Lacrimal System

Meibomian (tarsal) glands

-30 to 40 separate glands in upper lid -20 to 30 in lower lid -secretion is sebum - lipid (oily) portion of tears

Situations leading to insufficient tear layer

-atrophy of lacrimal gland (e.g. Sjogren's syndrome) -increase in age -compromised innervation

Mucous layer (innermost) of the Tear Film

-goblet cells (primarily) -epithelial cells -lacrimal gland (very little)

Causes of dry eye complaints in CLs wearers

-insufficient ears -poor tear quality (decreased/short tear break up time TBUT) -infrequent blinking -incomplete blinking -reflex tears (low osmolarity) -poor CLs wetting -blepharitis

What is the arterial supply to the lacrimal gland comes from?

-lacrimal branch of the ophthalmic artery -branch of infraorbital artery -meningeal artery

Nasolacrimal sac

-large sac in which tears drain after passing through the nasolacrimal duct -Hasner's Valve is present at the opening of the duct into the nose

Superior and Inferior Canaliculi

-lined w/ stratified squamous epithelium -surrounded by the orbicularis oculi muscle

Thickness of tears

-lipid layer = 0.1 microns -aqueous layer = 7 microns -mucous layer = 0.02 to 0.05 microns

What are the two levels of tear secretion?

-low order secretion (normal activities) -high order secretion (reflex tearing, crying)

What are the proteins secreted in the tear secretion?

-lysozyme -lactoferrin -lipocalin

What are the three layers of the tear film?

-outer lipid layer -middle aqueous layer -inner mucous layer

What are the two main stimulation of tear production?

-physical (ocular stimulation) -psychological (emotional stimulation)

Lipid layer (outermost) of the Tear Film

-sebaceous -Meibomian gland -sebaceous gland (of Zeiss)

Aqueous layer (middle) of the Tear Film

-serous -lacrimal gland -fornix accessory lacrimal gland (krause) -palpebral accessory lacrimal gland (Wolfring)

What is neural controlled by (2)?

-vasoactive intestinal peptide (VIP) -neuropeptide Y (NPY)

What are the two possible steps where meibomian regulation could occur?

1. by controlling the rate of lipid synthesis in the ER 2. by regulating the rupture of alveolar cells (holocrine secretion)

What % of secreted tear volume is lost to evaporation?

10-25%

Volume of tears drained per blink

2.0 mL

More/fewer accessory glands are present in the lower eyelid compared to the upper eyelid?

Fewer

What type of cell layer are acini and why is it important?

Myoepithelial cell layer --> requires muscle movement to allow the contents to be drained

What is the final tear fluid a mixture of?

NaCl-rich fluid --> aqueous layer

Lacrimal gland dysfunction (atrophy) may indicate a dry eye condition known as

Sjogren's Syndrome

T/F tears contain many antibacterial properties that protect the cornea and conjunctiva from infection

True

What is the secondary provider of the aqueous secretory component to the tear film?

accessory lacrimal glands (glands of Krause and Wolfring) historical perspective - basal (takes time) and reflex (fast) tearing

What is the lacrimal gland made up of?

acini that drain into progressively larger tubules

What allows for the secretion of mucin?

activation of sensory nerves

When does the lacrimal gland become functional?

after birth *typically 2 weeks after birth--may be as long as 2 mo.

Normal tear production varies with _____

age

How can neurotransmitters (from nerves surrounding the acini) regulate meibomian secretion?

alters synthesis or rupture of cells

What is the thickest tear film layer?

aqueous layer (7 microns)

The lacrimal excretory system usually functions far _____ capacity

below *theoretically allow for absorption of some of the volume en route to the nasal cavity

What releases the meibomian material from the ducts onto the tear film?

blink

mucins

collection of glycoproteins

What did Doane emphasize?

compression of the canaliculi and to a lesser extent, in the nasolacrimal sac during eyelid closure

What accounts for most of the conjunctival active transport?

conjunctival Cl- secretion

Where is the gland of krause located?

conjunctival fornix

What is the lacrimal gland surrounded by?

connective tissue

What is lost due to severe dry eyes?

corneal transparency

What is released by the corneal and conjunctival epithalia?

electrolytes and water

What results from the facial paralysis of the orbicularis muscle?

epiphora (excessive eye watering)

Common canaliculus

formed in 90% of individuals from superior and inferior canaliculi

What accessory lacrimal gland plays the least role in lacrimal secretion?

gland of krause

Conjunctival glands

goblet cells

What is the primary source of the mucin secretion?

goblet cells

What process assists with the movement of tears from lacrimal gland?

gravity, blinking

What secretory action allows sebaceous glands to secrete lipids onto the tear film?

holocrine secretion

Punctal plug

insertion of a small plug into the lacrimal canaliculus to cause more tears to remain on eye

What carries sensory stimuli from the lacrimal gland?

lacrimal branch of the trigeminal nerve (CN V)

Where is the lacrimal gland located?

lacrimal fossa of frontal bone

What is the lacrimal gland divided by?

lateral horn of the LPS and extensions of Muller's muscle into: -orbital lobe (above/larger) -palpebral lobe (below)

What is the outermost layer of the tear film?

lipid

How does the tear film act as a lubricant?

lubricant between the lids and ocular surface

What is the primary provider of the aqueous secretory?

main lacrimal gland

How can androgen sex steroids regulate meibomian secretion?

may regulate synthesis and secretion (ex. testosterone --> female)

1 blink can transport more/less tears than 1 min of basic secretion

more

What happens if meibomian glands do not function well?

more evaporation of tear film --> inflammation, dryness

How are foreign bodies, debris, and exfoliated cells removed from the eye?

movement of tears

What is the thinnest tear film layer?

mucous layer (0.02-0.05 microns)

When does the lacrimal gland start developing?

near 3rd month of fetal life

What is found in the tear secretion?

nutrients, growth factors, glycoproteins, immunoglobulins in an isotonic Na+, K+, and Cl- rich solution

What does the tear film cover?

ocular surface -conjunctiva -cornea

Punctum

opening of the medial portion of each eyelid

What is NOT released by the corneal and conjunctival epithalia?

proteins

When does siphoning by the lacrimal sac occur?

relaxation of the blink

Holocrine secretion

release of a secretion caused by the rupture of the gland, which becomes part of the secretion cell dies

What is the secretion of the lacrimal gland (+ accessories) primarily made up of?

sero-mucous mainly serous

What kind of secretion is found in the gland of Wolfring?

serous (middle layer of tear film)

What is the secondary source of mucin secretion?

stratified squamous cells of conjunctiva and corneal epithelia

Where is the gland of Wolfring located?

tarsal plate of eyelid

What is the primary source of oxygen to the cornea?

tear film *cornea = avascular

What secretes proteins, electrolytes, and water into the tear secretion?

tubuloacinar exocrine glands (lacrimal gland)