Lens Flashcards
The Lens is specialized __epithelial/endothelial__ tissue that is responsible for fine-tuning the image that is projected on the __cornea/retina_____.
epithelial, retina
T/F Intraocular lens can be compared to a camera lens.
T
The lens must be transparent, have a higher refractive index than the medium in which it is suspended, and have refractive surfaces with the proper curvature to allow what to occur?
for the lens to fine-tune an image that is projected onto the retina
T/F The curvature of surfaces must not be variable to permit the optical system to focus on objects that are far or near.
False, curvature of surfaces must be variable
Disruption of the precise organization of the lens _fiber/protein__ cells or damage to the fiber/protein within them can destroy the _opaqueness/transparency__ of the lens, a process known as _cataract/pterygium__ formation.
fiber, proteins, transparency, cataract
The lens is formed from __two/three populations of specialized epithelial cells.
two
- cuboidal cells
- elongated fiber cells
On the anterior/posterior surface of the lens closest to the __cornea/iris__ is a sheet of __cuboidal/fiber__ cells, the lens epithelium.
anterior, cornea, cuboidal
The bulk of the lens consists of elongated _cuboidal/fiber__ cells.
fiber
T/F The lens capsule, an elastic extracellular matrix that surrounds the entire lens is secreted by the epithelial and superficial fiber cells.
True
Most epithelial cells __do/do not__ divide.
DO NOT
In the germanitive zone __near/away__ the equatorial margin of the lens epithelium/endothelium cells do/do not proliferate and do so slowly/quickly
near, epithelium, DO, slowly
T/F The junctions between the apical and basal ends of cells from the opposite sides of the lens are called the common junction.
False, they are called sutures.
Fiber cells __stop/start__ elongating when they reach the sutures and their basal ends __detach/attach__ from the capsule.
stop, detach
Mature/Young fiber cells are gradually buried deeper in the lens as successive generations of fibers elongate/shorten and differentiate
mature, elongate
Lens increases/decreases in size; synthesis ceases/continues just before organelle degradation.
increases, ceases
The components of mature fiber cells in the lens, must be much less stable than those in cells found in other parts of the body.
False, much more stable
The lens is suspended in the posterior/anterior of the eye by a band of inelastic/elastic microfibrils, the Zonules/Iris, which insert into the lens capsule near the equator.
anterior, inelastic, zonules
The zonule fibrils originate in the pigmented/nonpigmented layer of the ciliary/corneal epithelium, a tissue that is located just posterior to the iris.
non-pigmented, ciliary
T/F Changes in the tension applied to the zonules are responsible for the alterations in lens curvature during accommodation.
True
The refractive properties of the lens are the result of the high/low concentration of crystallins in the cytoplasm/nucleus of the lens fiber cells/wing cells and the curvature of the lens surfaces.
high, cytoplasm, fiber cells
Lens crystallins accumulate to concentrations that are ________higher than in typical cells.
a. 1x
b. 2x
c. 3x
d. 4x
c. 3x
The lower/higher concentration of crystallins give lens fiber cells a significantly lower/higher refractive index than the fluids around the lens.
higher, higher
T/F In younger individuals, refractive error is often caused by defects in the curvature or refractive index of the lens.
False, In younger individuals, refractive error is often caused by abnormalities in corneal curvature or the length of the globe, RARELY by defects in the curvature or refractive index of the lens.
Transparency depends on maximizing/minimizing light scattering and absorption.
minimizing
Pick the best answer, may pick more than one answer.
Light passes smoothly through the lens as a result of:
1. the regular structure of lens fibers
2. the absence of membrane-bound organelles
3. the small and uniform extracellular space between the fiber cells.
All three statements are correct
1. the regular structure of lens fibers 2. the absence of membrane-bound organelles 3. the small and uniform extracellular space between the fiber cells.
In young/older individuals, the human lens is nearly colorful/colorless.
young, colorless
The human lens absorbs _increasing/decreasing _ amounts of short/long wavelengths as it ages.
increasing, short
T/F Pseudophakia is the absence of the natural crystalline lens, either from natural causes or because it has been removed.
False, Aphakia
T/F Aphakia is the substitution of the natural crystalline lens with a synthetic lens.
False, Pseudophakia
Pseudophakic IOLs are used in cataract surgery/cornea surgery.
Cataract surgery
Congenital aphakia can be caused by a Hepatitis/Rubella infection in first 4/8/10 weeks of pregnancy.
Rubella, 4
can also be caused by a mutation in the PAX-6 gene
T/F Lens fiber differentiation depends on synthesis and accumulation of large amounts of crystallins.
True
As much as ____of the wet weight of the lens fiber cell can be accounted for by crystallins, a protein concentration that is approximately ___ higher than that in the cytoplasm of typical cells
a. 25%, 3x
b. 40%, 2x
c. 50%, 2x
d. 40%, 3x
d. 40%, 3x
Crystallins are classified into 2 categories:
1. Classical and Taxon-specific 2. Classical and Non-classical 3. Taxon-specific and Taxon-nonspecific 4. Type1 and Type 2
- Classical and Taxon-specific
T/F All vertebrae lenses accumulate the Classical crystallins, which consist of α-crystallin family and β/γ-crystallin superfamily.
