BCSC Cornea & External Disease (Structure and Function of the External Eye and Cornea)

Question 1

The eyelid skin blends into the surrounding periorbital skin, varying from 0.5 mm thick at the eyelid margin to I mm thick at the orbital rim

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Question 2

Except for fine vellus hairs, the only hairs of the eyelids are the eyelashes, or cilia, which are twice as numerous along the upper eyelid margin as along the lower.

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Question 3

Cilia are replaced every 3-5 months; they usually regrow in 2 weeks when cut and within 2 months if pulled out.

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Question 4

The cilia catch small particles and also work as sensors to stimulate reflex eyelid closure.

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Question 5

Blinking augments the lacrimal pump to rinse tears over the eye and flush off foreign material.

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Question 6

The epidermis of the eyelids abruptly changes to nonkeratinized stratified squamous epithelium at the mucocutaneous junction of the eyelid margin, along the row of meibo- mian gland orifices.

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Question 7

Near the eyelid margin are the apocrine sweat glands (the glands of Moll) and numerous sebaceous glands (the glands of Zeis)

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Question 8

The conjunctival sac includes the bulbar conjunctiva, a fornix on 3 sides and a medial semilunar fold, and the palpebral conjunctiva.

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Question 9

Smooth-muscle fibers from the levator muscle maintain the superior fornix, and fibrous slips extend from the horizontal rectus tendons into the temporal conjunctiva and plica to form cul-de-sacs during horizontal gaze.

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Question 10

The caruncle is a fleshy tissue mass containing hairs and sebaceous glands.

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Question 11

The tar- sal conjunctiva is tightly adherent to the underlying tarsus, and the bulbar conjunctiva is loosely adherent to the Tenon capsule.

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Question 12

These tissues blend at the limbus, where a series of radiating ridges called the palisades of Vogt appear. This area contains corneal stem cells.

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Question 13

The cell morphology of the conjunctival epithelium varies from stratified cuboidal over the tarsus to columnar in the fornices to squamous on the globe. Multiple surface folds are present.

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Question 14

Goblet cells account for up to l0% of basal cells of the conjunctival epithelium; they are most numerous in the tarsal conjunctiva and the inferonasal bulbar conjunctiva.

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Question 15

The substantia propria of the conjunctiva consists of loose connective tissue. Conjunctiva-associated lymphoid tissue (CALT), which consists of lymphocltes and other leukocytes, is present, especially in the fornices. Lymphocltes interact with mucosal epithelial cells through reciprocal regulatory signals mediated by growth factors, clto- kines, and neuropeptides.

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Question 16

The palpebral conjunctiva shares its blood supply with the eyelids. The bulbar con- junctiva is supplied by the anterior ciliary arteries branching off the ophthalmic artery. These capillaries are fenestrated and leak fluorescein just as the choriocapillaris does.

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Question 17

Sensory innervation is controlled by the lacrimal, supraorbital, supratrochlear, and infra- orbital branches of the ophthalmic division of cranial nerve V.

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Question 18

The lacrimal functional unit (LFU; Fig 1-2) is a highly complex apparatus, comprising the lacrimal glands, ocular surface (cornea, conjunctiva, and meibomian glands), and eyelids, as well as the sensory and motor nerves that connect them. It is responsible for the regula- tion, production, and health of the precorneal tear film. The afferent component of the LFU is mediated through nociceptors found in the cornea and ocular surface and passing through the trigeminal nerve (cranial nerve V). These nociceptors synapse in the brain- stem with autonomic and motor (efferent) nerves. The autonomic nerve fibers innervate the meibomian glands, conjunctival goblet cells, and lacrimal glands. The motor nerve fibers innervate the orbicularis muscle to initiate blinking, with the blink rate in adults being about 15 times per minute. During blinking, the meibomian glands express lipid, and the tears are replenished from the inferior tear meniscus and spread across the cornea while excess tears are directed into the lacrimal puncta. See also Chapter 6.

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Question 19

The tear film contains proteins that control the normal ocular flora; it serves as a protec- tive layer to wash away irritants and pathogens; and it dilutes toxins and allergens. The tear film is the most important refractive interface of the eye and is primarily responsible for maintaining a smooth optical surface between blinks. Our understanding of the struc- ture of the tear film has gradually evolved. Formerly described as a trilayered structure composed of lipid, aqueous, and mucin layers, the tear film is now thought of as a uniform gel consisting of soluble mucus, which is secreted by conjunctival goblet cells, mixed with fluids and proteins secreted by the lacrimal glands (Fig I -3).

