Download Ocular Anatomy, PO 112 - UAB School of Optometry

Study Guide for Lens and Vitreous
LENS
Lens Facts:
-2 clinical Issues of the lens are cataracts and presbyopia (loss of accommodation)
-lens only has a variable refractive power (compared to the cornea) ~20D
-the refractive power of the lens varies form one part of the lens to another
-the lens Is not thin like the cornea, and this thickness contributes to the overall lens power
-the lens does not have a single, uniform refractive Index
-lens is 66% water and 33 % protein (the highest protein concentration in the body!)
-the lens grows throughout life
Structures:
Development-Embryonic nucleus – represents the lens after the first 6 weeks and is made of nothing
but primary lens fibers
-Fetal nucleus – represents the lens at birth and made up of secondary lens fibers that are
added in concentric circles throughout life
-the cuboidial epithelium becomes columnar and the cells undergo posterior migration,
rotation and elongation to form secondary lens fibers
-Lens suture in fetal nucleus –sutures of the anterior surface create an erect “Y” suture
and the posterior surface contains an inverted “Y” suture
-Tunica vasculosa lentis – branch of hyloid artery that surrounds the lens during
development and nourishes it until aqueous humor is available; deteriorates prior to birth
Anterior Pole- center of anterior surface; covered with epithelium
Posterior Pole-center of posterior surface; NOT covered with epithelium
Equator-circumferential area midway between poles
Zonules (of Zinn) - also known as suspensory ligaments; see fig. 12.15 pg 510
-collection of small fibers that arise from non pigmented epithelial of the pars plana and
the valleys between the ciliary process
-function is to suspend the lens in the visual axis
-the tension exerted by the zonules change the shape of the capsule of the lens
-the locations at which the zonule inserts onto the lens change with age, but remains at
the thickest portion of the lens capsule, and is primarily at/near the lens equator
-the primary component is microtubules
Capsule- acellular, elastic capsule surrounding the lens that is secreted by the lens epithelium
-produced throughout life from the inside, which is in contact with the anterior epithelium
-contains inner layer and outer dense layer (zonular lamellae)
-the outer layer merges with the zonules from the ciliary body
-made of primarily type IV collagen in GAG matrix that makes it flexible
-thickest at the anterior and posterior regions near the equatorial zone (attachment site for
zonules), with age this shifts anterior (because of new additions from the
epithelium).
-THINNEST at the posterior pole (because it is secreted by the epithelium.
-molds the lens shape in response to the tension on the zonules
Eg. ciliary muscle contraction (accommodation) = releases tension on the zonule fibers
and the capsule and the lens rounds up and the anterior portion bulges forward
-another function is to act as a diffusion barrier
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UAB School of Optometry
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Lens Epithelium-layer of cuboidal epithelium; but closer to the equator epithelium is more columnar
-located beneath the lens capsule, absent on posterior of lens
-central zone – non-proliferative and comprises almost half the epithelial cells
-Germanitive zone – proliferation; cells continually migrating toward the equator
-Transitional zone – start producing cell membranes so they can elongate; differentiate
into lens fibers cells
*see fig. 12.8 pg 502
-anterior epithelium is responsible for most of the ion and metabolite transport for the
lens
-epithelia cells have distinct basal and apical membranes; the lateral membranes are
infolded and communication occurs through desmosomes and gap junctions
-the gap junctions help move substances into and among cells since the lens relies on the
aqueous for nutrition
-elongating epithelial cells at the equator become long lens fibers that form new shells in
the lens
Lens Fiber Cells- epithelial cells at the equator elongate and also rotate so that the long axis of
the cell is parallel to the lens surface—one of the growing tips of the cell extends forward next to
the overlying epithelium while the other tip extends back next to the capsule
-these new lens fiber cells continue to extend in both directions until they meet other
growing fiber tips and interdigitations form = SUTURES = SHELLS
-aprox. 5 new shells are added each year and each shell adds ~ 4uM to the lens thickness
-an ages lens has aprox. 2500 shells and 3.6 million lens fibers!
-young lens fibers are more uniform in shape and look more like hexagonal prisms with
two broad sides and 4 narrow sides…UNIFORMITY and precise ALIGNMENT reduce
scatter and contribute to lens transparency!
-as old fibers move deeper due to new fibers being added, the nuclei and organelles
disintegrate, so only the youngest-superficial lens fibers are nucleated and have
organelles
-the LENS BOW is a characteristic pattern of cell nuclei form the superficial lens fiber
cells that are displaced inward as new lens fibers are added on top
-fibers have BALL and SOCKET JOINTS where fibers are packed closely together and
there are finger-like protrusions --most common in the equatorial cortical regions—where
there are shape changes due to accommodation
Lens Sutures – see above about fetal sutures (REMEMBER the embryonic nucleus is only
composed of primary fiber and does NOT have sutures!)
