The lens is a biconvex structure located directly behind the posterior chamber and pupil (Fig 2-28). The lens contributes 20.00 D of the 60.00 D of focusing power of the average adult eye. The equatorial diameter is 6.5 mm at birth; it increases in the first 2–3 decades of life and remains approximately 9–10 mm in diameter in adults. The anteroposterior width of the lens is approximately 3 mm at birth and increases after the second decade of life to approximately 6 mm by age 80 years. This growth is accompanied by a shortening of the anterior radius of curvature of the lens, which would increase its optical power if not for a compensatory change in the refractive gradient across the lens substance.
In youth, accommodation for near vision is achieved by ciliary muscle contraction, which moves the ciliary muscle mass forward and inward. This contraction relaxes zonular tension and allows the lens to assume a globular shape, causing its anterior radius of curvature to shorten (Fig 2-29). With age, accommodative power is steadily lost. At age 8 years, the power is 14.00 D. By age 28 years, the accommodative power decreases to approximately 9.00 D, and it decreases further to 1.00 D by age 64 years. Causes of this power loss include the increased size of the lens, altered mechanical relationships, and the increased stiffness of the lens nucleus secondary to changes in the crystalline proteins of the fiber cytoplasm. Other factors, such as alterations in the geometry of zonular attachments with age and changes in lens capsule elasticity, may also play a role.
The lens lacks innervation and is avascular. After regression of the hyaloid vasculature during embryogenesis, the lens depends solely on the aqueous and vitreous for its nourishment. From embryonic life on, it is entirely enclosed by a basal lamina, the lens capsule. See also BCSC Section 11, Lens and Cataract.
The lens capsule is a product of the lens epithelium (see Fig 2-28). It is rich in type IV collagen and other matrix proteins. Synthesis of the anterior lens capsule (which overlies the epithelium) continues throughout life so that its thickness increases. Because there are no lens epithelial cells posteriorly, the thickness of the posterior capsule remains constant. Values of 15.5 μm for the thickness of the anterior capsule and 2.8 μm for the posterior capsule have been cited for the adult lens, although these values may vary among individuals and based on the location within the capsule.
Figure 2-28 Microscopic appearance of the adult lens.
(Courtesy of Tatyana Milman, MD, except for lower right image, which is courtesy of Nasreen A. Syed, MD.)
Morphologically, the lens capsule consists of fine filaments arranged in lamellae, parallel to the surface (see Fig 2-29). The anterior lens capsule contains a fibrogranular material, identified as laminin, which is absent from the posterior capsule at the ultrastructural level. The thinness of the posterior capsule creates a potential for rupture during cataract surgery.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.