Simplified Phacoemulsification Navneet Toshniwal
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Anatomy and Development of LensCHAPTER 1

 
INTRODUCTION
  • The lens is a transparent, biconvex crystalline structure in the eye devoid of blood vessels and nerves that, along with the cornea, helps to refract light to be focused on the retina
  • The lens, by changing shape, function towards changing the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina
  • This adjustment of the lens is known as accommodation
  • In humans, the refractive power of the lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power
  • The lens is part of the anterior segment of the eye
  • Anterior to the lens is the iris, which regulates the amount of light entering into the eye
  • The lens is suspended in place by the zonular fibers, which are attached to the lens near its equatorial line and connect the lens to the ciliary body
  • Posterior to the lens is the vitreous body
  • The lens has an ellipsoid, biconvex shape. The anterior surface is less curved than the posterior
  • In adult, the lens is 10 mm in diameter and has an axial length of about 4–5 mm
  • Figure 1.1 shows the anatomy of the lens.
 
LENS CAPSULE
  • The lens capsule is a smooth, transparent basement membrane that completely surrounds the lens2
  • The capsule is elastic and composed of collagen
  • It is synthesized by the lens epithelium and its main components are Type IV collagen and sulfated glycosaminoglycans (GAGs)
  • As the capsule is very elastic it causes the lens to assume a more globular shape when not under the tension of the zonular fibers, which connect the lens capsule to the ciliary body
  • The capsule varies from 4 μm to 20 μm in thickness, being the thickest near the equator and the thinnest near the posterior pole
  • The lens capsule may be involved with the higher anterior curvature than posterior of the lens.
 
LENS EPITHELIUM
  • The lens epithelium is located in the anterior portion of the lens between the lens capsule and the lens fibers
  • It is a simple cuboidal epithelium
  • The cells of the lens epithelium regulate most of the homeostatic functions of the lens
  • The cells of the lens epithelium also serve as the progenitors for new lens fibers
  • The Epithelium constantly lays down fibers in the embryo, fetus, infant, and adult, and continues to lay down fibers for lifelong growth.
 
LENS FIBERS
  • The lens fibers form the bulk of the lens
  • They are long, thin, transparent cells, firmly packed, with diameters between 4 μm and 8 μm and length of up to 12 mm long
  • The lens fibers stretch lengthwise from the posterior to the anterior poles and, when cut horizontally, are arranged in concentric layers
  • The middle of each fiber lies on the equator
  • These tightly packed layers of lens fibers are referred to as laminae
  • The lens fibers are linked together via gap junctions and interdi­gitations of the cells that resemble “ball and socket” forms
  • The lens is split into regions depending on the age of the lens fibers of a particular layer
  • Moving outwards from the central, the oldest layer, the lens is split into an embryonic nucleus, the fetus nucleus, the adult nucleus and the outer cortex
  • New lens fibers, generated from the lens epithelium, are added to the outer cortex. Mature lens fibers have no organelles or nuclei.3
 
LENS PROTEINS
  • These proteins are class of Crystallins
  • Crystallins are water-soluble proteins that compose over 90% of the protein within the lens
  • The three main crystallin types found in the human eye are α-, β- and γ-crystallins
  • Crystallins tend to form soluble, high molecular weight aggregates that pack tightly in lens fibers, thus it increases the index of refraction of the lens while maintaining its transparency
  • β and γ crystallins are found primarily in the lens, while subunits of α-crystallin have been isolated from other parts of the eye and the body
  • α-crystallin proteins mainly function to keep the lens transparent
  • Another important factor in maintaining the transparency of the lens is the absence of light scattering cellular organelles
  • Lens fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the lens fibers
  • Disruptions of certain cytoskeleton elements can lead to the loss of transparency.
 
LENS METABOLISM
  • The lens is metabolically active
  • Compared to other tissues in the eye; however, the lens has less energy demand
  • Metabolism is by:
    • Glycolysis (80%)
    • Hexose monophosphate shunt (18%)
    • Krebs cycle (1–2%)
  • The lens receives all of its nourishment from the aqueous humor
  • Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the lens and out of the equatorial regions
  • Glucose is the primary energy source for the lens.
 
DEVELOPMENT OF LENS
  • Development of the human lens begins at 4-mm embryonic stage
  • The lens is derived from the surface of ectoderm4
  • The first stage of lens differentiation takes place when the optic vesicle, which is formed from out pocketing in the neural ectoderm, comes in proximity to the surface ectoderm
  • The optic vesicle induces nearby surface ectoderm to form the lens placode
  • At 4-mm stage, the lens placode is a single monolayer of columnar cells
  • As development progresses, the lens placode begins to deepen and invaginate. As the placode continues to deepen, the opening to the surface ectoderm constricts and the lens cells form a structure known as the lens vesicle
  • By 10-mm stage, the lens vesicle is completely separated from the surface ectoderm
  • After the 10-mm stage, signals from the developing neural retina induces the cells closest to the posterior end of the lens vesicle begin to elongate towards the anterior end of the vesicle
  • These signals also induce the synthesis of crystallins. These elongating cells eventually fill in the lumen of the vesicle to form the primary fibers, which become the embryonic nucleus in the mature lens
  • The cells of the anterior portion of the lens vesicle give rise to the lens epithelium
  • Additional secondary fibers are derived from lens epithelial cells located towards the equatorial region of the lens
  • These cells lengthen anteriorly and posteriorly to encircle the primary fibers. The new fibers grow longer than those of the primary layer, but as the lens gets larger, the ends of the new fibers cannot reach the posterior or anterior poles of the lens. The lens fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. These seams are readily visible and are termed sutures. The suture patterns become more complex as more layers of lens fibers are added to the outer portion of the lens
  • The lens continues to grow after birth, with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the germinative zone. The lens epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin, and then finally lose their nuclei as they become mature lens fibers5
  • From development through early adulthood, the addition of secondary lens fibers results in the lens growing more ellipsoid in shape; after about the age of 20 years; however, the lens grows more round with time.
 
KEY POINT
Put the picture of anatomy of lens in operation theater in first few days of phaco practice.
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FIGURE 1.1: Anatomy of lens