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Ocular anatomy > Crystaline lens > Flashcards

Crystaline lens Flashcards

1
Q

What are the dimensions of the lens

A

Biconvex
- Anterior curvature (10mm)
- Posterior curvature (6mm)

Posterior is curvier (more convex) than anterior

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2
Q

What is the poles of the lens

A

The center point on the anterior and posterior surfac

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3
Q

What is the axis of the lens

A

Line adjoining the poles

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4
Q

What is the equator

A

The marginal circumference of the lens (end of the lens)

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5
Q

What is the equator encricled by and what keeps the lens in posistion

A

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

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6
Q

What are the three structure in the crystaline lens

A

Lens capsule

Epithelium

Lens fibres

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7
Q

What is the acellular transparent elastic basement membrane that envelops the entire lens

A

The lens capsule

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8
Q

Where are the lens capsule thickest and thinest

A

THnnest at the posterior pole (3um) Thickest at the anterior and psoterior surfaces closest to the equator where ciliary zonules are inserted

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9
Q

What is the function of the lens capsule

A
  • 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)
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10
Q

What is the single layer of cuboidal epithelium in the lens

A

Lens epithelium

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11
Q

Where is the lens epithelium located

A

At the anterior surface of the lens (behind capsule), where mitosis (cell division occurs)

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12
Q

What is the function of the lens epithelium

A
  • Centeral epithelium play a role in transporting a variety of solutes between the lens and the aqueous
  • Secretes capsular material
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13
Q

What is the strcutre that is the main mass of the lens

A

Lens fibres

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14
Q

How are lens fibres formed and what does it elongate to form

A

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

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15
Q

What is the lens bow/ vortex

A

The pattern of cells of the posterior process passing along the inner surface of the posterior capsule

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16
Q

What happens to the older lens fibres

A

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

17
Q

What are the orientation of Y sutures anterior and posteriorly

A

Erect Y anterioly, inverted Y posteriorly

18
Q

What happens to Y sutures as the lens increase in size

A

The pattern of sutures become more complicated

19
Q

The three layers of the nuclues in the lens fibres

A

Emboynoic nuclues: earliest fibre mass in the center

Adult nuclues: formed after birth

Cortex: recently nucleated fibres and surrounds the nuclues

20
Q

What is the composistion of the crystalline lens

A

35% of lens is protein
65% is water content
Lipid, amino acid, electrolytes, peptide, carbohydrates less than 1% of lens

21
Q

What are the two types of proteins in the lens

A

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

22
Q

What are the functions of the crystalline lens

A

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

23
Q

What are the reasons for transparency in the lens

A

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

24
Q

What is the dioptric power of the lens and how does it change

A

About +15.00 D

Will decrease with age, lens has the abilty to change its dioptric power

25
Q

What happens duing accommodation

A

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

26
Q

When does accommodation take place

A

When trying to focus on a near object

27
Q

What happens when the lens is at a relaxed state

A

Relaxation of ciliary muscle, pulling backwards on zonules, flattening of lens in unaccommodated form

28
Q

Why is metabolism important to the crystallines lens

A

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

29
Q

Which structure supplies the crystalline lens

A

From the aqueous and viterous, the glucose diffuses into the lens

30
Q

Name the age related changes to the lens

A

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

31
Q

Causes of changes in transparency of lens as one ages

A
  • 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

Ocular anatomy (15 decks)

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