Podcast
Questions and Answers
What is the direction of lens thickness increase?
What is the direction of lens thickness increase?
What is the primary function of the lens?
What is the primary function of the lens?
What is the source of energy for cellular metabolism and replication in the lens?
What is the source of energy for cellular metabolism and replication in the lens?
What is the role of glutathione in the lens?
What is the role of glutathione in the lens?
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What is the effect of hexokinase absence on glucose metabolism?
What is the effect of hexokinase absence on glucose metabolism?
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What is the age-related change in soluble lens proteins?
What is the age-related change in soluble lens proteins?
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What is the primary cause of blindness worldwide?
What is the primary cause of blindness worldwide?
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What is the characteristic of posterior subcapsular cataract?
What is the characteristic of posterior subcapsular cataract?
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What is the purpose of the Na/K pump in the lens?
What is the purpose of the Na/K pump in the lens?
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What is the effect of oxidative stress on the lens?
What is the effect of oxidative stress on the lens?
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What is the main function of the lens in the eye?
What is the main function of the lens in the eye?
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What is the structure that suspends the lens from the surrounding ciliary body?
What is the structure that suspends the lens from the surrounding ciliary body?
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What is the term for the process by which the lens changes shape to focus on near objects?
What is the term for the process by which the lens changes shape to focus on near objects?
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What is the protein that makes up the majority of the lens fibers?
What is the protein that makes up the majority of the lens fibers?
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What is the term for the junction formed by the lens fibers when they reach the poles?
What is the term for the junction formed by the lens fibers when they reach the poles?
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What is the region of the lens where cell mitosis occurs?
What is the region of the lens where cell mitosis occurs?
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What is the term for the layer of cuboidal cells that forms new fiber cells?
What is the term for the layer of cuboidal cells that forms new fiber cells?
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What is the shape of the lens fibers in cross-section?
What is the shape of the lens fibers in cross-section?
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What is the term for the barrier function that prevents large molecules from entering the lens?
What is the term for the barrier function that prevents large molecules from entering the lens?
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What is the term for the region of the lens that contains fibers formed after sexual maturation?
What is the term for the region of the lens that contains fibers formed after sexual maturation?
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Study Notes
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
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Description
Understand the formation and development of the lens in embryology, including the lens placode, lens pit, and lens vesicle stages.
