1. Building blocks collagen (pp. 325-333)



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Study outline for Cornea/Sclera

1. Building blocks

collagen (pp. 325-333)

Most of the sclera and cornea are primarily connective tissue.


Collagen is made up of 3 single strands of alpha and/or beta chains to form a triple helix (Fig. 8.1, p. 326).
Collagen fibrils are 25-230 nm in diameter and are arranged into bundles of fibrils that are highly disorganized and variable in size in the scleral stroma, and very organized and uniform in size in the corneal stroma. Type 1 is the most common collagen found in the cornea and sclera. The random arrangement and the amount of interweaving in the scleral stroma probably contributes to strength and flexibility of the eye.
The intertwined helices in a collage molecule have non-helical portions on the ends of the strand. The individual molecules from natural linkages, creating long assemblies of parallel molecules that are the collagen fibrils. The structure of collagen fibrils is created through intermolecular cross-linking (Fig. 8.2, p. 327).
The collagen in the cornea and sclera is associated w/ polysaccharide molecules called glycosaminoglycans (GAGs). A proteoglycan is a core protein to which many GAG are attached, and they form a matrix around the collagen fibrils. The dominant GAGs in the cornea and sclera are dermatan sulfate and keratan sulfate. The collagen fibrils in the cornea and sclera are then surrounded by and embedded in proteoglycans. (See page 327 – fig. 8.3)
GAGs are rather large molecules. They also have a very negative charge and, therefore, attract positively charged molecules such as sodium. Sodium comes along with water, so tissues with large amounts of GAGs will take up considerable water if left tot heir own devices. The combination of H2O creates a gel around the collagen fibrils creating the ground substance. The corneal stroma has a higher affinity for water, whereas the cornea has very narrow limits because it must remain transparent. In cornea spacing of collagen is key to its transparency. Water content needs to be maintained at a steady level to keep spacing of collagen regular.
2.Sclera (pp. 328-331)
Function: The sclera maintains the shape of the eye and resists deforming forces both internal (IOP) and external. The sclera also provides attachment points for extraocular muscles and the optic nerve.
The opacity of the sclera is due to many factors including the number of GAG’s (glycosaminoglycans --complex sugars that attach covalently to collagen. (Fig. 8.3, p. 327)), the amount of water present, and the size and distribution of the collagen fibrils.
The sclera has only 25% of the total GAG’s that are present in the cornea. Because the GAG’s attract water, the sclera is less hydrated than the cornea (but not 75 % less; due to several structures that carefully maintain a lower hydration level in the cornea).
The large variation in fibril size and the irregular spacing between scleral components leads to light scattering and opacity.
The color of the sclera is white when healthy, but can discolor over time or due to illness (eg. hepatitis).
Internally, the sclera merges with the choroidal tissue in the suprachoroid layer. The innermost scleral layer is called the lamina fusca (see below).
The sclera contains a number of holes where structures pass through or interrupt the expansion of the sclera. At the posterior pole of the eye the optic nerve passes through the posterior scleral layer. This area is bridged by a network of scleral tissue called the lamina cribosa. (Fig. 16.17, p. 720) The lamina cribosa is the weakest part of the sclera. Elevated IOP could lead to a bulging out at the optic nerve and subsequent tissue damage.
The scleral blood supply is very limited, the tissue is largely avascular. It contains no capillary beds, only a few small branches from the episclera and choroid, and branches of the long posterior ciliary arteries.
Scleral thickness varies from 1.0 mm at the posterior pole to 0.3 mm behind rectus muscle insertions.
The sclera covers ~5/6 of the entire eye (about 85%).
The sclera consists of 3 layers:

(1) episclera, consists of loose vascularized connective tissue. Branches of the anterior ciliary arteries form a capillary network anterior to the rectus muscle insertions. Surrounds the peripheral cornea and is physically linked to Tenon’s capsule (see Orbit study guide) by connective tissue strands. The sclera thins towards the back of the eye.


(2) scleral stroma thick dense connective tissue layer that is continuous with the corneal stroma at the limbus.
(3) lamina fusca refers to the few pigmented cells that remain adherent to sclera after removal of choroids.
3. Tear layer

The tears consist of three layers (Fig. 8.24, p. 352) that together are 7 um thick. The outer or most anterior layer (1) is a lipid layer, the middle layer (2) is an aqueous layer that originates from the lacrimal gland. The mucous layer(3) is in contact with squamous cells (posterior layer).


4. Cornea


Function: The eye’s primary refractive element. Most important feature is transparency. The cornea comprises about 1/6 of the outer layer of the eye.

