Tear Production (M1)

Question 1

what does the tear film overlay

Answer 1

ocular surface (cornea and conjunctiva)

Question 2

where is tear film

Answer 2

anterior to cornea

Question 3

what are the 2 layers of the tear film

Answer 3

lipid and mucoaqueous layer

Question 4

what are the glands/cells that produce the components of the tear film

Answer 4

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

Question 5

Location of meibomian glands (aka tarsal glands)

Answer 5

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

Question 6

Function of acini

Answer 6

Secrete meibum, by holocrine method, into a central duct

Question 7

Composition of acini

Answer 7

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

Question 8

what regulates meibum production

Answer 8

hormones: androgen and estrogen

Question 9

what do androgen and estrogen stimulate and what does it lead to

Answer 9

stimulate hormone receptors, which leads to an increase in gene transcription for enzymes associated with fatty acid and cholesterol synthetic pathways

Question 10

where is meibum stored

Answer 10

central duct

Question 10

when is meibum secreted

Answer 10

when blinking (mechanical pressure)

Question 11

where are goblet cells found

Answer 11

nasal and inferior nasal region of conjunctiva

Question 11

what do goblet cells secrete and what method do they use

Answer 11

gel-forming mucins (glycoproteins).
merocrine and apocrine methods

Question 12

what layer of tear film are glycoproteins incorporated into

Answer 12

mucoaqueous layer

Question 13

MUC5AC

Answer 13

predominant mucin

Question 14

what are glycoproteins rich in and what do they do

Answer 14

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.

Question 15

where does mucin conc. decrease

Answer 15

near superficial lipid layer

Question 16

what is mucin secretion regulated by

Answer 16

neural control.
stimulation of sensory nerves (CN5 - trigeminal) in the cornea/conjunctiva contribute to secretion.
unknown motor innervation.

Question 17

location of accessory lacrimal glands

Answer 17

Krause: forniceal conjunctival stroma
Wolfring: palpebral conjunctiva stroma along posterior border of tarsal plate

Question 18

what method do accessory lacrimal glands secrete by

Answer 18

secrete aqueous by merocrine method - incorporated into the mucoaqueous layer of tear film

Question 19

what divides the lacrimal gland

Answer 19

levator aponeurosis - divided into 2 lobes.
orbital lobe: in the lacrimal fossa of the frontal bone temporally
palpebral lobe: in the superior temporal eyelid

Question 20

what cells make up the lacrimal gland

Answer 20

acinar cells - 80%
ductal cells - 10-15%
myoepithelial cells - 5-10%

Question 21

what do acinar cells secrete and how

Answer 21

secrete aqueous by merocrine method - aqueous moves into ducts that coalesce into 1+ excretory ducts - about 12 excretory ducts open into conjunctival fornix

Question 22

what layer of the tear film is aqueous incorporated in and what is it composed of

Answer 22

incorporated in mucoaqueous layer of tear film.
composed of water, electrolytes, soluble proteins, glucose, lactate, and urea.
98-99% of mucoaqueous layer is water

Question 23

what electrolytes are in the aqueous

Answer 23

Na+, K+, Mg2+, Ca2+, Cl-, HCO3^- (bicarbonate - acts as a buffer), PO4^3-

Question 24

purpose of electrolytes in the aqueous

Answer 24

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

Question 25

purpose of soluble proteins in aqueous

Answer 25

immune response: - lysozymes, lactoferrin, lipocalin, immunoglobulins, cytokines corneal wound healing: - growth factors, matrix metalloproteinases (MMP9)

Question 26

purpose of glucose in aqueous

Answer 26

energy metabolism

Question 27

purpose of lactate in aqueous

Answer 27

energy metabolism

Question 28

purpose of urea in aqueous

Answer 28

waste product of protein metabolism

Question 29

what is aqueous secretion regulated by

Answer 29

neural control. stimulation of sensory nerves in the cornea/conjunctiva (CN5) contribute to secretion. motor innervation is through the parasympathetic system

Question 30

why are basal tears secreted

Answer 30

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.

Question 31

why are reflex tears secreted

Answer 31

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

Question 32

why are emotional tears secreted

Answer 32

in response to stimulation of limbic system (emotions) and subsequent activation of the parasympathetic system