Tear Production (M1) Flashcards
what does the tear film overlay
ocular surface (cornea and conjunctiva)
where is tear film
anterior to cornea
what are the 2 layers of the tear film
lipid and mucoaqueous layer
what are the glands/cells that produce the components of the tear film
meibomian glands: meibum of the lipid layer
goblet cells: mucin of the mucoaqueous layer
lacrimal gland and accessory lacrimal glands of Krause and Wolfring: aqueous of the mucoaqueous layer
Location of meibomian glands (aka tarsal glands)
Extend vertical length of tarsal plate.
Ciliary portion of eyelid margin.
Openings located in a row along eyelid margin.
- posterior to gray line and anterior to mucocutaneous junction
Function of acini
Secrete meibum, by holocrine method, into a central duct
Composition of acini
Mixture of lipids and proteins:
- amphipathic lipids (phospholipids, fatty acids) - in contact with mucoaqueous layer; provide structure stability to tear film
- non-polar lipids (waxes, cholesterol, and triglycerides) - in contact with air; create hydrophobic barrier
what regulates meibum production
hormones: androgen and estrogen
what do androgen and estrogen stimulate and what does it lead to
stimulate hormone receptors, which leads to an increase in gene transcription for enzymes associated with fatty acid and cholesterol synthetic pathways
where is meibum stored
central duct
when is meibum secreted
when blinking (mechanical pressure)
where are goblet cells found
nasal and inferior nasal region of conjunctiva
what do goblet cells secrete and what method do they use
gel-forming mucins (glycoproteins).
merocrine and apocrine methods
what layer of tear film are glycoproteins incorporated into
mucoaqueous layer
MUC5AC
predominant mucin
what are glycoproteins rich in and what do they do
serine, threonine, cysteine.
serine and threonine are sights for O-glycosylation - heavy glycosylation gives a neg charge to the glycoprotein –> makes mucins hydrophilic –> allows them to mix with aqueous component of tear film.
cysteine-rich areas form disulfide bonds - responsible for enhanced mucoadhesive properties.
where does mucin conc. decrease
near superficial lipid layer
what is mucin secretion regulated by
neural control.
stimulation of sensory nerves (CN5 - trigeminal) in the cornea/conjunctiva contribute to secretion.
unknown motor innervation.
location of accessory lacrimal glands
Krause: forniceal conjunctival stroma
Wolfring: palpebral conjunctiva stroma along posterior border of tarsal plate
what method do accessory lacrimal glands secrete by
secrete aqueous by merocrine method - incorporated into the mucoaqueous layer of tear film
what divides the lacrimal gland
levator aponeurosis - divided into 2 lobes.
orbital lobe: in the lacrimal fossa of the frontal bone temporally
palpebral lobe: in the superior temporal eyelid
what cells make up the lacrimal gland
acinar cells - 80%
ductal cells - 10-15%
myoepithelial cells - 5-10%
what do acinar cells secrete and how
secrete aqueous by merocrine method - aqueous moves into ducts that coalesce into 1+ excretory ducts - about 12 excretory ducts open into conjunctival fornix
what layer of the tear film is aqueous incorporated in and what is it composed of
incorporated in mucoaqueous layer of tear film.
composed of water, electrolytes, soluble proteins, glucose, lactate, and urea.
98-99% of mucoaqueous layer is water
what electrolytes are in the aqueous
Na+, K+, Mg2+, Ca2+, Cl-, HCO3^- (bicarbonate - acts as a buffer), PO4^3-
purpose of electrolytes in the aqueous
maintain fluid balance
- measured through osmolarity - total conc. of all solutes in the solution
- normal osmolarity of tear film is about 300 mOsm/L
- hyperosmolarity is linked to dry –> higher conc. of solutes in tear film vs. ocular surface cells –> less water; occurs due to abnormal tear evaporation (evaporative dry eye) or low tear volume (aqueous deficient)
acts as buffers (bicarbonate):
- maintain constant pH of 6.8-8.2
purpose of soluble proteins in aqueous
immune response:
- lysozymes, lactoferrin, lipocalin, immunoglobulins, cytokines
corneal wound healing:
- growth factors, matrix metalloproteinases (MMP9)
purpose of glucose in aqueous
energy metabolism
purpose of lactate in aqueous
energy metabolism
purpose of urea in aqueous
waste product of protein metabolism
what is aqueous secretion regulated by
neural control.
stimulation of sensory nerves in the cornea/conjunctiva (CN5) contribute to secretion.
motor innervation is through the parasympathetic system
why are basal tears secreted
to coat the ocular surface.
mucin and aqueous are secreted in response to stimulation of CN V and subsequent activation of the parasympathetic system travels with CN 7.
- stimulation of CN V occurs during normal blinking
Meibum is secreted upon blinking.
why are reflex tears secreted
in response to an irritant.
mucin and aqueous are secreted in response to stimulation of CN V (ex. sneeze, cutting onions) and subsequent activation of the parasympathetic system
why are emotional tears secreted
in response to stimulation of limbic system (emotions) and subsequent activation of the parasympathetic system
