The Human Lens

The lens is a biconvex, avascular, colourless and almost transparent structure. It consists of closely packed transparent cells enclosed in a capsule. It lies behind the pupil and suspended by suspensory ligaments (zonules).

• It has the highest protein content in the body and is the only solid intraocular structure.
• The lens size is 4-5mm anteroposteriorly. It grows throughout life and is 35% protein and 65% water.

• The epithelium is the only sensitive part of the lens.
• The lens fibres formed at the anterior epithelium migrate to the posterior capsule. They lose their nuclei and water and are progressively pushed centrally to from the nucleus of the lens.
• Lens fiber production continues throughout life, with the new lens fibers being laid down around the older fibers; growth results in concentric layers of secondary lens fibers.
• The structure of the lens is similar to an onion; each layer of fibers approximates a layer of an onion.
• Older lens fibers form the nucleus and the younger ones form the cortex.
• The anterior lens capsule is twice as thick as the posterior lens capsule.
• The zonules arising from the ciliary body suspend the lens in place.
• The lens derives its nutrients from the aqueous and vitreous humors and excretion is also via the same.
• The lens has more K+ than Na+ (about 120mmol/kg K, 20mmol/kg Na).

Physiology of the symptoms the lens

• First of all, the primary function of the lens is the refraction of light, and it is important that the transparent lens have minimal light scatter.
• Transparency is a function of:
  • The absence of blood vessels,
  • Few cellular organelles in the light path,
  • An orderly arrangement of fibers,
  • The short distance between components of differing relative to the wavelength of light.

Now the physiology of symptoms of the lens.

• The nucleus being more converging than the cortex leads to short sightedness in nuclear sclerosis (the nucleus becomes harder and yellow).
• If the nucleus is less converging than the cortex, it leads to long sight.
• Presbyopia symptoms are due to reduced accommodative capability with aging and results in longsightedness and diminished ability to clearly see nearby objects.
• Loss of lens behind pupil leads to longsightedness (aphakia).
• Displacement/subluxation of the lens leads to astigmatism.

Functions of the lens

• To focus a clear image on the retina
• It protects the eye by filtering out UV light.
• For accommodation.