2020–2021 BCSC Basic and Clinical Science Course™
2 Fundamentals and Principles of Ophthalmology
Part I: Anatomy
Chapter 2: The Eye
The lens epithelium lies beneath the anterior and equatorial capsule but is absent under the posterior capsule (see Fig 2-28). The basal aspects of the cells abut the lens capsule without specialized attachment sites. The apices of the cells face the interior of the lens, and the lateral borders interdigitate, with practically no intercellular space. Each cell contains a prominent nucleus but relatively few cytoplasmic organelles.
Regional differences in the lens epithelium are important. The central zone represents a stable population of cells whose numbers slowly decline with age. An intermediate zone of smaller cells shows occasional mitoses. Peripherally in the equatorial lens bow area, there are meridional rows of cuboidal preequatorial cells that form the germinative zone of the lens (see Figs 2-28, 2-29). Here, cells undergo mitotic division, elongate anteriorly and posteriorly, and form the differentiated fiber cells of the lens. In the human lens, cell division continues throughout life and is responsible for the continued growth of the lens.
Figure 2-29 Organization of the lens. At areas where lens cells converge and meet, sutures are formed. A, Cutaway view of the adult lens showing an embryonic lens inside. The embryonal nucleus has a Y-shaped suture at both the anterior and posterior poles; in the adult lens cortex, the organization of the sutures is more complex. At the equator, the lens epithelium can divide, and the cells become highly elongated and ribbonlike, sending processes anteriorly and posteriorly. As new lens cells are formed, older cells come to lie in the deeper parts of the cortex. B, Cross section and corresponding surface view showing the difference in lens fibers at the anterior (A), intermediate (B), and equatorial (C) zones. The lens capsule, or basement membrane of the lens epithelium (d), is shown in relation to the zonular fibers (f) and their attachment to the lens (g). C, The diagram shows a closer view of lens sutures. D and E, Optical sections of the lens of a young adult human (25-year-old woman) demonstrated by Scheimpflug photography. The cornea is to the right. The lens is in the nonaccommodative state in part D. The lens is shown during accommodation in part E. Note that the anterior radius of curvature is shortened in the latter case.
(Parts A–C reproduced with permission from Kessel RG, Kardon RH. Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy. San Francisco: WH Freeman; 1979. Parts D and E courtesy of Jane Koretz.)
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.