When visual function or acuity is declined because of cataract, a general method to improve the impaired vision is a surgery. In the current cataract surgeries, an intraocular lens is inserted into the capsular bag after removal of the central anterior lens capsule, lens nucleus and cortex, so as to substitute for the refractive function of the crystalline lens.
As one of the postoperative problems, opacification frequently occurs along the capsular bag, and visual function or acuity is impaired if the opacity reaches the pupillary area. This is called secondary cataracts or after-cataracts. The reduction of visual function due to the secondary cataracts has been a serious problem in the medical field though there have been no effective preventions for surely avoiding occurrence of the secondary cataract.
Clinically significant secondary cataracts are classified into two types, namely, capsular fibrosis and Elschnig's pearls. The capsular fibrosis is white-color fibrous opacity composed of extracellular matrices such as collagens. Some lens epithelial cells transdifferentiate into myofibroblast-like cells after cataract surgery, and they produce a large quantity of extracellular matrices including collagens, which cause fibrous opacity to appear inside and outside of the capsular bag.
The lens epithelial cells exist only under the anterior lens capsule in normal conditions. During cataract surgery, a circular opening is made at the center of the anterior lens capsule. This is called an anterior capsulotomy. The transdifferentiation of lens epithelial cells into myofibroblast-like cells mainly occurs around the incised edge of the anterior capsule. The anterior capsulotomy margin adheres to the posterior lens capsule through the extracellular matrices secreted by the transdifferentiated cells. When a large number of cells transdifferentiate around the anterior capsulotomy margin, the fibrous opacity composed of extracellular matrices can reach the pupillary area and impair the visual function.
Since the entire anterior capsulotomy margin adheres to the posterior capsule, a closed space surrounded by the anterior and posterior lens capsules is formed outside the adhered region. In this closed space, the lens epithelial cells can proliferate, differentiate into lens fiber cells, and a crystalline lens gradually regenerates. The bulk of regenerated lens is composed of newly formed lens fiber cells. If the closed space is filled with the regenerated lens fiber cells, these cells are extruded from the closed space to the open space on the posterior capsule (the posterior chamber) through the holes in the fibrous net of extracellular matrices at the adhesive region around the capsulotomy margin. They expand on the posterior lens capsule, where they are exposed to the aqueous humor. Due to the influence of the aqueous humor, the extruded lens fiber cells swell and induce fusion of plasma membrane, and form so-called Elschnig's pearls, which undesirably scatter the light passed through the cornea and disturb visual function.