L42 Lacrimal & Tears

Question 1

What does the lacrimal system consist of?

Answer 1

The lacrimal system consists of the main accessory lacrimal glands and the lacrimal drainage system.

Question 2

Describe the lacrimal system

Answer 2

This simplified diagram shows the sac and gland.

Question 3

Where is the lacrimal gland located?

Answer 3

Lacrimal gland is located in the upper lateral portion of the orbit.

Sits in fossa in the frontal bone.

Question 4

Describe the arrangement of the lacrimal gland and why it is this way.

Answer 4

The lacrimal gland exists as two lobes; the Orbital lobe and the Palpebral lobe. It exists this way as it is almost divided by the aponeurosis of the Levator Palpebral Superioris.

You can see the two sections in the diagram marked OL

(orbital lobe) and PL (palpebral lobe).

Question 5

Describe the structure of the lacrimal gland

Answer 5

Tubuloacinar gland.

Composed of tubules (ducts) and secretory units (acini).

10-12 ducts discharge into upper lateral fornix.

Question 6

What do fibroblasts produce?

Answer 6

Collagen

Question 7

What are the predominant cells in the lacrimal gland?

Answer 7

Lymphocytes (B-cells which can turn into plasma cells for IgA production)

Question 8

Describe the Structure of secretory acini of the lacrimal gland.

Answer 8

• they are composed of secretory cells
• secretory cells contain large amounts of Rough Endoplasmic reticulum
• cells are linked together by tight junctions
• surrounded by myoepithelial cells.

Question 9

What is a myoepithelial cell?

Answer 9

Epithelial cells containing muscle- thus is able to contract ( usually found near glandular epithelium).

Question 10

How to ductal cells of the lacrimal gland modify the composition of the tear film?

Answer 10

They do this by secreting electrolytes.

(Background context: Tears freshly secreted from the acini have a different composition to those found in the duct).

Question 11

Do lacrimal ducts have secretory cells in them?

Answer 11

No, they are attached to acini which have secretory cells.

The duct is yellow in the picture and the acini are the green circles attached to it.

Note that the blue fibres are actually myoepithelial cells (contractile muscles) that can ‘squeeze’ the gland.

Question 12

How can one instantly cry a large volume of tears?

Answer 12

The lacrimal gland cannot instantly produce that many tears instead it actually acts as a tear reservoir. Such that when myoepithelial cells contract they can release those tears quickly.

Question 13

What makes tears salty?

Answer 13

High level of electrolytes in them. (sodium , potassium chloride).

Question 14

What are the three principal components of the tear film layer?

Answer 14

Upper layer-m Lipid layer from meibomian glands

Middle layer- aqueous layer comes from the lacrimal gland

Bottom layer - mucous

Question 15

What is tear PRODUCTION (not output) controlled by?

Answer 15

Autonomic control. Innervated by the parasympathetic system by fibres that run in the Facial (V) Nerve.

Question 16

What is the output of the lacrimal gland dependent on?

Answer 16

The output from glands is dependent on the level of corneal stimulation (by sensation experienced on surface of the eye) from the trigeminal nerve.

Question 17

Which antibody is present in tears?

Answer 17

IgA

Question 18

Where is IgA produced in the eye?

Answer 18

Secretory IgA is produced by plasma cells in the interstices (means in-between spaces) of the gland.

(IgA is transported across the acini).

Question 19

How much of the total lacrimal tissue do accessory lacrimal glands make up?

Answer 19

5%

Question 20

What are accessory lacrimal glands and how is it different from the main lacrimal gland?

Answer 20

Accessory lacrimal glands are small ducts and their respective acini spread across the conjunctiva whereas the main lacrimal gland is the same structured ducts and their respective acini just grouped together in one place.

Question 21

Why is it when you cry you also tend to get a runny nose?

Answer 21

The tear outflow path consists of the puncta, canaliculi, lacrimal sac, and nasolacrimal duct.

Furthermore, tear outflow is an active process mediated by the contraction of the Orbicularis in blinking.

Question 22

Describe the Puncta (Size and locations).

Answer 22

0.2-0.3 mm aperture

You have one in the upper eyelid and one in the lower eyelid.

Angled backwards towards the eye.

Question 23

Where is the lacrimal sac located?

Answer 23

In a fossa formed between the lacrimal and maxillary bone.

Question 24

Where does the nasolacrimal duct pass through?

Answer 24

Through a canal in the maxillary bone opening in the nasal cavity beneath inferior. nasal turbinate

Question 25

How is tear drainage affected by blinking - describe the process.

Answer 25

Prior to blinking tear drainage system is open.

As the eyelid close on each other to blink . The puncta meet and close. Canaliculi and sac are compressed (no tears move into puncta). Fluid moves into the duct.

As the eyelids begin to reopen (in the same blink) , the canaliculi and sac re-expand. Relative negative pressure develops in sac.

As blink ends and eye is open again, the Puncta reopens and fluid flows into puncta and canaliculi.

Question 26

The tear film perform several important functions that can be broadly classified as…?

Answer 26

Optical - in the sense that it is clear to allow light through and highly polished to create a smooth highly refractive surface

Metabolic - exchange of gasses

Protective - stops cornea drying out

Lubricative- because eyelids are constantly moving across the surface of the cornea.

Question 27

What is the comfort of contact lenses often dependant on?

Answer 27

The integrity of the tear film layer- there are some people that may never be able to wear contact lenses because they have problems with their tear film.

Question 28

Where does the majority of tear volume lie?

Answer 28

70-80% of tear volume lies within marginal tear strips. - this is highlighted in green.

(There is a small portion behind the lids and the remainder covers the cornea and exposed bulbar)

Question 29

What’s the pH of tears?

Answer 29

Same as blood - 7.45

Question 30

Define Trilaminar structure and give an example of something that has a trilaminar structure.

Answer 30

Means to consist of three layers

An example of a trilaminar is the tear film as it has three layers; lipid,aqueous and mucous.

Question 31

What are tears made of?

Answer 31

They are a complex secretion composed of electrolytes, proteins, lipids, and mucins.

Question 32

What is thought to be the principal function of the lipid layer?

Answer 32

Is to retard (slow down) tear evaporation and to form a hydrophobic barrier preventing overspill of tears.

Question 33

Tear film results from the secretion of which glands?

Answer 33

Main and Accessory Lacrimal glands

Meibomian Glands

Conjunctival Goblet cells.

Question 34

What proteins exist in the aqueous layer of the tear film?

Answer 34

IgA, Iyzozome and lactoferrin which have an antimicrobial function, and lipocalin which contributes to tear stability.

Question 35

By what is secretion of the aqueous layer regulated?

Answer 35

It is regulated by the autonomic nervous system.

Question 36

What does the aqueous layer contain?

Answer 36

Contains water, electrolytes and proteins

Question 37

What does the Mucin layer of the tear film derive from?

Answer 37

Conjunctival goblet cells and (as recently discovered) the glycocalyx of the ocular surface (basically glycoproteins on the surface of the eye).

Question 38

What is the role of the mucin layer of the tear film?

Answer 38

Has a lubricative role and maintains a hydrophilic ocular surface.

Question 39

What does the mucin layer contain?

Answer 39

•Contains a variety of mucin types (soluble and cell anchored)