Anterior Segment of the Eye

• Anterior segment of the eye structures include the eyelids, which consist of the superior, inferior, and nictitans.
• Meibomian glands are within the eyelids and secrete the oily layer of the tear film.
• Blinking maintains the thickness of the tear film.
• Blinking moves tears to and through the nasolacrimal system.
• Blinking helps to remove small particles from the corneal surface.
• The nictitating membrane contains tear-producing glands which contribute to the aqueous portion of the tear film.
• The nictitating membrane protects the globe by moving over the cornea when the globe is retracted.
• The nictitating membrane supports the position of the lower eyelid.
• The nictitating membrane forms part of the lacrimal pool in the medial canthus.
• Facial nerve (VII) efferent fibers mediate eyelid closure and innervate the corrugator supercilia which opens eyelids.
• Oculomotor nerve (III) innervates the levator palpebral superioris.
• Eyelid reflexes exist.

Eyelids of Different Species

• Dog eyelids have cilia on the upper eyelid.
• The upper eyelid is more mobile than the lower eyelid in dogs.
• The blinking frequency of dogs is normally 3-5 times per minute, but can increase to 10-20 times per minute when restrained.
• Incomplete blinks occur 50% of the time in dogs.
• Eyes open 10 – 15 days of age in dogs
• Cat eyelids lack cilia on both eyelids.
• Complete blinks only occur 1-5 times every 5 minutes in cats.
• Kittens usually open their eyes between 10-15 days of age.
• The nictitating membrane is more active in cats and can extend at least 2/3 of the way across the cornea.
• Smooth muscle draws the membrane across this is innervated by post ganglionic adrenergic sympathetic nerve fibres, cell bodes in anterior cervical ganglion.
• Each muscle cell is innervated by one or more axons as the muscle cells do not have tight junctions
• Horse eyelids have long and numerous cilia on the upper eyelid.
• The blinking frequency of horses is 5-25 times per minute, with approximately 30% of blinks being incomplete.
• Horses have Vibrissae on dorsal brow and lower eyelid, providing additional sensation for the eyelids.
• Cattle, sheep, and pigs have a majority of cilia present on the upper eyelid.
• The Harderian gland is present in pigs, rabbits, rodents, and some ruminants, assisting in eye lubrication through secretions of lipids, porphyrins, indoles, and growth factors.
• In pigs, the meibomian glands are poorly developed, and the main eyelid glands are sweat glands.
• Bird and reptile lower eyelids are larger and more mobile than the upper eyelid.
• Chicks have their eyes open when hatched.
• The superciliary or supraorbital ridge refers to the unfeathered bony protuberance just dorsal to the orbital rim that is seen in many raptors, such as eagles and hawks.
• The nictitating membrane in birds is nearly transparent and replaces eyelids for blinking; movement is under direct muscular control via the pyramidalis through a loop of quadratus.
• Nictitans also contain a superficial tear gland, and some species also have a harderian gland.

Tear Production and Drainage

• Corneal function and clarity depend on the integrity of the lacrimal system.

Tear Film Function

• Maintains a uniform corneal surface by smoothing out minor irregularities.
• Removes foreign matter from the cornea and conjunctiva.
• Lubricates the cornea and conjunctiva.
• Provides nutrients to the cornea.
• Controls local bacterial flora (in some species).
• The rate of evaporation of the tear film is directly related to the rate of blinking.
• Horses turn over their tear film every 7 minutes.

Layers of the Tear Film

• Lipid
• Mucin
• Aqueous

Lipid Layer

• Produced by sebaceous glands (tarsal and meibomian glands) via holocrine secretion, where the entire cell contributes to the secretion.
• Contains waxy and cholesteryl esters, polar and neutral lipids, and long-chain omega-hydroxy fatty acids that stabilize the tear film.
• Prevents evaporation of underlying layers and overflow of tear film onto eyelids.

Aqueous Layer

• Approximately 7 μm thick.
• Secreted by the lacrimal gland, the superficial gland of the nictitating membrane, the harderian gland, and accessory lacrimal glands in the conjunctiva.
• Contains inorganic salts, glucose, urea, proteins, glycoproteins, and soluble mucins.
• Performs the primary functions of the tear film.
• Affected by topical anesthetics, anticholinergics, and gas anesthesia.

