Immediately posterior to the anterior lens capsule is a single layer of epithelial cells. These cells are metabolically active and carry out all normal cell activities, including biosynthesis of DNA, RNA, protein, and lipid. They also generate adenosine triphosphate to meet the energy demands of the lens. The epithelial cells are mitotic; the greatest activity of premitotic (replicative, or S phase) DNA synthesis occurs in a ring around the anterior lens known as the germinative zone. The newly formed cells migrate toward the equator, where they differentiate into fibers. This area, called the bow region, is where the epithelial cells begin the process of terminal differentiation into lens fibers (Fig 2-5).
During this differentiation, perhaps the most dramatic morphologic change occurs when the epithelial cells elongate to form lens fiber cells. This elongation is associated with a tremendous increase in the mass of cellular proteins in the fiber cell membrane. At the same time, the cells lose organelles, including nuclei, mitochondria, and ribosomes. The loss of these organelles is optically advantageous, because light passing through the lens is no longer absorbed or scattered by these structures. However, because these new lens fiber cells lack the metabolic functions previously carried out by the organelles, they are now dependent on glycolysis for energy production (see Chapter 3 in this volume).
Figure 2-5 Schematic of the mammalian lens in cross section. Arrow indicates the direction of cell migration from the epithelium to the cortex.
(Illustration by Mark Miller.)
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.