Figure 9-15 Crystalline deposits of calcium oxalate are noted within the lens (arrows). A cortical cleft with morgagnian

globules (arrowheads) is also seen. (Courtesy of Tatyana Milman, MD.)

The crystalline lens is a soft, elastic, avascular, biconvex structure that in the adult measures approximately 9–10 mm in diameter and 5 mm anteroposteriorly (Fig 9-1). The lens is derived from surface ectoderm. See BCSC Section 11, Lens and Cataract, for discussion of the structure, embryology, and pathology of the lens.

Capsule

The lens capsule surrounds the entire lens (Fig 9-2). It is a thick basement membrane elaborated by the lens epithelial cells and composed, in part, of type IV collagen fibers. The lens capsule is thickest anteriorly (12–21 μm) and peripherally near the equator and thinnest posteriorly (2–9 μm). The capsule provides insertions for the zonular fibers and plays an important part in molding the lens shape in accommodation.

Epithelium

The lens epithelium is derived from the cells of the original lens vesicle that did not differentiate into primary lens fibers. The anterior or axial lens epithelium forms a single layer of cuboidal cells with their basilar surface toward the anterior lens capsule, whereas the equatorial, mitotically active cells appear more elongated as they differentiate into lens fibers. Epithelial cells are not normally observed posterior to the lens equator (see Fig 9-2).

Cortex and Nucleus

The center of the lens contains the oldest lens fibers, the embryonic and fetal lens nucleus, while the outer cortical fibers are derived from postnatally differentiated lens epithelial cells. New lens fibers are continuously laid down from the outside as the lens epithelial cells differentiate. Differentiation of lens epithelial cells into cortical lens fibers occurs at the equator. In this region, termed the equatorial lens bow, the lens epithelial cells move centrally, elongate, acquire crystallins, lose organelles, and transform into cortical lens fibers. As is the case clinically, the histologic demarcation between the nucleus and cortex is not well defined (see Fig 9-2).

Zonular Fibers

The lens is supported by the zonular fibers that attach to the anterior and posterior lens capsule in the midperiphery (see Fig 9-2 and Chapter 7, Fig 7-7). These fibers hold the lens in place through their attachments to the ciliary body processes.

Congenital Anomalies

See BCSC Section 6, Pediatric Ophthalmology and Strabismus, and Section 11, Lens and Cataract, for discussion of ectopia lentis and congenital cataract, as well as additional discussion of the following topics.