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Question 20

The cornea is a transparent, avascular tissue that measures l1-12 mm horizontally and 10-l I mm vertically. Its refractive index is 1.376

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Question 21

The cornea is aspheric, although its ra- dius of curvature is often recorded as a spherocylindrical convex mirror representing the central anterior corneal surface, also called Ihe corneal cap.

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Question 22

The average radius of curvature of the central cornea is 7.8 mm.

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Question 23

The cornea contrib- utes 74o/o, or 43.25 diopters (D), of the total 58.60 dioptric power of a normal human eye. The cornea is also the major source of astigmatism in the optical system

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Question 24

For its nutrition, the cornea depends on glucose diffusing from the aqueous humor and oxygen diffusing through the tear film. In addition, the peripheral cornea is supplied with oxygen from the limbal circulation.

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Question 25

The cornea has one of the body’s highest densities of nerve endings, and the sensitiv- ity of the cornea is 100 times that of the conjunctiva. Sensory nerve fibers extend from the long ciliary neryes and form a subepithelial plexus.

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Question 26

Neurotransmitters in the cornea include acetylcholine, catecholamines, substance B calcitonin gene-related peptide, neu- ropeptide Y, intestinal peptide, galanin, and methionine-enkephalin.

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Question 27

The corneal epithelium is composed of stratified squamous epithelial cells and makes up approximately 5%-l0% of the total corneal thickness (Fig 1-a; see also Chapter 2,Fig2-l).

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Question 28

The epithelium and tear film form an optically smooth surface. Tight junctions between superficial epithelial cells prevent penetration of tear fluid into the stroma.

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Question 29

Continuous pro- liferation of perilimbal basal epithelial cells flinbal stem cells; see Chapter 3) gives rise to the other layers that subsequently differentiate into superficial cells. With maturation, these cells become coated with microvilli on their outermost surface and then desquamate into the tears. This process ofdiflerentiation takes about 7-14 days.

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Question 30

Basal epithelial cells secrete a continuous, 50-nm-thick basement membrane, composed of type IV collagen, laminin, and other proteins.

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Question 31

The clarity of the cornea depends on the tight packing of epithelial cells to produce a layer with a nearly uniform refractive index and minimal light scattering.

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Question 32

The regular arrangement of stromal cells and macromolecules is also necessary for a clear cornea.

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Question 33

Keratocytes vary in density and size throughout the stroma and form a 3-dimensional network throughout the cornea. They are flattened fibroblasts, located be- tween the collagen lamellae (Fig 1-5). These corneal fibroblasts continually digest and manufacture stromal molecules. The density of keratocltes declines with age but to a lesser degree than does that of endothelial cells. The density also declines with corneal surgery and may not recover completely.

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Question 34

Beneath the acellular Bowman layer (see Fig 1- ), the corneal stroma is composed of an extracellular matrix formed of collagens and proteoglycans. Type I and type V fibrillar collagens are intertwined with filaments of type VI collagen. The concentrations and ratio of proteoglycans vary from ante- rior to posterior

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Question 35

Similarly, the posterior corneal stroma is “wetter” than the anterior (3.S5 mg HrO/mg dry weight vs 3.04). Other water-soluble proteins, analogous to lens crystal- lins, may be secreted by keratocltes or contained in the epithelial cells to control the optical properties of the cornea. The lamellae of the anterior stroma are short, narrow sheets with extensive interweaving between layers, whereas the posterior stroma has long, wide, thick lamellae extending from limbus to limbus with minimal interlamellar connections.

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Question 36

The human cornea has little elasticity and stretches only 0.25o/o at normal intraocular pressure (IOP).

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Question 37

The lattice arrangement of collagen fibrils embedded in the extracellular matrix is partly responsible for corneal transparency. This pattern acts as a diffraction grating to re- duce light scattering by means of destructive interference. Scattering is greater anteriorly resulting in a higher refractive index that decreases from 1.401 at the epithelium to 1.380 in the stromaand 1.373 posteriorly. The cornea is transparent because the size of the lat- tice elements is smaller than the wavelength of visible light.

