Average measurements of adult lens
- Thickness = 4-5 mm
- Diameter = 10 mm
- Radius of curvature of the anterior surface = 10 mm
- Radius of curvature of the posterior surface = 6 mm
- Dioptric power = 17 D
- Refractive index for Cortex = 1.386 & Nucleus = 1.406
STRUCTURE OF LENS
- The lens is a biconvex, transparent structure located immediately in front of the vitreous body and behind the iris.
- The center points on its anterior and posterior surfaces are referred to as the anterior and posterior poles, respectively. A line joining these two poles forms the axis of the lens.
- The marginal circumference of the lens is called the equator.
- Male lenses are heavier than female lenses.
The lens is made up of 3 parts
- Lens capsule
- Lens epithelium
- Lens fibres
- It is a transparent acellular elastic basement membrane that grows in size to accommodate the ever-enlarging lens.
- It is thickest on the anterior and posterior surfaces close to the equator (20 microns) and thinnest at the posterior pole (3 microns).
- The thick basement membrane is formed by the lens epithelium anteriorly and by superficial lens fibres posteriorly.
- By light microscopy the capsule appears homogeneous, but electron microscopy reveals a fibrillar structure with a more dense outer layer (the zonular lamella) into which the zonules penetrate.
- The lens capsule is basically composed of type IV collagen combined with about 10% glycosaminoglycan.
- Lens epithelium is a single layer of cuboidal cells that lie immediately beneath the anterior and equatorial region of the lens capsule.
- At the equator, these cells elongate and form columnar cells, which become arranged in meridional rows. It is at the equator that lens epithelial cells become transformed into lens fibres.
- At the equator, lens mitotic activity is at a maximum.
- Lens fibrres consitute the main mass of the lens. The fibres are formed by multiplication and differentiation of the lens epithelial cells at the equator. .
- Each elongated lens cell is called a fiber. It is hexagonal in cross section and very long measuring abt 10 nm.
- As each lens fiber moves to the interior of the lens, they lose their organelle.
- As the lens increases in size, the lens fibres are unable to stretch the anteroposterior distance so that progressively more complicated suture patterns are formed.
- Embryonic nucleus: the earliest fibre mass in the center of the lens – first 3 months of gestation.
- Fetal nucleus: formed later with its Y-shaped sutures. – 3 to 8 months of gestation.
- Adult nucleus: is composed of fibres formed after birth.
Suspension of the lens
- The lens is held in position by a series of delicate radially arranged fibres collectively known as the suspensory ligament of lens or zonule.
- The zonlues arise from the epithelium of the ciliary processes and are attached into the outer prt of the lens capsule near the equator of lens.
- Accommodation and presbyopia: When viewing near objects, the ciliary muscle contracts and the meridional fibres pull the choroid and ciliary body forward and the circular fibres; acting as a sphincter move the ciliary body inward, the zonules relax and the elastic lens assumes a more globular shape.
BUT with advancing age, the lens becomes less elastic, leading to so decreased accommodation, which causes presbyopia.
2. Cataract – senile, congenital, DM, trauma, complicated etc..