Biology: The Lens and Accommodation
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Questions and Answers

What is required to achieve greater refractive power in a lens?

  • Decreased lens size
  • Increased water content
  • Lower protein concentration
  • Higher protein concentration (correct)
  • What is the primary function of the Lens in the human eye?

  • To allow the eye to focus at different distances (correct)
  • To regulate the amount of light entering the eye
  • To protect the eye from external damage
  • To change the color of the light entering the eye
  • In which type of fish does the lens move closer to the retina for near vision?

  • Both a and b
  • Neither a nor b
  • Teleost (Bony) fish
  • Cartilaginous fish (correct)
  • What is the main function of the Pecten in the avian eye?

    <p>To nourish the retina and control vitreous body pH</p> Signup and view all the answers

    What is the shape of the lens in adults, and what is its diameter?

    <p>Biconvex, 10 mm</p> Signup and view all the answers

    What is the shape of the lens in fish?

    <p>Spherical</p> Signup and view all the answers

    How do some species, like fish, accommodate for near and far vision?

    <p>By moving the lens or retina laterally</p> Signup and view all the answers

    What is the composition of the lens in diving ducks?

    <p>Lower protein content, more aqueous</p> Signup and view all the answers

    What is the primary component of the zonules of Zinn?

    <p>Fibrillin, a type of connective tissue with elastic properties</p> Signup and view all the answers

    What happens to the shape of the lens when the ciliary muscles contract during accommodation?

    <p>The lens becomes more globular</p> Signup and view all the answers

    What is the structure of the lens capsule?

    <p>A layer of collagen fibrils embedded in a glycosaminoglycan matrix</p> Signup and view all the answers

    Why does the cornea have very little refractive power underwater?

    <p>Because the cornea is less dense than water</p> Signup and view all the answers

    What is the shape of the lens fibres in cross-section?

    <p>Hexagonal</p> Signup and view all the answers

    What is the main difference between cortical and nuclear fibres?

    <p>The presence or absence of a nucleus</p> Signup and view all the answers

    What is the purpose of the high concentration of gap junctions in lens fibres?

    <p>To facilitate metabolic communication</p> Signup and view all the answers

    What is the reason for the difference in protein concentration between the lens nucleus and cortex?

    <p>To regulate refractive index</p> Signup and view all the answers

    How does the lens grow throughout life?

    <p>By continually adding new fibres to the central portion</p> Signup and view all the answers

    What is the diameter of the lens at birth?

    <p>6.5 mm</p> Signup and view all the answers

    What is the process by which lens epithelial cells transform into lens fibres?

    <p>The process involves the elongation and differentiation of epithelial cells at the equator, where they form columnar cells that become arranged in rows.</p> Signup and view all the answers

    How do lens fibres grow and push under the epithelium and capsule?

    <p>The apical surface of the cell grows and pushes anteriorly, while the basal surface grows and pushes posteriorly, eventually meeting opposite fibers at irregular sutures.</p> Signup and view all the answers

    What is the term for the alignment of newly produced lens fibre nuclei in the lens cortex?

    <p>Lens bow</p> Signup and view all the answers

    How do lens fibres in the adult lens differ from those in the embryonic lens?

    <p>Adult lens fibres are secondary lens fibres, whereas embryonic lens fibres are primary lens fibres.</p> Signup and view all the answers

    What is the significance of the equatorial region in lens fibre formation?

    <p>The equatorial region is where epithelial cells elongate and differentiate into lens fibres.</p> Signup and view all the answers

    What is the relationship between lens fibre growth and the capsule?

    <p>Lens fibres push under the capsule, with the anterior surface pushing anteriorly and the posterior surface pushing posteriorly.</p> Signup and view all the answers

    What is the significance of the distinct arrangement of lens fibres at the poles, and how does this arrangement facilitate the lens's function?

    <p>The lens fibres do not meet at the poles, but instead, form irregular lines called sutures. This arrangement allows for a smooth, continuous curvature of the lens, which is essential for its refractive function.</p> Signup and view all the answers

    What is the primary difference between cortical and nuclear fibres, and how do these differences impact their functions?

    <p>Cortical fibres have a nucleus and organelles, whereas nuclear fibres lack these structures. This difference affects the metabolism and function of the fibres, with cortical fibres being more active and nuclear fibres being more compact and dense.</p> Signup and view all the answers

    How do the lens fibres communicate with each other, and what is the significance of this communication?

    <p>Lens fibres communicate with each other through a high concentration of gap junctions, which enable metabolic communication and coordination between the fibres. This communication is essential for the maintenance of the lens's integrity and function.</p> Signup and view all the answers

    What is the mechanism by which the lens grows throughout life, and what is the significance of this growth?

