The lens is enclosed in an elastic basement membrane called the lens capsule (Fig 10-1; see also Chapter 2, Fig 2-28). The capsule is acellular and is composed primarily of type IV collagen; it contains smaller amounts of other collagens and extracellular matrix components (including glycosaminoglycans, laminin, fibronectin, and heparan sulfate proteoglycan). The capsule is thicker on the anterior surface of the lens, where the epithelial cells continue to secrete capsular material throughout life. On the posterior surface of the lens, where there is no epithelium, the posterior fiber cells have limited capacity to secrete such material and the capsule is relatively thinner. The zonular fibers, from which the lens is suspended, insert into the capsule near the equator on both the anterior and posterior aspects. The capsule is not a barrier to diffusion of water, ions, small molecules, or proteins up to the size of serum albumin (which has a molecular weight of 68,000).
Figure 10-1 Cross section of the mammalian lens, demonstrating the central nucleus and its growth over time. Near the equator is the bow region, where the lens-fiber cells elongate until their 2 ends meet. At this point, they are fully mature and as they are pushed inward by newer fibers, they lose their nuclei and organelles, making up the lens cortex. There is no shedding of these fibers over time. Thus, the lens continues to increase in size throughout life.
(Reproduced with permission from Friedman NJ, Kaiser PK, Trattler WB. Review of Ophthalmology. 3rd ed. Edinburgh: Elsevier; 2018:288.)
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.