Lens Development in Embryology

Lens Development

• Lens development begins on the 27th day, while optic vesicles are forming
• The adjacent surface ectoderm thickens to form the lens placode, which invaginates to form the lens pit
• The lens pit separates from the surface ectoderm to form the lens vesicle
• The lens vesicle contains anterior lens epithelium and posterior lens epithelium
• Posterior lens epithelium differentiates into primary lens cells and elongates anteriorly as fibers to fill the lumen
• Primary lens fibers produce crystallins

Lens Structure

• The lens is a highly organized, transparent structure
• Comprises three parts: capsule, lens epithelium, and lens fibers
• The lens is avascular, transparent, and elliptic in shape
• Located in the posterior chamber, anterior to the vitreous chamber, and posterior to the iris
• Suspended from the surrounding ciliary body by zonular fibers

Lens Dimensions

• Biconvex, with an anterior radius of curvature of 8-14 μm and a posterior radius of curvature of 5-8 μm
• Thickness is 3.5-5 mm, with an increase of 0.02 mm per year
• Lens diameter is 6.5 mm in infants, 9 mm in teenagers, and does not change significantly after that

Lens Composition

• Refractive power is 20 D in the unaccommodated lens
• 1/3 of the lens is protein, and 2/3 is water
• pH is 6.9
• Protein content varies among the lens, with a higher refractive index in the nucleus (1.50) and less in the outer cortical surface (1.37)

Lens Capsule

• Elastic acellular envelope that allows passage of small molecules
• Provides a barrier function, preventing large molecules from entering the lens
• Thickness varies among regions, with the thickest region at the equator (21-23 μm) and the thinnest region at the posterior pole (4 μm)

Lens Epithelium

• Anterior lens epithelium is cuboidal, with cells that secrete the anterior capsule and have metabolic transport mechanisms
• Posterior lens epithelium is absent, but was used during embryologic development to form primary lens fibers
• Germinal zone is the region of cell mitosis and differentiation in the lens fibers

Lens Fibers

• All fibers are formed from mitosis in the germinative zone
• Fibers lose their nuclei and organelles as they elongate and become compacted
• Fibers are hexagonal in shape, arranged in concentric rings, and have dimensions of 3 x 9 μm
• Fiber cytoplasm contains high concentrations of crystallins (90%), which contribute to the gradient refractive index

Divisions of the Lens

• Embryonic nucleus is the center of the lens, formed by the elongating posterior epithelium of primary lens fibers
• Fetal nucleus includes the embryonic nucleus and fibers formed before birth
• Adult nucleus includes the embryonic and fetal nuclei and fibers formed from birth to sexual maturation
• Lens cortex contains fibers formed after sexual maturation, divided into superficial, internal, and deep zones

Sutures

• Anterior suture is formed by the apical aspects of the fibers, with an upright Y shape
• Posterior suture is formed by the basal aspects of the fibers, with an inverted Y shape

Zonules of Zinn

• Group of thread-like fibers that attach the lens to the ciliary body
• Formed from the basement membrane of the nonpigmented ciliary epithelium in the pars plana
• Divide into primary and secondary zonules

Accommodation

• Change in lens shape by contraction of the ciliary muscle, increasing the lens power
• Lens thickness increases anterior to posterior, and the lens thins along the equator
• Anterior lens surface moves forward, and the anterior chamber becomes shallower

Lens Physiology

• Primary function of the lens is refraction of light and transparency
• Transparency is due to the absence of blood vessels, few cellular organelles, and orderly arrangement of fibers
• Metabolic activity occurs mostly in the anterior epithelium, maintaining cell and fiber function

Lens Metabolism

• Obtains glucose from the aqueous humor
• ATP production is mostly via anaerobic metabolism
• ATP activity is highest in the epithelial cells and newer fibers of the cortex
• Lens is constantly pumping out water to maintain the correct optical constituents

Regulation of Lens Proteins

• Glutathione is the primary protector against oxidative damage
• Ascorbic acid prevents oxidative damage and has an anti-cataract effect

Age Changes in Lens

• Decrease in soluble lens proteins, ATP content, and glutathione activity
• Increase in Ca, Na, and H2O, leading to disruption of ion balance and cell damage

Clinical Manifestations of Aging

• Presbyopia: loss of accommodative ability, inability to focus at near distances
• Cataract: any lens opacity, with multiple factors influencing lens metabolism to cataract development