Key Features

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  Normally transparent at birth.

  
  
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  Increasing opacity with age, infection, surgery, trauma, various metabolic states.

  
  
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  Can alter shape and refractive power to allow for accommodation.

Associated Features

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  Asymmetric oblate spheroid shape.

  
  
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  Avascular.

  
  
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  Located posterior to the iris and anterior to the vitreous body.

  
  
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  Spectral filter.

  
  
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  Suspended by the zonules.

The lens is a transparent structure that has evolved to alters the pathway of the light entering the eye. In 2002, the World Health Organization estimated that lens pathology (cataract) was the most common cause of blindness worldwide, affecting more than 17 million people across the globe. Cataract surgery is the most common surgical procedure performed in the developed world.

The lens is an asymmetric oblate spheroid that is avascular and lacks nerves and connective tissue. It is located posterior to the iris with its anterior surface in contact with the aqueous and the posterior surface with the vitreous. The lens is suspended by the zonular fibers that arise from the ciliary epithelium and insert 1–2 µm into the outer part of the capsule. Histologically the lens consists of three major components: capsule, epithelium, and lens substance ( Fig. 5.1.1 ).

Gross Anatomy of the Adult Human Lens.

Note the different regions are not drawn to scale.

The lens capsule is an acellular envelope that is continuously synthesized by the lens epithelium anteriorly and fiber cells posteriorly. It is composed of a number of stacked lamellae, which contain major structural proteins and fibronectin. The lens epithelium is a single layer of cuboidal cells approximately 10 µm high and 15 µm wide, located beneath the anterior capsule that extends to the equatorial lens bow. Their basal surface adheres to the capsule, whereas their anterior surface abuts the newly formed elongating lens fibers. The proliferative capacity of epithelial cells is greatest at the equator, and cells in the germinative zone are dividing constantly. Here, newly formed cells are forced into the transitional zone where they elongate and differentiate to form the fiber mass of the lens. The bulk of the lens is composed of the nucleus and cortex, which comprise densely packed lens cytoplasm (“fiber cells”) with very little extracellular space.

The lens grows throughout life but at a slower rate with increasing age. The rate of increase in lens weight and equatorial diameter is greater than that of lens thickness. Newly formed fibers are internalized as more are added at the transitional zone of the lens, and thus the newest fibers are in the outer cortex, and the oldest fibers are found in the center of the nucleus. Each growth shell, therefore, represents a layer of fibers that are younger than those in the shell immediately preceding it.

The metabolic needs of the lens are met by the aqueous and the vitreous humor, with the majority of glucose and amino acids coming from the aqueous. The capsule is freely permeable to water, ions, other small molecules, and proteins with a molecular weight up to 70 kDa. In addition, epithelial cells and fibers possess a number of channels, pumps, and transporters that enable transcellular movement.

The lens acts as spectral filter and readily absorbs the energetic ultraviolet (UV) component of the electromagnetic spectrum that, if transmitted, has the potential to damage the retina. The overall transmission of visible light decreases with increasing age, a feature that arises largely from age-related changes and brunescence. The refractive index of the lens increases from 1.386 in the peripheral cortex to 1.41 in the central nucleus. In addition, the curvature of the lens increases in a similar manner. Thus each successive layer of fibers has more refractive power and can bend light rays to a greater extent. When visible light passes through the lens, it is split into all the colors of the spectrum. The different wavelengths of these colors result in differences in refraction (chromatic aberration). As a consequence, yellow light (570–595 nm) normally is focused on the retina, blue (440–500 nm) anteriorly, and red (620–770 nm) posteriorly. The lens is designed to minimize spherical aberration (defocus caused by greater refraction of light striking the peripheral lens compared to the center) in these ways:

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  The refractive index increases from the periphery to the center of the lens.

  
  
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  The curvature of both the anterior and the posterior capsule increases toward the poles.

  
  
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  The curvature of the anterior capsule is greater than that of its posterior counterpart.

  
  
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  Modulation of pupillary size prevents light from striking the periphery of the lens under nonmydriatic conditions.

Accommodation is the process by which the lens changes its optical power by altering its shape and thus its focusing ability. At rest, the ciliary muscle is relaxed and the zonules pull on the lens keeping the capsule under tension and the lens flattened. Accommodation occurs when the ciliary muscle contracts, relaxing the zonules, thus increasing the curvature of the anterior surface and decreasing the radius of curvature from 10 mm to 6 mm. The increase in curvature of the anterior surface increases the refractive power. Accommodation is accompanied by a decrease in pupil size (miosis) and convergence of the two eyes.

Adenosine triphosphate (ATP) is the principal source of energy of the lens, the majority of which comes from the anaerobic metabolism of glucose. Approximately 90%–95% of the glucose that enters the normal lens is phosphorylated to glucose-6-phosphate (G6P) in a reaction catalyzed by hexokinase. G6P is used either in the glycolytic pathway (80% of total glucose) or in the pentose phosphate pathway. The 5%–10% of glucose that is not converted to G6P either enters the sorbitol pathway or is converted into gluconic acid.

The protein concentration within the lens is the highest in the body. The majority of ongoing synthesis generates crystallins and major intrinsic protein 26 (MIP26). The water-soluble crystallins constitute approximately 90% of the total protein content of the lens. The three groups of crystallins can be divided into the α-crystallin family and the β/γ-crystallin superfamily.

The continuous entry of optical radiation into the lens, especially UV (295–400 nm), makes the lens particularly susceptible to photochemical reactions leading to generation of reactive oxygen species (ROS). Protection against damage induced by ROS is achieved by a complex antioxidant system that relies heavily on superoxide dismutase, ascorbate, catalase, and glutathione peroxidase.

Numerous morphological, biochemical, and biophysical changes occur to the lens with age. Most notable are the age-related changes in color (more yellow), light transmission (decreased), consistency (increased hardness), loss of accommodative ability (manifested clinically as presbyopia), and protein aggregation. The resultant lens opacification, referred to as cataract, results in loss of light transmission.

While cataract surgery is safe and commonly performed, a major complication is development of a secondary cataract (posterior capsular opacification or Soemmerring’s ring). Posterior capsular opacification (PCO) is the most common and can be further divided into fibrosis type and pearl type (Elschnig’s pearls). Vision can be affected by blockage of the visual axis (both) or by progressive decentration of the intraocular lens (IOL) due to remnant lens epithelial cell proliferation and migration, epithelial–mesenchymal transition, collagen deposition, and generation of aberrant lens fiber cells. Soemmerring’s ring is often less visually significant, as the trapped and proliferating residual lens epithelial cells are located in the periphery, behind the iris. While currently no definitive prevention exists, newer surgical techniques and IOLs may help to decrease the incidence of new cases. The current standard treatment involves the use of a neodymium:yttrium–aluminum–garnet (Nd:YAG) laser to perform a capsulectomy in the clinical setting.

In conclusion, the lens is a deceptively complex structure that allows for the transmission and refraction of light. An orderly structure, stable metabolic state, and intact antioxidant system are mandatory to maintain clarity. A full understanding of the basic science related to the lens allows for appreciation of the numerous pathologies that affect it and thus their medical and surgical treatment.