Anatomy & Physiology Of Lens

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.

Exam Important

  • Crystalline lens develops from Surface ectoderm.
  • Water content in lens is 64%.
  • Lens has a respiratory quotient of 1.
  • Crystalline lens absorbs light rays shorter than 350 nm.
  • Diameter of an adult crystalline lens is 9-10 mm.
  • Thickness of the adult crystalline lens is about 5 mm.
  • The capsule of the crystalline lens is thinnest at posterior pole.
  • Epithelium is absent in lens on Posterior surface.
  • Oldest lens fibres are found in Nucleus.
  • The youngest lens fibres are present in Superficial layer of the cortex.
  • The lens fibres meet around the Y-shaped sutures in Embryonic nucleus of the crystalline lens.
  • Infantile nucleus of the crystalline lens refers to the nucleus developed from Birth to puberty.
  • Hyaloideocapsular ligament of Weiger or retrolental ligament is a strong adhesion b/w the lens & posterior capsule.
  • Strongest & thickest attachment of zonule is at just anterior to the equator.
  • The crystalline lens derives its nourishment from Aqueous and vitreous humor.
  • Vitamin A does not handle free radicals inside lens.
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