Crystaline lens Flashcards
What are the dimensions of the lens
Biconvex
- Anterior curvature (10mm)
- Posterior curvature (6mm)
Posterior is curvier (more convex) than anterior
What is the poles of the lens
The center point on the anterior and posterior surfac
What is the axis of the lens
Line adjoining the poles
What is the equator
The marginal circumference of the lens (end of the lens)
What is the equator encricled by and what keeps the lens in posistion
The equator is encricled by ciliary processes of the ciliar body
Zonular fibres connects to these ciliary processes at the eqautor to keep the lens in posistion
What are the three structure in the crystaline lens
Lens capsule
Epithelium
Lens fibres
What is the acellular transparent elastic basement membrane that envelops the entire lens
The lens capsule
Where are the lens capsule thickest and thinest
THnnest at the posterior pole (3um) Thickest at the anterior and psoterior surfaces closest to the equator where ciliary zonules are inserted
What is the function of the lens capsule
- Moulds the shape of the lens in response to changing zonular fibre tension during accommodation
- Acts as a barrier to maintain lens transparency (impremeable to large molecules)
- Permeable to lower molecular weight compunds (amino acids etc)
What is the single layer of cuboidal epithelium in the lens
Lens epithelium
Where is the lens epithelium located
At the anterior surface of the lens (behind capsule), where mitosis (cell division occurs)
What is the function of the lens epithelium
- Centeral epithelium play a role in transporting a variety of solutes between the lens and the aqueous
- Secretes capsular material
What is the strcutre that is the main mass of the lens
Lens fibres
How are lens fibres formed and what does it elongate to form
Epithelial cells at the eqautor elogate to become lens fibre cells
Elongates to form:
One end of the cell elongates anteriorly and the other end elongates posteriorly, formaing and anterior and posterior process
The anterior process passes between the anteior epithelium and previous layer of process
What is the lens bow/ vortex
The pattern of cells of the posterior process passing along the inner surface of the posterior capsule
What happens to the older lens fibres
The posterior process passes along inner surface of posteior capsule, this pattern of cells at the equator is called lens bow.
The anterior and posterior process force the older fibres inwards towards the lens nuclues
As the fibre cells move inwards, growing into the lens substance, the cel nucleus eventually fragments and disappears
Their organells become specialized in product of crystallins (lens proteins mainly in the cortex)
The newer cells form the cortex, but as arent long enough to reach from one pole to another, the ends of their processes meet anteriorly and posteriorly at a junction called sultural lines
What are the orientation of Y sutures anterior and posteriorly
Erect Y anterioly, inverted Y posteriorly
What happens to Y sutures as the lens increase in size
The pattern of sutures become more complicated
The three layers of the nuclues in the lens fibres
Emboynoic nuclues: earliest fibre mass in the center
Adult nuclues: formed after birth
Cortex: recently nucleated fibres and surrounds the nuclues
What is the composistion of the crystalline lens
35% of lens is protein
65% is water content
Lipid, amino acid, electrolytes, peptide, carbohydrates less than 1% of lens
What are the two types of proteins in the lens
Crystallins (90% of total lens protein)
- found mostly in the lens cortex
-the lens refractive index varies across the lens depending on the crystallin composition
Albuminoids (10%)
- mainly in lens nuclues
- make up the membrane of proteins n cytoskeletal proteins
What are the functions of the crystalline lens
Transparency
Highly refractive
- refracts light entering the eye through the pupil and focues it on the retina
- absorbs some ultraviolet radiation
- changes focus for accommodation
Accomodation
What are the reasons for transparency in the lens
Scatters about 5% of light falling upon it
transparency due to
- acellullarity
- single layer of epithelium beneath the anterior capsule
- loss of nucleus and other intracellular organells in lens fibres
- avascular
- dense packing of the fibres which reduces the intercellular spaces thus reducting scattering
What is the dioptric power of the lens and how does it change
About +15.00 D
Will decrease with age, lens has the abilty to change its dioptric power
What happens duing accommodation
The ciliary muscle contracts, relaxing tension in the zonular fibres, releases traction on lens capsule, increases lens elasticity which increases the curvature espcially at the anterior surface
When does accommodation take place
When trying to focus on a near object
What happens when the lens is at a relaxed state
Relaxation of ciliary muscle, pulling backwards on zonules, flattening of lens in unaccommodated form
Why is metabolism important to the crystallines lens
Required for the production of energy to
- maintain correct lens hydration to ensure lens remains transparent. Dehydration is mainted by an active Na+/K ion water pump that resides withing the capsule, the membranes of cells in the lens epithelium and each lens fibre
- Development and growth of new fibres
- For continous protein and glutathoine (protein) synthesis
Which structure supplies the crystalline lens
From the aqueous and viterous, the glucose diffuses into the lens
Name the age related changes to the lens
Slowly increases in size as new fibres form (6mm, adult 10mm)
Lens thickness increases with age (from chil 3mm, 80y/o 5mm)
Transparency affected
- Older lens fibres in the center of the lens become dehydrated, compacted and sclerosed
- Yellow brown pigment accmulates, due to denatured crystallin proteins
Causes of changes in transparency of lens as one ages
- Protein modification (aggregation) which results in membrane disruption
- Enzymes may be damaged by oxidation- electrolyte imbalance
- Disruption of membrane lipid which contributes to membrane permeability
