Topic 7: Crystalline Lens

Question 1

describe the lens and its properties

Answer 1

BICONVEX body that refracts the light entering the eye through the pupil and focuses it on the retina

involved in accommodation and absorbs most UV radiation

anterior (10mm) < posterior (6mm)

avascular, transparent, highly refractive, able to change shape

Question 2

explain the dimensions of lens

Answer 2

centre points on anterior/posterior surfaces are ‘poles’

axis of lens: line adjoining poles

equator: marginal circumference of lens (zonules surround the equator)

diameter (adult): 10mm

thickness (adult): 4mm

Question 3

state lens structures from outer to innermost

Answer 3

(1) lens capsule
(2) anterior cuboidal epithelium
(3) lens fibres

Question 4

explain structures of lens

Answer 4

(1) capsule: envelops lens, Moulds shape of lens (accommodation), acts as barrier to maintain structural integrity
(2) epithelium: helps with transport of nutrients, anterior region only, single layer cuboidal epi centrally, cuboidal epi transforms to columnar cells at equator, columnar cells elongate (lose nuclei & organelles) to form lens fibres, pattern of displaced nuclei = lens bow, organelles from cells get transformed into crystallin proteins
(3) fibres: newer fibres in cortex (periphery), older fibres pushed to centre (nucleus)

Question 5

where are Y-sutures seen

Answer 5

in the nucleus

anteriorly: erect Y
posteriorly: inverted Y

Question 6

explain chemical composition of lens

Answer 6

lens protein: 33%

water content: 66%

others (lipids, carbohydrates, amino acids, peptides, electrolytes): 1%

Question 7

state the lens proteins

Answer 7

(1) crystallins
(2) albuminoids
(3) glutathione

Question 8

explain the lens proteins

Answer 8

(1) crystallins: 90%, in lens cortex, water soluble, formed by displaced organelles of epi cells
(2) albuminoids: 10%, in lens nucleus, water insoluble
(3) glutathione: a polypeptide made of 3 amino acids (glycine, cysteine, glutamic acid), synthesised in lens, prevents oxidation damage to lens (can lead to transparency loss of lens/cataract), amount reduces with age (hence cataract develops ~age 60)

Question 9

state source of nutrition of lens

Answer 9

aqueous humour and vitreous humour

Question 10

name the metabolic pathways of lens

Answer 10

(needed for glutathione synthesis)

(1) anaerobic glycotic pathway
(2) aerobic kreb’s cycle
(3) anaerobic hexose monophosphate shunt
(4) sorbitol pathway

Question 11

name aging changes in the lens

Answer 11

(1) increase in lens size (diameter, thickness)
(2) sclerosis/hardening of older fibres in the nucleus
(3) yellowing of crystalline proteins
(5) loss of lens transparency
(6) loss of accommodation