-after birth but prior to sexual maturation, non-identical but SYMMETRIC sutures are
produced
-after sexual maturation the lens exhibits a “branch star pattern” with sutures every 40
degrees
-due to continued growth and aging, the lens suture patterns become more and more
complex
-because of the irregular structure of the sutures, they scatter more incident light than
does the rest of the lens
Lens Proteins- 66 percent of the lens is proteins-a higher percent than in any other tissue in the
body!
-membrane and cytoskeletal proteins make of ~10% and are the INSOLUBLE proteins
-Crystallins are the soluble proteins that make up the remainder of the lens proteins
(~90%)
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UAB School of Optometry
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-the crystallins determine the refractive index of the cells and the lens
-crystallins are synthesized by the anterior cuboidal epithelium and elongating lens fiber
cells
-cells contain a different concentration of crystallins depending on when the cells were
produced, creating a gradient in the refractive index (adult nucleus 70% and cortex 10%)
-3 main crystallins groups include , , 
-the structures of  and  and not well known, however the structure for  is
-dense, uniform packing of the crystallins within the lens cells is responsible for the lens
transparency
-Regular spacing permits re-radiation “IN PHASE”-this means constructive interference
and TRANSMISSION
-Irregular spacing causes re-radiation “OUT OF PHASE”-this means destructive
interference and REDUCED TRANSMISSION
-crystallins are highly stable molecules, but they can be changed by light absorption and
altered chemical environments
Presbyopia- the loss of ability to accommodate
-this is a normal, age-related change and it happens to most everyone
-usually by the age of 45 most people realize that their near point of accommodation is
farther away than the normal reading or working distance, but only the realization is
sudden!
-loss of elasticity restricts muscle movement and decreases accommodation
Cataracts:
o Defined technically as any opacity in the lens
o Major types (3 common age-related variations) (Classified by location)
1. Cortical
 Initial opacity is confined to the outer part of the lens
 Can be peripheral, which makes it an equatorial cataract, meaning
it is more central to either the anterior or posterior pole
 Most common up to age 65
2. Nuclear sclerosis
 Initial opacity occurs in center of the lens, initially at embryonic
level and include fetal and adult deep cortex
 Spoke-like opacities at periphery enlarge to the center
 Cause hardening
 Dominant type of cataract after 65
3. Posterior subcapsular
 Initial opacity begins as a spot opacity near center of visual axis in
posterior lens
 Enlarges and becomes more diffuse
 Least common of the three age-related
Surgical options for cataracts:
o Intracapsular cataract extraction
 entire lens removed with capsule intact to avoid lens proteins from
entering body's system, triggering inflammation and possible blindness
 Historical method for the most part
o Extracapsular cataract extraction
 Posterior lens capsule is left to support IOL
 All other parts of lens removed completely
 Common method of modern cataract surgery
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UAB School of Optometry
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o Phacoemulsification
 Most common
 Basic steps:
 Ultrasonic probe used to break up the dense nucleus
 Fragments are vacuumed out
 IOL is placed in the capsule
Risk factors with cataracts
– Age : 95% of people over age 65 have some degree of cataract
– become a concern only when the severity is such that it affects the
patient’s lifestyle
– UV exposure (free radical production, direct protein damage)
– Medications
– Trauma
– Oxidation (changes in protein oxidation, solubility status)
• Brunescence (brownish coloration)
• Aphakic (absence of lens)
• Pseudophakic cystoid macular edema
(http://www.szp.swets.nl/szp/journals/oi062121.htm has more info
on this)
• Presbyopia
VITREOUS
Vitreous facts
 Largest component of eye, comprising about 80%
 Primary structural components = collagen (Type II) and hyaluronic acid
 Vitreous cortex (outer layer) attaches vitreous to surrounding structures
 Changes with age
Vitreous base
• Strongest attachment
• 1.5-2mm anterior and 1-3mm posterior to the ora serrata, and several mm into the vitreous.
• Fibers embedded firmly in the basement membrane of the non-pigmented epithelium of the
ciliary body and the ILM of the peripheral retina
• Retina can be torn while removing vitreous from the eye
Patella fossa
 Center of anterior surface contains the patellar fossa- the indentation where the lens sits
Hyaloideocapsular ligament (of Weiger), a.k.a. retrolental ligament (of Berger)
 Forms annular attachment 1-2mm wide and 8-9mm in diameter between posterior surface
of the lens and anterior face of the vitreous
 Firm attachment site in young people, weakens with age.
