Lens

lens

avascular, transparent elliptic structure that aids in focusing light rays on the retina; located w/in posterior chamber anterior to the vitreous and posterior to the iris; suspended from ciliary body and zonular fibers; malleable

embryologic development

the structure of the adult lens is determined during _________

lens vesicle

first lens like structure observable in developing embryo; composed of a layer of epithelial cells that form a hollow sphere

posterior

which cells of the lens differentiate and elongate to form the primary lens fibers?

no

is there posterior epithelium in the adult lens?

germinative zone of epithelium anterior to lens equator

where does cell division occur during the post developmental life of the lens?

biconvex

what is the shape of the lens?

posterior

the ______ surface of the lens has the steeper curve

lens thickness

distance from anterior to posterior pole

lens diameter

nasal to temporal measurement of the lens

surface curvatures, refractive index, change in index, length of optical path

what factors affect lens refraction?

20D

what is the refractive power of the unaccommodated lens?

gradient

the lens has a _____ refractive index

optical density

the lens has a gradient refractive index because of the changes in _____ throughout the lens

center

where is optical density the highest in the lens?

protein concentration

the refractive index is a factor of __________ within lens fibers

14D, 8-12yo

what is the maximum power of the lens in accommodation and what age is it reached at?

presbyopia

decreasing accommodative power with age

0

what is the accommodative power of the lens at age 50?

anterior pole

thickness of lens capsule at ______ increases with age

lens capsule

consists primarily of collagen; elastic; encloses all lens components and helps mold shape of the lens; provides immune privilege to the lens

B

lamellar arrangement of fibers

why is the lens capsule elastic?

no

does the lens capsule contain elastic fibers?

anterior epithelium

what part of the lens secretes the lens capsule?

yes

does the lens expand through life?

water & small solutes

what is the lens capsule permeable to?

zonular fibers

what counteracts the spherical shape the lens prefers to take?

capsule

where do zonular fibers insert?

zonular lamella

outer superficial zone of lens capsule that consists of zonules interconnected with matrix

anterior lens epithelium

adjacent to anterior lens capsule; cuboidal epithelium; secrete anterior capsule and are site of metabolic transport mechanisms

A

capsule

the basal aspect of anterior epithelium is oriented towards what?

center of lens

the apical portion of anterior epithelium is oriented towards what?

desmosomes & gap junctions

what joins the lateral membranes of the epithelial cells?

no

are tight junctions present in the lateral membranes of the anterior epithelium?

germinal zone

band of cells in preequatorial region that lies just anterior to equator; location of cell mitosis in lens

posterior pole

basal aspect of forming lens fibers stretch towards what?

anterior pole

apical aspect of forming lens fibers stretch towards what?

apical side

where do the cellular nuclei move as each layer of forming lens fibers elongates?

lens bow

arcuate shaped line that connects the dots of cellular nuclei in the lens, towards the anterior aspect

loses cellular organelles

what occurs when an elongated cell becomes a lens fiber?

outer

new lens fibers are laid down ______ to the older fibers

longer

superficial lens fibers are _____ than deeper fibers

youngest

_______ cells lie directly below epithelium and capsule

secondary lens fibesr

all fibers formed from mitosis in germinative zone

nucleus

once the _____ is lost, the mature lens fiber has lost its attachment to basement membrane

growth factors

accumulate in lens capsule, concentration & distribution direct cellular processes

anterior

growth factors that influence proliferation and migration are concentrated along the _______ surface

equator

growth factors that influence differentiation are concentrated at the _______

fiber mass

biomolecules that regulate interactions among actin filaments, adhering junction integrins, and ECM increase what?

significant protein synthesis

___________________ must occur to form crystallins, aquaporin channel proteins, and gap junction components as fibers elongate

increases

as fiber elongates, cell membrane permeability _______

suture

joining of elongating fibers from opposite sides of the lens forms this

suture formation

what triggers cell organelle death in lens fibers?

interdigitate

membranes of adjacent fibers are said to __________, forming interlocking junctions along their lateral sides that help stabilize fibers

throughout life

lens fiber production continues _____________

concentric layers

how is growth of lens fibers organized?

crystallins

proteins lens fiber cytoplasm that make up 40% of the net weight of the fiber; distribution contributes to the gradient refractive index; soluble

epithelium fiber interface

border b/t apical membrane of anterior epithelium and apical membrane of elongating fiber; site of nutrient and ion exchange

A

gap junctions

extensive network of these throughout lens along lateral fiber membranes account for facility with which nutrients and ions move within the lens

no

are gap junctions evenly distributed throughout the lens?

towards equator and in outer layers

where are gap junctions more concentrated?

primary lens fibers

form very core center of lens, embryonic nucleus

secondary lens fibers

lens fibers outer to the embryonic nucleus core

higher

lens cortex has a ______ water content than nucleus

1.38

refractive index of lens cortex

1.41

refractive index of lens nucleus

actin

insoluble protein in the lens that is a key component of cytoskeleton of the lens fiber

microtubules

part of cytoskeleton and help stabilize fiber membrane; role in transporting vesicles to ends of elongating fibers

fetal nucleus

includes embryonic nucleus and the fibers surrounding it that are formed prior to birth

adult nucleus

includes embryonic and fetal nucleus and fibers formed from birth to sexual maturation

lens cortex

contains fibers formed after sexual maturation

upright Y

describe the structure of the anterior suture

inverted Y

describe the structure of the posterior suture

zonules of zinn

threadlike fibers that attach lens to ciliary body; microfibrils

basement membrane of nonpigmented ciliary epithelium

where do zonules of zinn arise from?

lens capsule

most zonule of zinn fibers attach to what?

primary zonules

zonules that attach to the lens

secondary zonules

join primary zonules with each other or connect processes to one another or to the pars plana

tension fibers

anchor primary zonules to ciliary valleys to form a fulcrum

relaxed, stretched

when emmetropic eye is viewing a distance object, the ciliary muscle is ______ and the zonules are ______

accommodation

increase in power that occurs to view a near object

increases

lens thickness _____ anterior to posterior in accommodation

thins

lens _____ along equator in accommodation

forward

anterior lens surface moves _____ in accommodation

no

does the posterior pole change positions in accommodation?

retinal blur

what is the stimulus that initiates the accommodative mechanism?

cone stimulation

accommodative mechanism is dependent upon what photoreceptors?

passive

vitreous has a _____ role in accommodation

widening, increasing, decreasing

accommodation can cause a _______ of intertrabecular spaces, ______ aqueous outflow and ______ IOP

0

what is the objective measurement of accommodation near age 50?

absence of blood vessels, few cellular organelles, orderly fiber arrangement, short distance b/t components of differing indices

what things contribute to transparency of the lens?

sodium potassium pumps

these things help maintain electrolyte balance, located in lens epithelium

anaerobic glycolysis

main source of energy/ATP required for cellular metabolism and replication within the lens

aqueous humor

where does the lens get glucose from?

epithelium/superficial fibers

where is aerobic glycolysis limited to in the lens?

100microns

thickness of lens cortex where organelles are still present

epithelial cells and new fibers

where is ATP activity higher in lens?

no

is the lens nucleus capable of protein synthesis?

facilitated diffusion

glucose uptake occurs by what process?

intercellular flow

removes metabolic waste from lens