Tears1 B(1).ppt

Lacrimal gland and tears Two Lectures Lecture 1 Where do tears come from? The lacrimal apparatus What are tears? Composition, structure and function of the tear film What happens when you blink? The dynamics of the tear film. Lecture 2 Why do you cry? Neurological control of tear film formation. Where do the tears go? The drainage system How good is a tear film? Methods of assessment of tear film integrity What can go wrong? Dry eyes, watery eyes, lacrimal gland disorders Lacrimal gland and tears Lacrimal Apparatus: production and drainage of tear film Drainage system Lacrimal gland and tears: 1 Tear film production: 1. Lacrimal gland 2. Accessory glands (Wolfrin; Krause; meibomian glands; mucous glands) 2 connected lobes – divided by levator aponeurosis tendon Large Orbital lobe Smaller Palpebral lobe – can be seen through the fornix conjuntiva Tubuloracinar gland with serous output Tubuloracinar - a gland having branching tubules which end in secretory alveoli (alveoli -Latin for small cavity) Serous - producing, or resembling serum (having a thin watery constitution) Weight: about 0.78g Size: approx. 20  12 5mm Made of many acini (cell clusters) with an exit duct Tall cone-shaped secretory cells – apices directed towards a central lumen. Cells contain numerous membrane-bound granules in apical cytoplasm. Secretory products accumulate in granules which grow and are excreted from apex into lumen 2 cell types: secretory and myoepithelial Myoepithelial cells surround each acinus and produce contractile force to push tears down duct. Central lumen of several lacunae combine to form larger ducts which themselves combine to form main drainage duct. Secretion contains mostly proteins, but also lysozymes, lactoferrin, B-lysin and IgA Blood supply: mainly from lacrimal artery, an early branch of the ophthalmic artery Histology & EM All so contain Goblet cells Function ? Secrete mucus Found in other parts of the body About 60 accessory lacrimal glands, 40 in lateral part of upper fornix (Krause), 6-8 in lower fornix 10-20 larger ducts open into superior fornix The accessory glands Accessory glands – minature versions of the main lacrimal gland Composition, structure and function of tears Functions of the tear film: 1. Maintains a smooth refracting surface over the cornea. 2. Transports metabolic products to and from the epithelial cells of the cornea (O2 in and CO2 out). 3. Prevents desiccation of the corneal epithelium. 4. Lubricates the eyelids. 5. Has bactericidal properties. 6. Provides a pathway for white blood cells in case of injury. 7. Provides nutrients (minor role) TEAR FLUID lacrimal gland - 90% accessory glands – 10% Resting level – l/min – 1.5 ml/day Tear production can increase to over 100x during stimulation of the lacrimal gland Thickness ~ 7-9 m (but don’t really know) Tear volume ~ 7l pH 7.5 Tear flow rate ~ 1.2l per minute Turnover ~ 16% per minute Structure of the tear film Classically: 3 layers – lipid layer – aqueous & mucus layer – mucous layer 0.1m thick Lipid bilayer Lipids from meibomian glands Lipid Layer : inhibits tear evaporation : protects aqueous layer from polar lipid contamination : at the meibomian gland orifices acts as hydrophobic barrier to prevent outflow of tears 60% thickness of the tear film Glycocalyx 40% thickness of tear film Mucous layer: Very hydrophilic Function 1: to enable tears to wet the epithelium Function 2: to maintain the stability of the tear film during blinking Mostly secreted by goblet cells of conjunctiva, although some comes from main lacrimal gland Mucins (glycoproteins) are present in sizes up to 50  106 kDa The dynamics of the tear film Immediately after blink, tear film thins by evaporation Immediately after blink, tear film thins by evaporation Hydrophobic lipids contaminate mucoid layer – mucoid layer ruptures Hydrophobic lipids contaminate mucoid layer – mucoid layer ruptures Tear film break-up

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