    <p>The lens grows throughout life by continually adding new lens fibres to the central portion of the lens, without discarding any of its cells. This growth allows the lens to maintain its function and adapt to changes in the environment.</p> Signup and view all the answers

    How do the lens fibres differ in terms of protein concentration, and what is the significance of this difference?

    <p>The lens nucleus has a higher protein concentration than the cortex, with a gradient of 200-300 mg/ml in the nucleus and 100-150 mg/ml in the cortex. This difference in protein concentration creates a refractive index gradient, which is essential for the lens's ability to focus light.</p> Signup and view all the answers

    What is the significance of the lens being transparent and biconvex in structure?

    <p>The transparent and biconvex structure of the lens allows it to focus light at different distances, enabling accommodation.</p> Signup and view all the answers

    What is the significance of the tight packing and interlocking of lens fibres, and how does this arrangement facilitate the lens's function?

    <p>The tight packing and interlocking of lens fibres allows for a compact and rigid structure, which enables the lens to maintain its shape and refractive function. This arrangement also facilitates the transmission of light through the lens.</p> Signup and view all the answers

    How does the lens grow throughout life, and what is the rate of this growth?

    <p>The lens grows throughout life, with the axis length increasing at a rate of 0.023mm/year.</p> Signup and view all the answers

    What is the purpose of the zonules of Zinn, and what are they composed of?

    <p>The zonules of Zinn are fibers that hold the lens in place, and they are composed of fibrillin, a connective tissue with elastic properties.</p> Signup and view all the answers

    What is the relationship between the equator of the lens and the ciliary processes of the ciliary body?

    <p>The equator of the lens is encircled by the ciliary processes of the ciliary body, and lies 0.5 mm from them.</p> Signup and view all the answers

    What happens to the shape of the lens when the ciliary muscles contract during accommodation, and what is the effect on the pupil?

    <p>When the ciliary muscles contract, the lens assumes a more globular shape, and the pupil becomes smaller.</p> Signup and view all the answers

    Why is the lens, unlike the cornea, able to change its dioptric power, allowing it to focus on distant or near objects?

    <p>The lens is able to change its dioptric power because it can change its shape, allowing it to focus on objects at different distances.</p> Signup and view all the answers

    What is the significance of the hexagonal cross-section of lens fibres, and how does it contribute to their compact packing?

    <p>The hexagonal cross-section of lens fibres allows for tight packing and interlocking, which facilitates the lens's function by minimizing space between fibres and reducing light scattering.</p> Signup and view all the answers

    How do the nucleated cortical fibres and non-nucleated nuclear fibres differ in structure, and what is the significance of these differences?

    <p>Cortical fibres have a nucleus and organelles, whereas nuclear fibres lack them; this difference is significant because it affects protein concentration and refractive index.</p> Signup and view all the answers

    What is the role of gap junctions in lens fibre communication, and how do they facilitate metabolic communication?

    <p>Gap junctions enable direct cell-to-cell communication, allowing for metabolic cooperation and coordination between lens fibres.</p> Signup and view all the answers

    How do lens fibres grow throughout life, and what is the significance of this continuous growth?

    <p>Lens fibres grow throughout life, with new fibres continually added to the central portion of the lens; this growth is significant because it allows the lens to maintain its function and adapt to changes in the eye.</p> Signup and view all the answers

    What is the significance of the alignment of newly produced lens fibre nuclei in the lens cortex, and how does this alignment facilitate the lens's function?

    <p>The alignment of lens fibre nuclei in the cortex enables the lens to maintain its shape and function, allowing it to focus light efficiently.</p> Signup and view all the answers

    How do the lens fibres differ in terms of protein concentration, and what is the significance of this difference?

    <p>The nucleus has a higher protein concentration than the cortex, which affects the refractive index and the lens's ability to focus light.</p> Signup and view all the answers

    Describe the process of lens fibre formation at the equator of the lens.

    <p>The epithelial cells elongate and form columnar cells, which become arranged in rows. The lens cells elongate and turn, so that their long axis is parallel to the surface of the lens.</p> Signup and view all the answers

    What is the significance of the alignment of newly produced lens fibre nuclei in the lens cortex, and what is this arrangement known as?

    <p>The alignment of newly produced lens fibre nuclei in the lens cortex is significant for the structure and function of the lens. This arrangement is known as the lens bow.</p> Signup and view all the answers

    How do lens fibres grow and push under the epithelium and capsule, and what is the significance of this process?

    <p>Lens fibres grow and push under the epithelium and capsule through a process of cell elongation and multiplication. This process is significant for the growth and development of the lens throughout life.</p> Signup and view all the answers

    What is the significance of the gap junctions in lens fibres, and how do they facilitate communication between fibres?