Radius of curvature of ~8 mm; overall the cornea is 0.52-0.53 mm thick at the center and 0.71 mm at the periphery (Fig. 8.25, p. 352). Posterior side (inner surface) of cornea has smaller radius of curvature than anterior.

The cornea is the major refractive component of the eye contributing over 40 diopters. It is avascular and transparent transmitting light very well. The anterior portion of the cornea is covered with tear film (see above). Optical zone is the circular region of the cornea that is 4mm around the corneal apex.
Central radius of curvature and refractive power:

Air/tear interface +43.6 D

Tear/cornea + 5.3 D

Cornea/aqueous - 5.8 D total central refractive power = 43.1 D


Cornea consists of five layers. From anterior to posterior they are:

Epithelial=> bowman’s => stroma => Decemet’s => endothelium.


Epithelial layer (1st and most complex)

  1. The epithelial cell layer is made up of ~ 6-8 rows of cells. The epithelial layer is about 50µm thick. [The entire cornea is about 500-700 microns (µm) thick (0.5 to 0.7 mm).]

  2. Surface layer (anterior) consists of squamous cells that are non-pigmented and have a flattened appearance. The surface of these cells consists of many microvilli that serve to increase the surface area and stabilize the tear film ‘layer”.

  3. The squamous cells are connected through tight junctions ie Zonulae Occludens. This creates an effective barrier to exclude foreign material that might cause damage.

  4. As the surface cells get older their attachments are lost and the cell is sloughed off in the tear film.

  5. New cells migrate outward from the more internal rows of epithelial cells (bowman’s) toward the tear film layer.

  6. The cornea epithelium is subdivided into 3 parts: The squamous cell layers at the surface of the cornea, wing cells that have an appearance of a wing, and columnar basal cells.

  7. All of the 3 cell types originally derive from the columnar basal cells. So cells are continually being renewed along the basal surface and will ultimately (in about 10 days) turnover an entire new cell layer. Basal cells communicate through gap junctions.

  8. The middle layer of wing cells is 2-3 layers thick. These cells are polyhedral and have convex anterior surfaces and concave posterior surfaces.

  9. The most posterior cell layer consists of a single row of columnar basal cells.

  10. Cells transform from columnar to cuboidal to squamous. [Programmed cell death is called apoptosis. This process occurs throughout the body including corneal epithelium cells.]

  11. The cells are connected to adjacent cells by desmosomes and the basement membrane by hemidesmosomes. For some nice images of these structures visit a website http://www.med.uiuc.edu/histo/medium/atlas/slidesr.htm , then look at images 37, 38, 38a

  12. The basement membrane (Bowman’s) is formed with secretions from the basal epithelial cells.

  13. Newly born epithelial cells are formed at the corneal periphery and then they migrate toward the center of the cornea.

  14. There are 325,000 nerve endings in epithelial layer of the cornea. These nerve endings arise from about 2000 nerves which arise from the medial and lateral long ciliary nerves.

Layer 2 of cornea



Bowman’s layer (formerly Bowman’s membrane) (2nd layer)


  1. This layer of cornea is about 10 um thick

  2. It is a dense, acellular fibrous sheet of interwoven collagen fibers that are randomly arranged. Fibrils are 20-25 um in diameter.

  3. Bowman’s layer is a transition layer between the basal epithelium and the stroma.

  4. This layer is produced by the epithelium; It does regenerate, but very slowly.

  5. Corneal nerves pass through the layer losing their Schwann cell covering and passing into overlying epithelium as unmylenated fibers.

  6. The Bowman’s layer ends at the corneal periphery.


Corneal Stroma (3rd layer)


  1. The next layer is the stroma (also known as substantia propria). It is 500 to 700 microns thick representing about 90% of the total cornea thickness.

  2. It is comprised of collagen fibrils and fibroblasts. The fibroblasts in the corneal stroma are often called keratocytes [old name, corneal corpuscles] and are specialized fibroblasts that produce collagen fibrils during development and maintain the connective tissue in the mature eye (pp. 327-331).

  3. Collagen fibrils of the cornea are 25-35nm in diameter and are grouped into flat bundles called lamellae.

  4. There are 200-300 lamellae distributed throughout the corneal stroma. All of the lamellae run parallel to the surface of the cornea. These stacked fibers account for 90% of the thickness and volume of the cornea.

  5. Adjacent lamellae lie at angles to one another; each lamellae extends across the entire cornea; each fibril runs from limbus to limbus. In the anterior 1/3 of stroma lamellae are 5-30 um wide and 0.2-1.2 um thick. Posterior 2/3 of the stroma is more regular and larger (100-200 um).

