Healthcare and Medicine Reference
In-Depth Information
The Glycocalyx
The glycocalyx is comprised of cell surface-associated mucins that extend
into the tear fl uid with large extracellular domains. It is assumed that the
glycocalyx is highly organized, most likely through interactions of surface
mucins with lectin-like proteins such as galectin-3 (Argüeso et al. 2009).
Other components of the glycocalyx include proteoglycans with their
associated glycosaminoglycans.
FUNCTIONS OF TEAR FLUID
Protection
The ocular surface epithelium is constantly confronted with disturbances
from the environment, many of which can be repelled by instinctive blinking
or averting the face from the source of danger. Drying, infection and minor
injuries, however, are some of the ever-present threats to the eye that may
be prevented by a healthy tear fi lm.
Physical Protection
Superfi cial lipids repel dust particles and some types of bacteria (Brauninger
et al. 1972). Hydrophilic particles cannot penetrate through this lipid layer,
while many of those hydrophobic particles able to penetrate are absorbed
and immobilized by mucins lying underneath. During the blink, the trapped
particles are removed toward the lower fornix and fi nally drained through
the lacrimal puncta (Adams 1979).
As an excess of lipids at the ocular surface may lead to the development
of hydrophobic, non-wettable spots and tear fi lm breakup, lipocalin, the
major lipid-binding protein in the tear fi lm, scavenges lipids in the tear
fl uid and delivers them to the aqueous phase. Thereby, it increases the tear
fi lm's surface pressure and preserves its integrity (Glasgow et al. 1999).
Surfactant proteins found in tears perform similar stabilizing functions
(Bräuer et al. 2007).
Antimicrobial Protection
The tear fi lm also contains major antimicrobial peptides and proteins.
While IgA and IgG are produced by plasma cells and secreted through
the epithelium, the cell-wall-degrading enzyme lysozyme is provided
by the lacrimal gland and fends off gram-positive bacteria through its
muraminidase activity (Fullard and Snyder 1990). Siderophilic bacteria,
which require free iron, are attenuated by lactoferrin and lipocalin, two
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