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Question 38

e elements is smaller than the wavelength of visible light. Transparency also depends on keeping the water content of the corneal stroma at 78o/o. Corneal hydration is largely controlled by intact epithelial and endothelial barriers and the functioning of the endothelial pump, which is linked to an ion-transport system controlled by temperature-dependent enzymes such as Na,K-ATPase. In addition, nega- tively charged stromal glycosaminoglycans tend to repel each other, produ cing a swelling pressure (SP). Because the IOP tends to compress the cornea, the overall imbibition pres- sure of the corneal stroma is given as IOP - SP. The total transendothelial osmotic force is calculated by adding the imbibition pressure and the various electrollte gradients pro- duced by the endothelial transport channels. Corneal hydration varies from anterior to posterior, with increasing wetness closer to the endothelium. See also BCSC Section 2, Fundamentals and Principle s of Ophthalmology.

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Question 39

The endothelium is made up of closely interdigitated cells arranged in a mosaic pattern of mostly hexagonal shapes. Human endothelial cells do not proliferate in vivo, but they can divide in cell culture.

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Question 40

Although some evidence cites the possibility of peripheral corneal endothelial stem cells, cell density declines throughout life. Cell loss results in enlarge- ment and spread ofneighboring cells to cover any defective area, especially as a result of trauma or surgery.

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Question 41

Cell density varies over the corneal endothelial surface; normally, the concen- tration is highest in the periphery. Normal endothelial cell density is between 2000 and 3000 cells/mm2.

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Question 42

The corneal endothelium maintains corneal clarity through 2 functions: by acting as a barrier to the aqueous humor and by providing a metabolic pump. Increased permeability and insufficient pump sites occur with decreased endothelial cell density, although the cell density at which clinically evident edema occurs is not an absolute. En- dothelial alterations can be reversible (eg, pseudoguttata) or permanent (eg, cornea guttae [cornea guttata]). See Chapter 2 for additional discussion on specular photomicroscopy and endothelial function.

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Question 43

The Descemet membrane is the basement membrane of the corneal endothelium. It increases in thickness from 3 prm at birth to 10-12 pm in adults, as the endothelium grad- ually lays down a posterior amorphous nonbanded zone.

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Question 44

The cornea provides a stiff, strong outer coating for the eye. The Bowman layer and the corneal stroma are key factors contributing to the cornea’s biomechanical properties. The Bowman layer, a woven, matlike layer composed of randomly dispersed tlpe I collagen fibrils, is stiffer and stronger than the underlying stroma proper, while the highly woven anterior stroma proper is stiffer and stronger than the nonwoven posterior stroma proper. This difference is evident clinically, as blunt dissection of a plane through the anterior stroma is much more difficult to achieve than that through the posterior stroma.

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Question 45

Biomechanical failure can occur in the cornea, as illustrated by corneal ectasia, or ker- atectasia. In keratectasia, the shape of the cornea is distorted, causing irregular astigma- tism and vision loss. Keratectasia can occur as a result of tissue fatigue, which is caused by chronic low-grade stress and leads to biomechanical failure. Potential sources of corneal stress include chronic eye rubbing, refractive surgery, elevated IOR and possibly external nocturnal pressure on the eye.

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Question 46

In the future, a better understanding of corneal biomechanics may enable clinicians to improve visual outcomes after refractive surgery, better identify corneas at risk of iatro- genic ectasia from refractive surgery, and more accurately measure the effects of collagen crosslinking on biomechanical enhancement. Methods to measure the biomechanical properties of the cornea are discussed in Chapter 2.

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Question 47

The sclera is composed primarily of type I collagen and proteoglycans (decorin, biglycan, and aggrecan). Other components include elastin and glycoproteins such as fibronectin. Fibroblasts lie along collagen bundles. The long posterior ciliary nerves supply the ante- rior sclera.

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Question 48

Normally a densely white tissue, sclera becomes more translucent when thinning oc- curs or the water content changes, falling below 40%o or rising above 80%0. For example, senile scleral plaques are areas of calcium phosphate deposits just anterior to the inser- tions of the medial and lateral rectus muscles that become dehydrated and reveal the blue color ofthe underlying uvea.

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