True
Taxon-specific crystallins are functional/non-functional enzymes or proteins that are structurally similar/dissimilar to enzymes but that lack/contain enzymatic activity.
functional, similar, lack
T/F Adult human lenses DO produce taxon-specific crystallins,
False Adult human lenses DO NOT produce taxon-specific crystallins,
T/F The enzyme betaine-homocysteine methyltransferase is present at high levels in the embryonic nucleus, indicating that this enzyme serves as a taxon-specific crystallin during the early development of the human lens.
True because Adult human lenses DO NOT produce taxon-specific crystallins,
Human lenses express ___ α-crystallin genes.
a. 1
b. 2
c. 3
d. 4
b. 2, αA and αB.
The role of the α-crystallins in promoting/preventing protein aggregation and precipitation has been demonstrated in experiments performed in vivo/in vitro.
preventing, in vitro
Analysis of αA knockout mice: protein aggregates show large/small amounts of αB-crystallin and larger/smaller amounts of other proteins.
Resulted in Lens smaller/larger than normal-structurally similar though and formation of cataract in first one/few weeks.
large, smaller
smaller, few
(αB-crystallin/αA-cryastallin) is present in a variety of cells throughout the body, especially in heart and skeletal muscle.
αB-crystallin
T/F In vitro tests showed that the mutant form of the protein αB-crystallin had chaperone activity and did not enhanced the aggregation of test proteins.
False, In vitro tests showed that the mutant form of the protein αB-crystallin had NO chaperone activity and even ENHANCED the aggregation of test proteins.
T/F Studies suggest that αB-crystallin has important chaperone functions in the lens and in other cells of the body.
True
The β/γ-crystallin superfamily is less/more diverse than the α-crystallins, and the function of its members in the lens is less/more evident.
more, less
___ β-crystallin polypeptides and ____ γ-crystallins are expressed in the human lens.
a. 3, 6
b. 3, 3
c. 6, 3
d. 6, 6
c. 6, 3
Classical crystallins have a composition of ____ water and ____ organic matter (of which ____ is structural proteins).
a. 35%, 65%, 33%
b. 33%, 35%, 65%
c. 65%, 35%, 33%
d. 65%, 33%, 35%
c. 65%, 35%, 33%
Microtubules are abundant/sparse beneath the plasma membranes of lens fiber cells.
abundant
Microtubules play Important roles in:
1. stabilizing the fiber cell membrane
2. transporting vesicles to the apical and basal ends of elongating fiber cells
3. gives lens its birefringent property
4. all of the above
- all of the above
The membranes of mature lens fiber cells have an usual/unsusal lipid composition
unusual
Human lens fiber cells have the lowest/highest proportion of _glucose/cholesterol and sphingomyelin of any plasma membrane in the body, and the amount of glucose/cholesterol increases/decreases as the fiber cells mature.
highest, cholesterol, cholesterol, increases
The cholesterol/phospholipid ratio is nearly ___ ___ greater in nuclear than in cortical fiber cells.
a. 2 fold
b. 3 fold
c. 4 fold
d. 5 fold
b. 3 fold
T/F The presence of high concentrations of cholesterol and sphingomyelin is likely to cause lens fiber cell membranes to be flexible.
False, cause lens fiber cell membranes to be rigid,
T/F The human lens grows rapidly in the embryo and during the first postnatal year
True
The rate of lens growth slows between ages ___ and ____years, then continues at a much slower, nearly linear rate throughout life.
a. 2 and 10
b. 1 and 5
c. 1 and 10
d. 2 and 5
c. 1 and 10
In the anterior/posterior of the lens, this network, the tunica vasculosa lentis, arises from the iris stroma/hyaloid artery.
posterior, hyaloid artery
T/F The capillaries at the anterior/posterior of the lens arise from blood vessels of the developing iris stroma to form the anterior/posterior papillary membrane.
anterior, anterior
During the _____ trimester of human development, the capillaries of the tunica vasculosa lentis and the anterior papillary membrane regress.
a. first
b. second
c. third
b. second
Absence of the lens at an early stage of embryogenesis leads to
1. the absence of the corneal endothelium
2. abnormal differentiation of the corneal stroma
3. absence of the iris, ciliary body, and anterior chamber.
4. all of the above
- all of the above
T/F The lens derives much of its energy from glycolysis.
True
The start/end product of glycolysis is lactic acid.
end
As a result of lactate accumulation, intracellular pH raises/drops significantly from peripheral/central to deeper fiber cells.
drops, peripheral
T/F If cells could survive in an atmosphere free of oxygen, most oxidative damage could be avoided.
True
Molecular oxygen/nitrogen is, directly or indirectly, the source of least/most oxidative damage.
oxygen, most
The oxygen tension around the lens in the living eye is low, approximately ____ mm Hg
a. 10
b. 15
c. 20
d. 25
b. 15
The low/high oxygen tension around and within the lens probably damages/protects lens proteins and lipids from oxidative damage.
low, protects
Despite the low/high oxygen level, the lens derives a substantial proportion of its ATP form glycolysis/oxidative phosphorylation- a process that generates free radicals.
low, oxidative phosphorylation
T/F The lens is not exposed to solar irradiation throughout its life.
False. it IS exposed
Much of the solar irradiation that reaches the eye is absorbed by the cornea/retina, but the remaining UV light _could have/could not have _ harmful effects on the lens
cornea, could have
T/F If light is not absorbed, it produces no damage.
true
