Anatomy & Physiology Of Lens

ANATOMY & PHYSIOLOGY OF LENS

• Lens is a transparent, bi-convex, crystalline structure 
• Placed b/w iris & the vitreous in a saucer shaped depression called patellar fossa.
• Its equatorial diameter is 10 mm.
• Refractive index is 1.39.
• Total power is 15-16 D.

ANATOMY OF CRYSTALLINE LENS

• There are 3 structural elements that make up lens-

1) Lens capsule:

• Thickest at pre- equator region & thinnest at the posterior pole.
• Hyaloideocapsular ligament of Weiger or retrolental ligament is a strong adhesion b/w the lens & posterior capsule.
• It is a circular adhesion 8-9 mm in diameter.
• This attachment is termed as Egger’s line.
• Within this circle is a potential space called as Space of Burger or Erggelet’s space.

2) Lens epithelium :

• Single layer on the anterior (front) surface of the lens.
• There is no epithelium on posterior surface.

3) Lens fibres 

• These are long protein fibers that are closely packed & parallel.
• Lens fibres are formed through out the life, these are arranged compactly as nucleus and cortex of the lens.

a) Nucleus : It is the central part containing the oldest fibres. Consists of different zones:

1. Embryonic nucleus: Formed at 1-3 months of gestation, therefore consists the oldest primary fibres.
2. Fetal nucleus : Lies around the embryonic nucleus & is formed from 3 months of gestation till birth.
3. Infantile nucleus: Corresponding the lens from birth to puberty & surrounds the fetal nucleus.
4. Adults: Corresponds to the lens fibres formed after puberty to rest of the life.

b) Cortex: It is the peripheral part contain the youngest fibres.

• The ciliary zonules (Zonules of zinn or suspensory ligaments of ligaments of lens) hold the lens in position & enable the ciliary muscle to act on it.

Fibres of zonules are divided into:

1) Hayloid zonule (orbiculo-posterior capsular fibres)

• These originate from the pars plana of the ciliary body & inser into the capsule just posterior to the equator at the edge of patellar fossa.

2) Anterior zonule (orbiculo-anterior capsular fibres or  anterior zonular sheet)

• These originate from the pars plana of the ciliary body & insert into the capsule just anterior to the equator.
• These are the strongest & thickest of the zonular fibres.

3) Posterior zonule (ciliary-posterior capsular fibres or posterior zonular sheet)

• These originate from the pars plicate of the ciliary body & insert into the capsule posterior to equator.
• These are most numerous.

4) Equatorial zonule (Cilio-equatorial fibres)

• These originate from the pars plicata of ciliary body & insert into the lens capsule at the equator.

Metabolic activity of lens

• Lens is an avascular structure but receives its nutrition mostly from aqueous humor.
• Some nutrition derived from vitreous humor too.
• Lens does not need much oxygen, but receives glucose as its main energy source.
• Glucose from the aqueous (mainly) & vitreous diffuses into the lens.

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