology, and their contraction
may play a role in expelling
secretory products from glandular
lumina into ducts.
Plasma cells in connective tissue
produce IgA and are involved in
the formation of secretory IgA in
tear fluid. Secretory IgA is the
best-defined effector component
of the mucosal immune system.
HISTOPATHOLOGY OF THE
HUMAN MAIN LACRIMAL
GLAND
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Acinar atrophy; periacinar fibrosis;
periductal fibrosis; interlobular
ductal dilatation; interlobular
ductal proliferation; lymphocytic
infiltration; and fatty infiltration.
Chronic graft-versus-host disease
(GVHD), stromal fibroblasts are
actively involved in the pathogenic
process of the lacrimal gland.
In Sjögren syndrome, the earliest
histologic finding in salivary glands
has been described as periductal
lymphocytic infiltration.
Aqueous Tear Components
• Contains proteins, and electrolytes
• 6–10 mg/mL total proteins and almost 500 proteins
• Major tear proteins include lysozyme, lactoferrin, secretory
immunoglobulin A (sIgA), serum albumin, lipocalin (previously
called tear-specific prealbumin), and lipophilin.
• Chloride and potassium are higher in the tears (tears, 120 mEq/L
and 20 mEq/L; serum, 102 mEq/L and 5 mEq/L, respectively)
• Glucose concentration is lower in the tears (about 2.5 mg/100 mL)
compared to plasma (85 mg/L).
• The osmotic pressure of the tears ranges between 280 and 305
mOsm/L.
• Tear proteins vary with the state of health of the ocular surface.
Comparison of Tear
Components
with that of Serum
Mucus part of tear film
Conjunctiva
• The conjunctiva is a mucous
membrane that protects the soft
tissues of the eyelid and orbit,
allows extensive movement of
the eye and is the main site for
the production of the aqueous
and mucous components of
tears.
Embryology
• The conjunctiva develop within
the lid folds from surface
ectodermal and neural crest
tissue along the posterior surface
of the lids and from similar
tissues around the developing
cornea.
Histology
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The conjunctival surface is composed
of stratified nonkeratinizing
squamous epithelium
The epithelium contains goblet cells,
Langerhans cells, and dendritic
melanocytes. The substantia propria,
or conjunctival stroma, is highly
vascularized and may contain
nonstriated muscle, sympathetic
nerves, and fatty tissue.
Apically, a glycocalyx is secreted from
mucin-containing intraepithelial
vesicles, consisting of
transmembrane mucins (MUC1,
MUC4, MUC16).
The long-chain glycoprotein
molecules maintain tear film stability
by anchoring the mucin produced by
the goblet cells (MUC5AC) to the
conjunctival surface and also bind to
immunoglobulins.
Lymphocytes, dendritic melanocytes,
and Langerhans cells may be seen in
the suprabasal region of the
epithelium.
Glycocalyx
• Mat of long chain glycoproteins integral to the superficial corneal epithelial cell
membranes.
• Helps create a hydrophilic surface on the hydrophobic cell membranes and protects
against bacterial pathogens
• Insufficient mucins can damage glycocalyx, causing the tear film to destabilize and
break up before a blink can occur, exposing the cornea to air and pathogens
Muc 16
(TEM)
The cytoplasmic tail of
MUC16 is tethered to the
actin cytoskeleton through
actin linking ERM family
of proteins (ezrin, rodixin
moesin, and merlin).
H185 carbohydrate epitope
of Muc16 (TEM)
Muc 16
(SEM)
SEM showing corneal
surface microplicae
MUC16 Facilitates the Ocular Surface Barrier Function
(A) siRNA knockdown of mucin-16 in a human corneal limbal epithelial cell line. HCLE cells
cultured to subconfluent or confluent stages do not express MUC16, and rose bengal
dye penetrates all cells. After the cells are cultured seven additional days, cells stratify
and express MUC16. Islands of differentiated cells exclude the Rose bengal dye. When
MUC16 was knocked down by 80% to 90%, the islands that exclude the dye were
diminished.
(B) Rose bengal stains areas of damaged ocular surface epithelium in dry eye
Conjunctival goblet cells
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Goblet cells are unicellular, mucinsecreting glands that account for
approximately 5% to 10% of basal
cells.
They are likely apocrine in nature,
with all the secretory granules
secreted once the cell has been
activated.
They are the primary source of the
large soluble mucins in the tear film.
Goblet cells release their secretory
granules in response to activation of
the parasympathetic nerves that
surround them. Sympathetic nerves
also surround the goblet cells, not
sensory.
The mucin synthesized by goblet cells
of normal human conjunctiva is
identified as MUC5AC
Goblet cells may also secrete MUC1,
MUC4, and MUC16. These mucins
function to protect, hydrate, and
lubricate the ocular surface.
Maintenance of tear film stability
Production and secretion of tear components, redistribution
and drainage of the tears are precisely regulated
Control of Tear Secretion
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The small sensory nerve endings
located just below the epithelial
surface of the cornea, lid margin,
and conjunctiva constantly
respond to drying and
temperature change as well as
contact and chemical changes, by
sending intensity-coded neural
signals to the spinal trigeminal
nucleus located in the brain stem.
A multisynaptic pathway to the
preganglionic parasympathetic
nuclei in the superior salivatory
nucleus forms the output to the
secretory tissue.
Dry Eye
2007 International Dry Eye Workshop
Dry eye is a multifactorial disease of the tears and ocular surface
that results in symptoms of discomfort, visual disturbance, and
tear film instability with potential damage to the ocular surface.
Dry eye is accompanied by increased osmolarity of the tear film
and inflammation of the ocular surface.
 Tear osmolarity increases by an average of ~13%
 HLA ClassII expression by conjunctival epithelial cells increases
 CD4+ T-Cells infiltrate conjunctiva
 Tear MMP-9 activity and expression of cytokines increases
 Lacrimal proteins and conjunctival goblet cell density decreases
Major etiological causes of dry eye
Report of the International Dry Eye Workshop. The Ocular Surface. 2007. 5(2) 69-201
Mechanisms of dry eye
Report of the International Dry Eye Workshop. The Ocular Surface. 2007. 5(2) 69-201
SUMMARY
 Ocular surface consists of the cornea, bulbar conjunctiva and the palpebral
conjunctiva, bathed in the tear film.
 Tear film consists of the anterior lipid layer (secreted by the meibomian glands),
central aqueous layer (secreted by the lacrimal glands) containing mucins
(secreted by conjunctival goblet cells), anti-bacterial peptides, growth
factors, antibodies, solutes, and the posterior glycocalyx (superficial
epithelial cell membrane bound mucins).
Dry eye is a widely prevalent multifactorial disorder of the break up of the tear film.