Cortex and Nucleus
Aside from the single layer of epithelial cells on its anterior surface, the lens is composed of lens fibers, which are long ribbonlike cells. These fibers are formed from epithelial cells at the lens equator; therefore, younger fibers are always exterior to older ones (see Fig 10-1 and Chapter 2, Fig 2-29). The lens structure can be equated with the growth rings of a tree: the oldest cells are in the center, and the progressively younger layers, or shells, of fiber cells are toward the periphery. Unlike the case with many tissues, no cells are sloughed from the lens, and cells produced before birth remain at the center of the lens throughout life. The fiber mass of the adult lens can be divided into the cortex (the outer fibers, laid down after approximately age 20 years) and the nucleus (the cells produced from embryogenesis through adolescence).
As new fiber cells elongate and differentiate into mature fibers, their cell nuclei form the bow zone, or bow region, at the lens equator (see Fig 10-1). Elongating fibers substantially increase their volume and surface area and express large amounts of both lens crystallins (discussed later) and a lens-fiber–specific membrane protein called the major intrinsic protein (MIP). As the fibers become fully elongated and make sutures at each end with fibers that have elongated from the opposite side of the lens, they become mature, terminally differentiated fiber cells. The cell nuclei disintegrate, as do mitochondria and other organelles. This process has been proposed to occur via autophagy, the degradation of the cell’s own unneeded and/or damaged components via a defined intracellular process. There are several types of autophagy, and in each, the degradation is directed toward certain intracellular components:
microautophagy: cytoplasmic material
chaperone-mediated autophagy: proteins that can be recognized by a heat shock protein complex
macroautophagy: cell organelles
mitophagy (a type of macroautophagy): mitochondria
Elimination of cellular organelles is necessary in the central portion of the lens because such bodies are sufficiently large to scatter light and thereby degrade visual acuity. Also, with the loss of cell nuclei, the mature fibers lose the machinery required for synthesis of proteins.
Chai P, Ni H, Zhang H, Fan X. The evolving functions of autophagy in ocular health: a double-edged sword. Int J Biol Sci. 2016;12(11):1332–1340.
Costello MJ, Brennan LA, Basu S, et al. Autophagy and mitophagy participate in ocular lens organelle degradation. Exp Eye Res. 2013;116:141–150.
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.