  6. In the innermost layer adjacent to the next corneal layer Descemet’s membrane the collagen fibrils interlace to form a dense but thin collagenous sheet which contributes to the maintenance of the attachment between the stroma & Descemet’s membrane.

  7. Keratocytes in the stroma produce fibrils that make up the lamellae. In between the fibrils is the ground substance that contains proteoglycans (protein with the carbohydrate glycosaminoglycan (GAG). The GAGs are hydrophilic negatively charged that are located around specific sites around each collagen fibril.

  8. The hydrophilic nature of the GAGs serves to keep the stroma well hydrated which helps to maintain the spatial arrangement of the fibrils.

  9. Corneal hydration and the regular arrangement of the fibrils contributes to corneal transparency.

  10. So proper hydration is critical to maintain transparency. Proper hydration is maintained by the actions of the epithelium and endothelium to maintain a balance (primarily by pumping water out of the cornea).


Descemet’s membrane (4th layer)
Function: structure and tough resistant barrier to perforation of the cornea.

Secreted by endothelium. Has 5 types of collagen with Type VIII dominant.




  1. considered to be the basement membrane of the endothelium. The layer is constitutively adding new materal so it becomes thicker with age; it is approximately 10 microns thick.

  2. the anterior portion exhibits a banded appearance like a latticework collagen fibrils.

  3. posterior Descemet’s membrane is non banded and is secreted by the endothelial cells throughout life.


Endothelium (5th layer)

Endothelium is about 5microns (a single cell layer) thick

No nerves in endothelium

Cell center is 3000 cells/mm2



Periphery is 1500 cells/mm2


  1. When irregularity increases the variability in cell area (polymegatheism) and number of cell sides (pleomorphism) also increases.

  2. Is the inner (posterior) most layer of the cornea; a single layer of flattened cells that is adjacent to the anterior chamber in direct contact with aqueous.

  3. the basal portion abuts Descemet’s membrane (10 um); the apical surface with extended microvilli line the anterior chamber. The cells are polyhedral; 70-80% are hexagonal; It’s appearance called the endothelial mosaic.

  4. There are no hemidesmisomes that secure Descemet’s membrane to the endothelium.

  5. the lateral surface of the cells show extensive interdigitation; contain many gap junctions for intercellular communication.

  6. Endothelial cells do not replicate; so if there is injury the existing cells spread out causing a reduction in thickness of the layer. (Fig. 8.19, p. 341).

  7. Endothelial cells are altered by contact lens wear and naturally die off with increased age (Figs. 8.17, 8.18, p. 341). The corneal epithelium is also altered in a corneal transplant surgery (Fig. 8.39, p. 370). The existing cells spread out to compensate leading to loss of their polyhedral appearance.

8. Resist movement of water molecules but less effective passive barrier than epithelium
Adjacent cells can attach together through desmisomes (cadherin mediated). Cells can also attach to cells at the basement membrane through hemidesmisomes (integrin mediated) (Fig. 8.9, p. 333; Fig. 8.13, p. 337). Hemidesmosomes connect on the inside of the cell at anchoring plaques that directly link an array of collagen type VII fibrils (Fig. 8.9, p. 333).
Tight junctions (or zona occludens) attach cells and limit the flow of foreign materials. Gap junctions (made up of connexins) allow cells to communicate (Fig. 8.14, p. 338).

Corneal/Scleral Disease
Guttata are formed when regions of Descemet’s membrane bleb out through the endothelium. (Fig. 8.21, p. 346). Localized thickening posteriorly-numerous. Hassel-Henle Bodies are thickening at the periphery.
Kerataconus is a genetic disease that has the prominent feature of a conical shaped cornea. This can be repaired through surgical intervention (Fig. 8.36, p. 365).
Corneal dystrophies [Table 13.1 from Genetic Diseases of the Eye (Traboulsi, E. ed.)].



Epithelium

Meesman's dystrophy







Epithelial basement membrane

Map/dot/fingerprint dystrophy







Bowman's membrane

Reis‑Bucklers dystrophy




Anterior membrane dystrophy of Grayson-Wilbrandt




Honeycomb dystrophy of Thiel and Behnke




Subepithelial mucinous corneal dystrophy







Stroma

Granular dystrophy




Granular-lattice dystrophy




Lattice dystrophy




Macular dystrophy




Gelatinous droplike dystrophy




Fleck dystrophy ,




Central crystalline dystrophy of Schnyder




Marginal crystalline dystrophy of Bietti




Central cloudy dystrophy of Francois




Parenchymatous dystrophy of Pillat




Posterior amorphous dystrophy







Pre‑Descemet's dystrophy

Typical pre‑Descemet's dystrophy




Associated with ichthyosis




Polymorphic stromal dystrophy




Cornea farinata

Endothelial dystrophies

Fuchs' endothelial dystrophy




Congenital hereditary endothelial dystrophy




Posterior polymorphous dystrophy


Corneal Reshaping (review pages and lecture noted below)
The primary focus is to compare and contrast basic procedural differences and outcomes
Keratoplasty corneal removal (Fig. 8.34, p. 363)
There are two basic procedures used in corneal transplantation.

