The lens

Question 1

What is the lens

Answer 1

A transparent biconvex structure that’s behind iris and pupil and in front of the vitreous.
Attached to CB by zonular fibres. The fibres arise from epithelium of the CB and run to equator of the lens.
Dioptric power - 15D- front surface more curved than back surface

Question 2

Dimensions and Orientation

Answer 2

power reduces with age at birth 15-16D of accommodation, 8D at 40 and 1-2D by 60.
Orientation- Centre points at anterior and posterior surfaces are anterior and posterior poles. Line joining poles is axis of lens.
Marginal circumference around the edge of the lens is called the equator.

Question 3

The Capsule- 1

Answer 3

An elastic basal membrane that envelopes the lens.
Thickest at anterior and posterior surfaces close to equator- where the annular fibres attach.
Thinnest at posterior pole.
Elastic nature allows accommodation - tries to make less as spherical as possible

Question 4

Epithelial layer- 2

Answer 4

Made up of cuboidal cells at ANTERIOR SURFACE ONLY. At equator cells elongate turn into columnar cells and transfer into lens fibres and get added to stroma.
Cell division os creates at equator and cells at anterior don’t normally divide but transfer solutes between lens and AH and secret capsular materials.
Metabolic and synthetic activity in epithelial is crucial fro lens survival and maintaining lens transparency.

Question 5

Stroma -3

Answer 5

Made up of lens fibres bulk of lens.
Columnar cells at equator elongate with basal part- bottom part going towards anterior surface and apical part- top part going towards posterior surface.A new lens fibre has its centre at equator . Nucleus moves anteriorly and gives rise to lens bow. Nucleus will eventually break up to maintain lens transparency , older fibres don’t have nuclei.
Process of adding lens fibres goes on throughout life. Ball and socket joints keep adjacent fibres in place.
Lens fibres from same layer join at sutures.In foetus there are few lens fibres give rise to simple Y shape

Question 6

Lens Metabolism

Answer 6

Lens has Hugh protein content and low water content.
Lens density creates in middle
Nourishment and oxygenation is from diffusion of Aqueous
Transparency is maintained by controlling hydration levels.
Epithelial cell membrane permeability and active transport are responsible. Lens is avascular

Question 7

Glucose

Answer 7

Active transport needs energy source- glucose enters by simple diffusion and provides 70% of energy lens needs to maintain hydration levels. 30% from aerobic metabolism in lens epithelium. Advantage of mostly having anaerobic metabolisms that not susceptible to oxygen starvation can survive periods without oxygen but needs glucose

Question 8

Water balance

Answer 8

osmotic control of water is achieved by sodium pump. pumps out sodium pumps in potassium. occurs only at anterior surface

Question 9

Ageing of lens

Answer 9

Lens grows throughout life o cells are lost. With age nucleus centre yellows and becomes sclerosed. Results in reduced VA, increased absorption of blue light.
Senile cataract- occurs if changes cause opacification of lens/ Accumulation of metabolic products.