    <p>Gap junctions in lens fibres facilitate communication between fibres, allowing for the exchange of ions and small molecules. This communication is significant for maintaining the homeostasis and osmolarity of the lens.</p> Signup and view all the answers

    Describe the structure of lens fibres in the adult lens, and how do they differ from those in the embryonic lens?

    <p>Adult lens fibres are elongated cells that form the main mass of the lens. They differ from embryonic lens fibres in terms of their structure and function, with adult fibres being more organized and specialized.</p> Signup and view all the answers

    What is the significance of the equatorial region in lens fibre formation, and how does it contribute to the growth of the lens?

    <p>The equatorial region is significant for lens fibre formation, as it is the site where epithelial cells elongate and form columnar cells. This region contributes to the growth of the lens by producing new fibres that push under the epithelium and capsule.</p> Signup and view all the answers

    What is the significance of the distinct arrangement of lens fibres at the poles, and how does it facilitate the lens's function?

    <p>The distinct arrangement of lens fibres at the poles allows for efficient transmission of light and facilitates the lens's function in focusing light.</p> Signup and view all the answers

    What is the mechanism by which lens epithelial cells transform into lens fibres, and what is the significance of this process?

    <p>Lens epithelial cells transform into lens fibres through a process of fibre formation, which involves cell elongation, nucleus migration, and fibre differentiation; this process is crucial for the growth and development of the lens.</p> Signup and view all the answers

    How do lens fibres grow throughout life, and what is the significance of this growth?

    <p>Lens fibres grow throughout life through the process of fibre formation and differentiation, which allows the lens to accommodate for near and far vision; this growth is significant for maintaining proper vision.</p> Signup and view all the answers

    What is the purpose of the high concentration of gap junctions in lens fibres, and how does it facilitate lens fibre communication?

    <p>The high concentration of gap junctions in lens fibres allows for direct cell-to-cell communication and coordination of fibre function, facilitating proper lens function.</p> Signup and view all the answers

    What is the significance of the alignment of newly produced lens fibre nuclei in the lens cortex, and how does it facilitate lens function?

    <p>The alignment of newly produced lens fibre nuclei in the lens cortex allows for proper fibre organization and facilitates the lens's function in focusing light.</p> Signup and view all the answers

    What is the relationship between lens fibre growth and the capsule, and how does it impact lens development?

    <p>Lens fibre growth is influenced by the capsule, which provides a framework for fibre organization and differentiation; this relationship is crucial for proper lens development.</p> Signup and view all the answers

    Study Notes

    Structure of the Lens

    • The lens is a transparent, biconvex structure situated between the iris and vitreous body
    • Anteriorly, the lens is less convex than posteriorly
    • In adults, the lens is about 10 mm in diameter and 4 mm thick

    Lens Capsule

    • The capsule is an elastic basal membrane that envelopes the entire lens
    • Thickest at the anterior and posterior poles (20 μm) and thinnest at the posterior pole (3 μm)
    • Composed of 40 lamellae, each resembling a unit basal lamina
    • Each lamella has collagen fibrils embedded in a glycosaminoglycan matrix

    Lens Epithelium

    • Found only on the anterior surface of the lens
    • Functions include homeostasis and osmolarity, synthesis of crystallin, and production of lens fibers
    • At the equator, epithelial cells elongate and form columnar cells, which become arranged in rows

    Lens Fibers

    • Constitute the main mass of the lens
    • Formed by the multiplication and differentiation of lens epithelial cells at the equator
    • Each fiber is 4 x 7 μm in size, hexagonal in cross-section, and up to 12 mm in length
    • Fibers are tightly packed together and interlocked through their plasma membranes
    • Older fibers form the nucleus, while younger fibers form the cortex
    • Fibers do not meet at the poles, instead meeting at irregular lines called sutures

    Microstructure of Lens Fibers

    • High concentration of gap junctions for metabolic communication
    • Cortical and nuclear fibers differ in structure
    • Cortical fibers have a nucleus and organelles, while nuclear fibers have no nucleus or organelles
    • Lens nucleus is more compact than the cortex, with a higher protein concentration (200-300 mg/ml vs 100-150 mg/ml)

    Accommodation

    • The lens allows the eye to focus at different distances
    • Human accommodation involves deformation of the lens
    • Some species accommodate through lateral movement of the lens or retina
    • Deformation itself is not accommodation

    Zonules of Zinn

    • Suspensory ligaments that hold the lens in position
    • Comprised of fibrillin (connective tissue with elastic properties)
    • Diameter of 1-2 μm
    • Involves in the accommodation process, allowing the lens to change its dioptric power

    Growth and Development

    • The lens grows throughout life
    • At birth, the lens is about 6.5 mm in diameter
    • In adults, the lens can reach 10 mm in diameter and up to 15 mm in older people
    • The lens continues to grow and does not discard any of its cells, adding new cells to the central portion of the lens as lens fibers

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