Chapter 2: Anatomy

Question 1

what is the shape of the lens?

Answer 1

binconvex

Question 2

what are the components of the lens?

Answer 2

capsule, epithelium, cortex, nucleus

Question 3

what is the axis that joins the anterior and posterior poles of the lens?

Answer 3

optic axis

Question 4

what are meridians?

Answer 4

hypothetical lines on lens surface passing from one pole to the other

Question 5

what are the functions of the lens?

Answer 5

maintain clarity, refract lights, accommodate in conjunction with CB/zonules

Question 6

when does the lens lack blood supply and innervation? where does it get metabolic requirements after?

Answer 6

after fetal development; aqueous humour

Question 7

what is the index of refraction of the lens?

Answer 7

1.4 centrally and 1.36 peripherally

Question 8

how is the lens able to refract light?

Answer 8

its n is different from that of the aqueous and vitreous surrounding it

Question 9

how much refractive power does the lens contribute in its nonaccomodative state?

Answer 9

20D

Question 10

where does the rest of the convergent refractive power come from and how much?

Answer 10

40-45 D from air-cornea interface

Question 11

what are the measurements and weight of birth vs adult lens?

Answer 11

birth: 6.4 mm equatorial; 3.5 mm AP; 90 mg.
adult: 9-10 mm equatorial; 5 mm AP; 255 mg.

Question 12

what happens to the thickness, shape, and refractive index of the lens with aging?

Answer 12

• cortical thickness increases
• more curved
• decreased index of refraction

Question 13

why does the index of refraction of the lens decrease with age?

Answer 13

increased insoluble proteins

Question 14

what happens to the overall refraction with aging?

Answer 14

depends on the interplay between index of refraction decreasing and increasing thickness/curvature.

Question 15

what is the lens capsule composed of?

Answer 15

elastic, transparent BM composed of type 4 collagen and other matrix proteins

Question 16

what lays down the capsule?

Answer 16

epithelial cells

Question 17

what part of the lens molds during accommodation?

Answer 17

capsule

Question 18

what part of the lens capsule attaches to the zonules?

Answer 18

the outer layer, called the zonular lamella

Question 19

what are the various thicknesses of the lens capsule?

Answer 19

• anterior 14 um; posterior 4 um.
• equator: 17 um.
• thickest in anterior (21) and posterior (23) pre-equatorial zones.

Question 20

where do the lens zonules originate from?

Answer 20

basal lamina of the nonpigmented epithelium of the pars plana/plicata of the CB

Question 21

what constitutes the zonular fibres?

Answer 21

microfibrils composed of elastic tissue

Question 22

where do the zonules insert?

Answer 22

capsule; 1.5 mm anterior to equator and 1.25 mm posterior to equator

Question 23

what happens to the zonules with age?

Answer 23

regress - leaving separate anterior and posterior layers that form triangle shape in cross sections

Question 24

what are the dimensions of the zonules?

Answer 24

• 5-30 um diameter; microfibrils are 8-10 nm in diameter

Question 25

what are the histo features of zonules?

Answer 25

PAS positive, eosinophilic

Question 26

where does the lens epithelium lie?

Answer 26

immediately posterior to the anterior lens capsule

Question 27

what is the function of the lens epithelium?

Answer 27

metabolically active - produces DNA, RNA, proteins, lipid, and adenosine triphosphate for energy

Question 28

what is the location of the greatest activities of premitotic DNA synthesis in the lens epithelium?

Answer 28

ring around the anterior lens - germinative zone

Question 29

describe lens differentiation in the epithelium.

Answer 29

newly formed cells migrate toward the equator –> elongate, lose organelles/gain protein mass –> differentiate into fibres (optically advantageous)

Question 30

what meets the energy requirements of the lens fibres?

Answer 30

glycolysis

Question 31

where do epithelial lens cells begin the process of terminal differentiation into fibres?

Answer 31

bow region (posterior pre-equatorial)

Question 32

where is the germinative zone?

Answer 32

anterior pre-equatorial

Question 33

what happens to the lens morphology as new lens fibres are laid down?

Answer 33

no cells are lost; new fibres crowd/compact the previously formed fibres –> older layers in centre, newer layers form the cortex

Question 34

what are the oldest lens layers?

Answer 34

embryonic and fetal lens nuclei

Question 35

what are lens sutures?

Answer 35

formed by interdigitation of anterior and posterior tips of spindle shaped fibres

Question 36

why do multiple optical zones/Y-shaped sutures form?

Answer 36

strata of epithelial cells of differing optical densities are laid down throughout life; gradual transition occurs