Function of the tear film
- cornea and conj is a bumpy rough surface, so tear film makes it smooth
- Optical quality of the eye
- Nutrition (provides a lot of O2 to cornea, so doesnt have to be vascularized)
- Mechanical (to avoid friction between tissues to avoid stress/act as a lubricant)
- Antibacterial (via lytic enzymes- break down proteases, bacteria, cell membrane (protect ocular surface as 1 defense))
- Immune response (immunoglobins IgA- innate protection& nasolacrimal drainage syst. and glands have a lot of inflammatory cells around it for good response)
Structure of tear film
- Outer layer- lipids
- Middle layer- aqueous layer
- Inner layer- mucin
There is a __A__ gradient from anterior to posterior into the aqueous layer, and a ___B__ gradient from posterior to anterior into the aqueous layer. The __B__ gradient is more substantial.
A. Lipid
B. Mucin
What is the function of the outer most layer, the lipid layer?
prevent evaporation of tears (and overflow)
-produced by meibomian glands along the lower and upper lids
What is the middle layer, the aqueous later produced by?
lacrimal gland, accessory glands of Krause and Wolfring
What is the function of the middle, aqueous layer?
transport principle tear components and provide moisture; serous layer; lytic enzymes, inflammatory components, and nutrients are found here for the most function of all layers; makes up about 90% of tear film; (deficiency in lacrimal or accessory glands can give aqueous dry eye)
What is the inner layer, mucin layer produced by?
Goblet cells- make the big fat glycosylated proteins:mucin;/ Goblet cells are specialized epithelium cells-holecrine cells, that live mostly in conjunctiva and move from basal surface of epithelium.
What is the function of the inner mucin layer?
hydrophilic coating for hydrophobic cornea, regulates surface tension of the ocular surface so that the tear film can spread out
Accessory lacrimal glands (Glands of Krause and Wolfring)
Both conjunctiva bound
secrete aqueous layer for basic secretion and majority of basal tear film
-Krause: have more of, but are smaller; located where we have a bend in the conj., the fornix.
-Wolfring: found in stroma of conj.; have less of but are bigger and found at margin of tarsal plate
Secretory granules within the acini of the lacrimal gland
secretory granules are released into the central lumen of the acini and travel down a series of ducts to conjunctival epithelium. The ducts are also lines with secretory epithelium which also adds to the aqueous layer.
Function and location of the lacrimal gland
F: secretes aqueous fluid for reflex secretion (also contributes to basal tear film)
L: located in superior temporal orbit roof; lies close to the lateral palpebral ligament
The lacrimal gland is divided into two portions by the levator aponeurosis:
- orbital (superior) lobe- closer to the orbit
- superior aspect- in close proximation to bony orbit
- anterior portion- is close up against orbital septum
- inferior aspect- is close to aponeurosis
- palpebral (inferior) lobe- closer to the eye
- superior aspect- in close proximity to aponeurosis
- anterior aspect- against orbital septum (keep in place)
- inferior- close to palpebral conjunctiva
When producing tears in either portion of the lacrimal gland, they must pass through a series of ducts in what portion to get to the ocular surface?
palpebral portion. This allows the gland to have dynamic secretion (division allows reflex, emotional stimulus, pain, ect)
The corneal epithelial surface cells have microvilli and are not a smooth surface. What allows for good attachment of the mucin layer to the epithelial cells?
Glycocalyx: big proteins that attach directly to epithelial cells of the cornea and conjunctiva
