Week 9: The Lens Flashcards
What is the lens?
> A transparent biconvex structure
What is the location of the lens?
- behind the iris and pupil
- in front of the vitreous body
- Attached to the ciliary body by means of the zonular fibres also known as the suspensory ligament of the lens
- Fibres arise from the epithelium of the ciliary body and run to the equator of the lens
What is the dioptric power of the eye and lens?
> Total eye = 58 diopters (D)
Lens = 15 D
Lens is important as it has ability to change its dioptric power
What is the range of accommodation at different ages?
- 15-16 D at birth
- 8 D at 40 years old
- 1-2 D by 60 years old
What is the dimensions of the lens?
Thickness
> 3.50 - 4.00 mm at birth
> 4.75 - 5.00 mm later in life
(measurements are for the unaccommodated lens – thickness varies with accommodation)
What is the orientation of the lens?
- Centre points on anterior and posterior surfaces are the anterior and posterior poles
- Line joining poles is called the axis of the lens
- The marginal circumference is called the equator
What is the three layers of the lens?
- The Capsule (outside elastic membrane of lens)
- The Epithelial layer (only on front surface of lens)
- The Stroma (makes up bulk of lens)
What is the capsule?
- Is an elastic basal membrane – envelopes the lens
- Thickest at the anterior and posterior surfaces close to equator (20µm) (where Zionillon fibres are)
- Thinnest at the posterior pole (3µm) (near vitreous)
- Elastic nature allows accommodation
What is the epithelial layer?
-Epithelium is made up of cuboidal cells below the capsule on the anterior surface only.
- At the equator the cells elongate and form columnar cells – transform into lens fibres, added to stroma of lens
> Mitotic activity is greatest at the equator – cells at the anterior pole do not normally divide, but are responsible for transporting solutes between the lens and aqueous humour and secreting the capsular material.
> Metabolic and synthetic activity of the epithelium is critical for the survival of the lens
What is the stroma?
-Is made up of lens fibres. Makes up the bulk of the lens.
> Fibres form from the epithelial cells in the germinal zone (at the equator).
What happens as the columnar cells elongate at the stroma?
- basal part (bottom) moves anterior of lens
- apical part (top near capsule) moves to the posterior of the lens
-A new lens fibre has its centre at the equator and its ends reach towards the anterior and posterior poles.
> The nucleus moves anteriorly – gives rise to the lens bow (nucleus migrates) - The nuclei will eventually break-up to maintain lens transparency
> Process goes on throughout life. Cells are pushed towards the axis of the lens. Each cell is now a lens fibre, is hexagonal in cross-section and very long (10mm)
> Ball and socket joints keep fibres in place
> The ends of the cells reach to the anterior and posterior surfaces
> No cell reaches from pole to pole
> Lens fibres from the same layer join at sutures
> In the foetus there are few lens fibres - gives rise to simple Y shape
What are the zones within the stroma?
Embryonic nucleus (deepest, forms very early)
> Foetal nucleus (with Y sutures, secondary fibres)
> Adult nucleus
-Made up of fibres formed after birth
> Lens Cortex
-Most recently formed cells – still nucleated
> Nucleated cells are alive deeper layers are dead.
> Lens density greater in the middle of the lens, is due to different protein concentrations and results in different refractive indexes
What is the lens metabolism?
> Lens has high protein content (1/3) and low water content (2/3)
Needs this to maintain high refractive index
Lens is avascular (no blood supply except hyoid artery which disappears)
Nourishment and oxygenation is by diffusion from the aqueous humour
As with cornea transparency is maintained by controlling hydration levels
Epithelial cell membrane permeability and active transport are responsible
How does glucose enter the epithelium?
-Active transport requires a source of energy – provided by glucose.
> Glucose enters by simple diffusion, is metabolised by anaerobic pathways – provides 70% of lens energy (low 02)
> 30% produced by aerobic metabolism of the lens epithelium
> Advantages of using mostly anaerobic metabolism is that lens is not susceptible to oxygen starvation – can survive periods without oxygen – must have glucose to maintain H2O levels
What is the water and electrolyte balance in the lens + epithelium?
> Osmotic control of water levels is achieved by a sodium pump in epithelium
Pumps sodium out and potassium in
Occurs only at the anterior surface of lens
