Lecture 7: Lens Flashcards
is the crystalline lens biconcave and biconvex?
biconvex
the biconvex crystalline lens is located anterior/posterior to what structures?
The biconvex lens is located posterior to the iris and pupil and anterior to the vitreous body
is the convexity of the crystalline lens is steeper at the posterior or anterior curvature?
the convexity of the posterior curvature is far steeper (5-8mm ROC). The anterior curvature is 8-14 mm ROC
where are the poles located?
the poles are at the center of the anterior and posterior curvature
what does the axis of the lens connect?
the poles
the lens diameter at birth is how many mm?
6.5 mm
what is the lens diameter of an adult lens in mm and how thick is it?
the adult lens diameter is 10 mm and 4 mm thick
T/F: the diameter of the lens increases significantly with age
FALSE: the lens does not increase significantly with age; HOWEVER, the thickness increases by 0.02 mm/year
what is the equator of the lens?
the marginal circumference of the lens
how many diopters do the lens account for?
the lens accounts for +15-20 Diopters of the total +60 D of the eye
The range of dioptric power is reduced to how many diopters by the age of 40
+8D (hence the onset of presbyopia)
by the age of 60, the range of dioptric power is reduced to how many diopters?
+1 to +2 D
The non-uniform index of refraction is due to what?
non-uniform distribution of crystalline proteins in the lens fibers
how far is the lens equator away from the ciliary processes of the pars plicata
0.5 mm
what are the three parts of the crystalline lens?
- elastic capsule
- lens epithelium
- lens fiber
what is the thickest part of the crystalline lens?
elastic capsule (thickest in the body 20 microns)
the lens capsule is how many microns thick anteriorly?
20 microns
the lens capsule is in direct contact with what?
anterior lens epithelium
anteriorly, the capsule is secreted by?
the anterior lens epithelium
T/F: posteriorly, there is no epithelium in the lens capsule
T
what secretes the posterior lens capsule (3 microns thick)
lens fibers
what are the two main functions of the lens capsule?
- Diffusion barrier freely permeable low-molecular weight compounds (metabolites from aqueous H)
- Makes the lens pliable to pull of zonular fibers during accommodation
the lens capsule is composed of?
Collagen Type IV embedded in matrix of glycoproteins and sulfated proteoglycans
Collagen Type IV embedded in matrix of glycoproteins and sulfated proteoglycans allow the lens to what?
stretch up to 60% of its circumference without tearing.
T/F: NO ELASTIC fibers in the capsule
T
T/F: anterior capsule decreases in age
F: Anterior capsule increases with age (1/3 larger by age 65)
what is the outer most layer of the lens capsule to which the zonules will attach?
Zonular lamella
lens epithelium is how many layers of what type of cells?
Monolayer of cuboidal cells
Epithelial cells proliferate throughout life at the equator at what zone?
germinative zone
lens epithelium cells stretch and become what shape?
columnar shape
lens epithelium are arranged in rows to form what?
secondary lens fibers
T/F: Lens epithelium are discarded
F: None are discarded
what are the 3 functions of the lens epithelium?
- Proliferates at germinative zone at the equator to form lens fibers
- Central anterior epithelium is involved in the transport
of substances from aqueous humor to lens interior - Secretes the lens capsule
Lens fibers that lose nuclei then detach from where?
from capsule and are compacted on older fibers towards the center
Each fiber is about how long and what shape?
10 mm and U shaped
Ends of the fibers meet ends of fibers from the other side of lens at what?
sutures
why does the lens have sutures?
As the lens increases in size, the lens fibers are unable to stretch the anteroposterior distance, so that progressively more complicated suture patterns are formed.
at the epithelium-fiber interface, nutrient and ion exchange occurs via?
gap junctions
what happens once fibers have lost their nuclei?
the organelles of the fiber become specialized to form lens crystallins (alpha and beta) that make up the bulk of the fiber ~40%
Non-uniform distribution of crystallins is thought to give the lens what?
a non-uniform refractive index
Embryonic Nucleus:
earliest mass at center of lens composed of primary lens fibers.
Formed at 2 months gestation.
Does not increase in size
Fetal Nucleus:
formed before birth. Contains the Y sutures.
Does not increase in size
Adult Nucleus:
fibers made between birth and sexual maturation.
Increases in size throughout life.
Will sclerose and become yellowish with age.
Lens Cortex:
Newly formed fibers that contain nuclei and surround the adult nucleus
what are the 10 divisions of the lens?
- Anterior lens capsule
- Anterior Epithelium
- Ant surface of adult nucl.
- Ant surface of fetal nucl
- Inner layer of ant half of fetal nucl (Y suture)
- Inner layer of Post half fetal nucl (Y suture upside-down)
- Post surf fetal nucl
- Post surf adult nucl
- Posterior line of disjunction 10. Posterior capsule
dislocation of the lens:
ectopia lentis
Marfan’s syndrome
the lens dislocates upward
homocysteinuria
the lens dislocates downward
PSC
An opaque spot in the posterior pole of cortex under the posterior capsule of the lens
NS: nuclear cataract causes an increase in hyperopia or myopia?
myopia