Mucous Layer

• Produced by apocrine conjunctival goblet cells, the underlying glycocalyx associated with corneal and conjunctival microvilli, and the Harderian gland.
• Human and canine goblet cells are most dense in the lower nasal and middle fornices.
• Contains glycoproteins: carbohydrate-protein complexes that contain hexosamines, hexoses, and sialic acid.
• Lubricates the corneal surface, making the hydrophobic surface more hydrophilic, permitting the spreading of tears.
• Lowers the surface tension of the tear film, allowing it to spread after blinking.

Glycocalyx

• Includes MUC1-21.
• MUC 1, 4, and 16 are secreted by the stratified squamous epithelium of the cornea and conjunctiva.
• Comprises polysaccharides produced by stratified squamous epithelial cells of the cornea and conjunctiva.
• Projects from the surface microvilli of these cells.
• Considered membrane-spanning mucins, unlike secreted mucins.
• In dogs, MUC16 is expressed at higher levels than MUC1 and MUC4.
• In rabbits, MUC1, MUC4, and MUC16 are expressed at relatively equal levels.
• Peripheral corneal epithelium has higher mRNA expression of MUC1, MUC4, and MUC16 compared to the central corneal epithelium.

Roles of Membrane-Associated Mucins

• Promotes water retention
• Provides a dense barrier to pathogens
• Participates in signal transduction
• Direct interaction with the actin cytoskeleton

Tear pH

• Tears are slightly alkaline.
• Cow: 8.3
• Dog: 8.1
• Horse: 7.8
• Horses, cows, rabbits, and humans have electrolyte compositions similar to plasma, with only a 3-6 times higher potassium level, indicating an active method of transport.

Tear Film Osmolarity/Osmolality

• Influenced by tear secretion rate, evaporation, and composition.
• Similar in cats (329 mOsm/L), dogs (356 mOsmol/L), and rabbits (376 mmol/kg).
• Lower in humans (283 mmol/kg) and horses (284 mmol/kg).

Antimicrobial Substances

• Non-specific: lysozyme, lactoferrin, α-lysine, complement.

Lysozyme

• Has antibacterial and antifungal properties.
• A ubiquitous antibacterial enzyme.
• Hydrolyzes bacterial cell walls.
• Produced by conjunctival goblet cells.
• Low levels are found in humans and NH primates.
• Absent in cats and cows.

Lactoferrin

• Found in humans, dogs, cats, cows, and other mammals.
• Reversibly binds iron, inhibiting bacterial growth.

Specific Substances

• Secretory immunoglobulins A, G, and M.
• Immunoglobulin A coats bacteria and viruses, leading to agglutination, neutralization, and lysis.
• IgA has a greater concentration in the PTF than IgG or IgM.
• Toll-like receptors are expressed by corneal and conjunctival epithelial cells.

Innervation

• The lacrimal nerve provides parasympathetic and sympathetic innervation to the lacrimal gland.
• Cholinergic fibers outnumber adrenergic fibers.
• It is a branch of CNV.

Secretion

• Acinar cells secrete proteins into the lacrimal gland fluid.
• Proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, and stored in secretory granules.

Types of Tears

• Continuous tears are part of the normally functioning preocular and tear film.
• Reflex tears are produced by exposure of the eye to irritants.
• Tears induced by drugs directly stimulating glands.
• Psychogenic tears (in sad humans).

Tear Evaluation

• Schirmer I evaluates the total and basal aqueous portions of tear formation.
• Schirmer II determines basal levels of tear formation using local anesthesia, with basal tear levels approximately ~50% of total tear formation in dogs and cats.

Drainage

• Occurs via the nasolacrimal drainage system.
• The tear film accumulates along the palpebral margin.
• Blinking moves tears medially into the lacrimal puncta.
• Tears move into the lacrimal pool, and facial muscle relaxation causes tears to flow into canaliculi via capillary action.
• Reflex blinking of eyelids closes the lacrimal sac, which acts as a pump.
• The nasolacrimal duct has pseudo peristaltic motion, which moves tears into the nasal cavity.