40 Questions
What is the primary function of the lacrimal gland and accessory glands?
To produce the tear film
What are the two main lobes of the lacrimal gland, and how are they divided?
The two main lobes are the Orbital lobe and the Palpebral lobe, which are divided by the levator aponeurosis tendon.
What is the term for the type of gland that has branching tubules ending in secretory alveoli?
Tubuloracinar gland
What is the term used to describe the type of secretion produced by the lacrimal gland, and what is its characteristic?
Serous; thin and watery
What is the approximate weight and size of the lacrimal gland?
0.78g and approximately 20 x 12 x 5mm
What are the characteristics of the secretory cells in the lacrimal gland?
Tall, cone-shaped cells with apices directed towards a central lumen, containing numerous membrane-bound granules in the apical cytoplasm
What are the components of the lacrimal apparatus?
The lacrimal gland and accessory glands, and the drainage system
What is the purpose of the drainage system in the lacrimal apparatus?
To drain the tear film
What type of cells surround each acinus and produce contractile force to push tears down the duct?
Myoepithelial cells
What is the main component of the secretion produced by the lacrimal gland?
Proteins
What is the name of the artery that supplies blood to the lacrimal gland?
Lacrimal artery
What is the function of the tear film in terms of the cornea?
Maintains a smooth refracting surface
What is the purpose of the bactericidal properties of the tear film?
To prevent infection
What is the function of the accessory lacrimal glands?
To secrete mucus
What is the primary source of tear fluid, and what percentage of it is produced by the lacrimal gland?
The primary source of tear fluid is the lacrimal gland, which produces 90% of it.
How many accessory lacrimal glands are located in the lateral part of the upper fornix?
Around 40
What is the function of the tear film in terms of the corneal epithelium?
Prevents desiccation
What is the normal resting level of tear production, and how much can it increase during stimulation?
The normal resting level of tear production is 1-1.5 μl/min, and it can increase to over 100 times during stimulation.
What is the thickness of the lipid layer, and what is its function?
The thickness of the lipid layer is 0.1 μm, and its function is to inhibit tear evaporation, protect the aqueous layer from polar lipid contamination, and act as a hydrophobic barrier at the meibomian gland orifices.
What is the primary function of the mucous layer, and where is it mainly secreted from?
The primary function of the mucous layer is to enable tears to wet the epithelium and maintain the stability of the tear film during blinking, and it is mainly secreted from the goblet cells of the conjunctiva.
What is the composition of mucins in the mucous layer, and what is their size range?
Mucins in the mucous layer are glycoproteins, and their size range is up to 50 × 10^6 kDa.
What happens to the tear film immediately after a blink, and why does it rupture?
Immediately after a blink, the tear film thins by evaporation, and hydrophobic lipids contaminate the mucoid layer, causing it to rupture.
What is the resting tear volume, and what is the tear flow rate?
The resting tear volume is approximately 7 μl, and the tear flow rate is 1.2 μl per minute.
What is the pH of the tear fluid, and what is the turnover rate of the tear film?
The pH of the tear fluid is 7.5, and the turnover rate of the tear film is 16% per minute.
What is the name of the tendon that divides the two lobes of the lacrimal gland?
Levator aponeurosis tendon
What is the shape of the secretory cells in the lacrimal gland?
Tall cone-shaped
What is the main component of the secretion produced by the lacrimal gland?
Proteins
What is the name of the type of gland that has branching tubules ending in secretory alveoli?
Tubuloracinar gland
What is the function of the myoepithelial cells in the lacrimal gland?
To push tears down the duct
What is the primary source of blood supply to the lacrimal gland?
Lacrimal artery
What is the characteristic of the secretion produced by the lacrimal gland?
Serous
What is the function of the Goblet cells?
To secrete mucins
What is the approximate contribution of the accessory glands to the total tear fluid production?
10%
What is the primary function of the lipid layer in the tear film?
To inhibit tear evaporation
What is the typical size range of mucins in the mucous layer of the tear film?
Up to 10^6 kDa
What happens to the tear film immediately after a blink?
It thins due to evaporation
What is the primary source of the mucous layer in the tear film?
Goblet cells of the conjunctiva
What is the approximate turnover rate of the tear film?
16% per minute
What is the function of the glycocalyx in the tear film?
To enable tears to wet the epithelium
What is the approximate pH of the tear fluid?
7.5
Study Notes
The Lacrimal Apparatus
- The lacrimal apparatus consists of the lacrimal gland and accessory glands, responsible for tear film production and drainage.
- The lacrimal gland has two connected lobes: the larger orbital lobe and the smaller palpebral lobe, which can be seen through the fornix conjunctiva.
- The lacrimal gland produces serous secretions, weighing approximately 0.78g and measuring 20 x 12 x 5mm.
Tear Film Production
- The lacrimal gland has two types of cells: secretory cells, which produce proteins, and myoepithelial cells, which surround each acinus and produce contractile force to push tears down the duct.
- The secretion contains mostly proteins, but also lysozymes, lactoferrin, B-lysin, and IgA.
- The lacrimal gland has a blood supply mainly from the lacrimal artery, an early branch of the ophthalmic artery.
- There are approximately 60 accessory lacrimal glands, with 40 in the lateral part of the upper fornix and 6-8 in the lower fornix.
Composition, Structure, and Function of Tears
- The tear film has several functions: maintaining a smooth refracting surface over the cornea, transporting metabolic products, preventing desiccation of the corneal epithelium, lubricating the eyelids, and having bactericidal properties.
- The tear film is composed of 90% lacrimal gland secretions and 10% accessory gland secretions.
- The resting level of tear production is approximately 1.5 ml/day, with a thickness of around 7-9 μm and a volume of 7 μl.
- The pH of tears is 7.5, with a flow rate of 1.2 μl per minute and a turnover of 16% per minute.
Structure of the Tear Film
- The tear film classically consists of three layers: lipid, aqueous, and mucous.
- The lipid layer is 0.1 μm thick, composed of lipids from the meibomian glands, and inhibits tear evaporation and protects the aqueous layer from polar lipid contamination.
- The mucous layer is very hydrophilic, enabling tears to wet the epithelium and maintaining the stability of the tear film during blinking.
Dynamics of the Tear Film
- Immediately after blinking, the tear film thins by evaporation, leading to hydrophobic lipid contamination of the mucoid layer, causing it to rupture.
- The tear film breaks up, and this process is repeated with each blink.
Learn about the lacrimal apparatus, composition and function of tears, and the dynamics of the tear film. Discover why we cry, the neurological control of tear film formation, and the drainage system. Understand what can go wrong, including dry eyes, watery eyes, and lacrimal gland disorders.
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