ANATOMY OF CRYSTALLINE LENS – High Yield Notes

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

  1. Lens capsule
  2. Lens epithelium
  3. Lens fibres

Lens capsule

  • 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

  • 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 fibres

  • 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.

APPLIED ANATOMY

  1. 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..

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