As new lens fibers form, they compact previously formed fibers, with the older layers toward the center, surrounding the central embryonic and fetal nuclei formed during embryonic development (see Fig 2-29). There is no definite morphologic distinction, but rather a gradual transition between the nucleus and cortex of the lens. The terms endonucleus, nucleus, epinucleus, and cortex refer to potential differences in appearance and behavior of the layers during surgical procedures.
In optical section with the slit lamp, lamellar zones of discontinuity are visible in the cortex. The fiber cells are hexagonal in cross section; have a spindle shape; and possess numerous interlocking, fingerlike projections. Apart from the most superficial cortical fibers, the cytoplasm is homogeneous and contains few organelles. The high refractive index of the lens results from the high concentration of lens crystallins (α, β, and γ) in the fiber cytoplasm.
Lens sutures are formed by the interdigitation of the anterior and posterior tips of the spindle-shaped fibers. In the fetal lens, this interdigitation forms the anterior Y-shaped suture and the posterior inverted Y–shaped suture. As the lens ages, further branches are added to the sutures; each new set of branch points corresponds to the appearance of a fresh optical zone of discontinuity.
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.