Fig. 2.1
Cross section of the crystalline lens showing its relationship to the adjacent ocular structures and the zones of optical discontinuity
2.2 Development
It develops from an invagination of the surface ectoderm of the fetus, so that what was originally the surface of the epithelium comes to lie in the center of the lens, the peripheral cells corresponding to the basal cells of the epidermis. The lens grows by the proliferation of the peripheral cells. The older cells undergo sclerosis and conglomerate together in the center, while the newly formed cells elongate into fibers. The lens has no innervation or blood supply after its fetal development. It receives nourishment and disposes off its metabolic waste via the surrounding aqueous and vitreous humor.
2.3 Structure
The lens is histologically composed of three structures, namely, capsule, anterior subcapsular epithelium, and lens substance (cortex and nucleus).
2.3.1 Capsule
The lens capsule is an elastic, transparent basement membrane composed of type IV collagen and sulfated glycosaminoglycans laid down by the epithelial cells. The capsule contains the lens substance and is capable of molding it during accommodative changes. The outer layer of the lens capsule, the zonular lamella, also serves as a point of attachment for the zonular fibers. The lens capsule is thickest in the anterior and posterior pre-equatorial zones (20 μm) and thinnest in the region of the central posterior pole (2–4 μm) [4]. This extreme thinness of the posterior capsule makes it more vulnerable for posterior capsular rupture during cataract surgery. Thickness of the anterior lens capsule increases with age, whereas thickness of the posterior capsule remains constant or changes slightly. The capsule is brittle, and though it has no elastic tissue, it is highly elastic in nature because of lamellar or fibrillar arrangement of fibers. This property of the lens gradually decreases with age.
2.3.2 Anterior Subcapsular Epithelium
Immediately beneath the anterior lens capsule is a monolayer of undifferentiated transparent, columnar, nucleated lens epithelial cells. These cells are metabolically active and carry out all normal cellular activities, including the DNA, RNA, protein, and lipid synthesis [5]. The energy demands of the lens are met by the adenosine triphosphate generated by these cells. The epithelial cells show greatest mitotic activity (replicative or S-phase) DNA synthesis in a ring around the anterior lens known as the germinative zone. These newly formed cells migrate toward the equator, where they differentiate into fibers. As the epithelial cells migrate toward the bow region of the lens, they begin the process of terminal differentiation into lens fibers. The fibers run in a curved manner from the septa on the anterior surface to that on the posterior surface. No fibers pass from pole to pole; they are arranged in such a way that those which begin near the pole on one surface of the lens end near the peripheral extremity of the plane on the other, and vice versa (Fig. 2.2).
