By Leos Kral
In order to better understand what cataracts are and how different types of cataracts are named, it is necessary to not only understand the anatomy of the eye lens but also how the lens develops. This article describes the main process of lens development.
During embryonic development, the lens first takes form as a hollow ball of cells as shown in the drawing below. Note that each cell contains a nucleus which contains chromosomes that contain DNA, the genetic material. The ball of cells is oriented in the developing eye in such a way that one surface is pointing toward the outside of the eye and the opposite surface is pointing toward the interior of the eye. The outside facing surface is the anterior side of the developing lens and the inside facing surface is the posterior side of the developing lens. Note that the cells are surrounded by a capsule of proteinaceus material. This lens capsule surrounds both the developing lens as well as the adult lens but is not shown in most of the following drawings.
The first step in lens development is the elongation of the posterior cells into the cavity towards the anterior cell layer. The elongating cells form the primary lens fibers. As the cells elongate, they also make a large quantity of a variety of proteins called crystallins. These proteins will become the primary structural component of each primary lens fiber.
The primary lens fibers soon loose their nuclei and other cellular organelles and become inert structures as indicated by a change in color from gray to blue (see drawing below). Note, however, the anterior cells persist in the developing lens as a living single layer of cells along the anterior surface. These cells will continue to divide and the cells along the edges (such as the red cell) will begin to form secondary lens fibers.
To form secondary lens fibers, the anterior cells at the edges of the cell layer begin to grow as shown in the drawing below. Specifically, the red cells at the top and bottom edges begin to form secondary lens fibers by elongating along the posterior surface of the primary lens fibers. These cells also elongate along the anterior surface of the primary lens fibers, between the primary lens fibers and the anterior layer of cells. As the red cells are elongating note that the next cells (colored light brown) are getting ready to also elongate.
In the drawing below the red cells have finishes forming the secondary lens fibers. Note that these secondary lens fibers form a complete ring around the primary lens fibers. The brown colored cells are elongating to form additional secondary lens fibers forming a second ring around the primary lens fibers. Note that the next set of cells (colored bright red) are getting ready to also elongate. Also note that the anterior cells have multiplied by cell division. This is necessary because new cells must be made available for secondary lens fiber formation and the layer of cells must continue to cover the entire anterior side of the developing lens as the lens increases in size.
The drawing below shows that two secondary lens fibers have formed and a third secondary lens fiber is elongating. This process repeats over and over until many layers of secondary fibers have formed. As the secondary fibers grow and mature, they also make a large quantity of a variety of proteins called crystallins. These proteins will become the primary structural component of each secondary lens fiber.
The last drawing below shows a developed lens. Note that the anterior cells continued to multiply as the lens grew in size thus an anterior cell layer continues to cover the entire anterior surface of the lens. The original primary lens fibers persist to form the nucleus (light blue) of the lens and the cortex of secondary lens fibers has increased in size to become the majority of lens material. For reference, the three original secondary lens fibers are color coded to the previous drawings. Newer lens fibers are colored darker blue. Note that only the outer most secondary lens fibers contain nuclei. As the secondary fibers produce more crystalline proteins and mature, they lose their nuclei and other cellular organelles and become inert structures.
New secondary lens fibers continue to form throughout the life of the individual. While the lens does not increase significantly in size in the adult, the density of secondary lens fibers increases. This also cause some compression of the primary lens fiber nucleus.
Copyright 1998, 1999 Leos Kral. Australian Shepherd Health and Genetics Information Resource and Health Registry. All rights reserved.