 Within ring is the retrolental space (of Berger), ‘potential space’, present because the lens
and vitreous are in contact but not connected
Retrolental space of Berger
 A small separation between the posterior lens capsule and the surface of the patellar fossa
often seen in histological sections
 Not clear if it exists in vivo or if it is an artifact of tissue shrinkage during histological
preparation
Peripapillary adhesion
 Attachment at optic disc, where the optic nerve leaves the eye
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UAB School of Optometry
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 Annular ring
 3-4 mm diameter
 Weakens with age
Macular adhesion
Premacular hole
 An area where there are no vitreal attachments present within; but they are concentrated
around.
 Occurs above the optic nerve.
Vitreal zones
 cortex a.k.a. hyaloid surface
o outermost, hyaloid surface
o 1-3mm wide
o Tightly packed collagen fibrils, some parallel, other perpendicular to retinal
surfaces
o Anterior cortex anterior to base and adjacent to the ciliary body, posterior
chamber and lens
o Posterior cortex extends posteriorly to the base and is in contact with the retina.
o Contains transvitreal channels, prepapillary hole, premacular hole, and
prevascular fissures
 Intermediate zone
o Contains firm, unbranched, continuous fibers that run anterior to posterior
o Arise in the region of the vitreal base and insert onto posterior cortex
o Peripheral fibers parallel cortex
o Central fibers parallel Cloquet’s canal
o Membrane like condensation called vitreous tracts can be seen in areas that have
differing fiber densities
Cloquet's canal
• AKA hyaloid channel in the retrolental tract
• Runs in the center of the vitreous body
• S-shape with 90 degree rotation at the center
• Site of the hyaloid artery system from embryonic development
• 1-2 mm wide and fluid filled
• Arises in retrolental space, base of patellar fossa
• Terminates in the area of Montegiani
Hyaloid artery
 Runs through the developing citreous chamber to supply a dense network of blood
vessels around the lens, the tunica vasculosa lentis
Area of Martegiani
 Funnel shaped space in front of optic disc
 Devoid of collagen fibrils.
 Bergmeister’s papilla – remnant of hyaloid artery in front of optic disc
Vitreal composition
• Highly transparent
• Dilute solution of salts, soluble proteins, hyaluronic acid
• Insoluble collagen meshwork contained in proteoglycan matrix (Fibrils, 8-16 nM in
diameter form a mesh throughout the vitreal body)
Hyaluronic acid
• Proteoglycans, core proteins (chondroitan and heparin sulfate) with GAGs
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UAB School of Optometry
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long unbranched chain of molecules that coil into network
Found at specific sites within collagen mesh
Maintain wide spacing between fibrils
Concentration and ratio highest in posterior cortex, decreases anteriorly and centrally
Interaction with collagen contributes to the viscoelastic properties and influences the gelliquid balance of the vitreous
Hyalocytes
 Vitreous cells in single layer in cortex near and parallel to vitreal surface
 Widely spaced
 In vitreal base:
o fibroblast-like anterior to the ora serrata
o macrophage-like posterior to it
 Possible functions:
o May synthesize HA, or glycoproteins for collagen fibrils
o May act as phagocytes
o Thought to synthesize collagen fibers that run anterior to posterior
 Note: Fibroblasts in vitreal base and near ciliary body may have been mistaken for
hyalocytes, >10% of population
Vitreal function
• Storage area for metabolites and catabolytes from retina and lens
• Provides medium for movement of these through the eye
• Acts as shock absorber to cushion retina from shock associated with rapid eye movement
or strenuous physical activity
**Sharp or repeated blows to the head can still result in hemorrhage or retinal
detachment
Floaters
• Disruption of HA collagen complex can cause collagen fibrils to aggregate into bundles
that can become large enough to see clinically
• More info at http://www.allaboutvision.com/conditions/spotsfloats.htm
Vitreal liquefaction
• With age, gel volume decreases and liquid increases
• Vitreous is homogeneous and gel like in infancy
• By age 40, vitreous is 80% gel, 20% liquid
• By age 70-80 ratio is 50/50, with most liquefaction occurring in central
vitreous
• Liquefaction thought to be due to conformational changes in HA molecule and
subsequent displacement of collagen
• HA-complex deteriorates
• Collagen coalesces into fibers then bands
• HA pools in adjacent areas forming lacunae of liquid vitreous
• Collagen fibril bundles can contract applying tension to the vitreous and
the retina
Syneresis a.k.a. vitreal collapse
• Tension can cause post vitreal detachment,
• Vitreous pulls away from retinal ILM at the peripapillary ring, forming a
retrocortical space
• Liquid vitreous can seep into space and cause syneresis or general vitreal
collapse
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UAB School of Optometry
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UAB School of Optometry
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