  1. Penetrating keratoplasty- graft involves full thickness of the cornea, this includes both the epithelium and the endothelium.

  2. Lamellar grafts- replace the anterior 2/3 of the cornea, leaving the recipients descemet’s membrane and endothelium in tact.

Trephine- circular cutting device used to cut the circular plug for corneal transplant.

The transplant should remove all of the damaged recipient cornea, but not come within 1.5 mm of the limbus.

The thickness of the transplant depends on if the trauma to the injured cornea is relatively superficial or not.

Keratoplasty is not considered refractive surgery (except in treating keratoconus) since its purpose is not to change the curvature of the cornea, but rather to increase corneal clarity.

Perscription of spectacles to a patient who has undergone a corneal transplant can be difficult due to corneal irregularities, hard contact lenses are often used.
Radial keratotamy (RK, Fig. 8.31, p. 359).
RK- is the first generation of refractive surgery. It is rarely used as the preferred method for refractive surgery today.

-It depends on making 4-16 radial incisions (beginning 1.5-2mm from the corneal apex and coming within .5mm of the limbus) in a symmetric pattern..

-Radialy asymmetric incisions along with tangential incisions can be made to improve astigmatism.

-Healing involves the formation of epithelial cell plugs that are pushed out of the incisions by the formation of new collagen; if these processes of repair do not alter the gaping of the incisions, the initial flattening of the cornea will be permanent.

-A significant portion of patients that have undergone this surgery experience a hyperopic shift years after the surgery. This is probably a result of continual widening of the surgical incisions. Exactly what is happening is not known, in addition it is not known whether these changes will continue or will stabilize.
Photorefractive Keratectomy (PRK, Fig 8.32, p. 360).
Uses laser to reshape the surface by controlled ablation of corneal tissue. The epithelium and stroma are both removed.

To correct refraction in:



  1. myopic eye- curvature is reduced flattened by removal of more tissue centrally than peripherally.

  2. Hyperopia- removing tissue peripherally and almost none centrally.

  3. Astigmatism- removal of more tissue in some meridians than in others.

Advantages of PRK-

  1. Tissue repair after PRK surgery is largely epithelial replacement to cover the ablated region rather than formation of new collagen to bridge the incisions.

  2. It is less dependent on the niceties of surgical technique.

  3. Is less likely to seriously weaken the structure of the cornea.

  4. In a study of patients with less than -4D 90% do not need a post surgical correction (though a substantial number accept a visual acuity of less than 20/20).

Disadvantages of PRK-

  1. Postsurgical haze

    1. transient haze expected with removal of epithelium and swelling of stroma.

    2. Haze permanent in some patients, could be a result of migrating fibroblasts.

  2. Myopic Regresion

    1. Theory- increased curvature of the thinned cornea in response to intraocular pressure.

Laser in situ keratomileusis (LASIK, Fig. 8.33, p. 361).


LASIK – new method, combines surgical removal and subsequent replacement of anterior corneal cap on a stromal surface whose shape has been changed by laser ablation of the tissue (flap and zap).

    1. Keratome- used to cut corneal flap, revealing stroma

Advantages

  1. keeping epithelium and Bowman’s layer intact in the optical zone.

  2. works better on high levels of refractive error.

  3. immediate results.

Disadvantages

  1. Opportunity for excess formation of new collagen along the interface between the reshaped stromal surface and the optical zone flap.

  2. Too early to know much about the long-term prospects for the procedure.



Laser Assisted Sub-Epithelial Keratectomy (LASEK) (2nd half of lecture 3)

LASEK- much like LASIK but optimal for patients with a cornea that is too thin for LASIK.



  1. Trephine- epithelial flap is cut with this devices instead of the microkeratome used in LASIK.

Disadvantages



  1. recovery time often a longer than in LASIK. It may take up to 4-7 days to achieve good vision.

Possible complications of all refractive surgery includes:

  1. night vision difficulties

  2. halo effects

  3. double or triple vision

  4. Dry Eye




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VS 112 Ocular Anatomy UAB School of Optometry




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