lens 1(2).ppt

The Lens 1  main function:  The Lens allows the eye to focus at different distances (accommodation)  Human accommodation – deformation of lens  Some species – lateral movement of lens or retina  Deformation itself is not accommodation  The lens - transparent, biconvex structure situated betwee...

The Lens 1  main function:  The Lens allows the eye to focus at different distances (accommodation)  Human accommodation – deformation of lens  Some species – lateral movement of lens or retina  Deformation itself is not accommodation  The lens - transparent, biconvex structure situated between iris & vitreous body.  Anteriorly, the lens is less convex than posteriorly. In adults, the lens is about 10 mm in diameter and 4 mm thick. 4 mm 10 mm Equator Anterior pole Equatorial diameter (adult)10mm Posterior pole Axis Anterior radius of curvature 8-14mm Posterior radius of curvature 4.5-7.5mm Axis length 4mm (unaccommodated) – grows at 0.023mm/year  Equator - encircled by ciliary processes of ciliary body and lies 0.5 mm from them.  The lens - kept in position by the suspensory ligaments. Zonules of Zinn Zonules of Zinn  Diameter 1–2 μm  Comprised of fibrillin (connective tissue with elastic properties)  Accommodation for close objects - ciliary muscles contract.  Relieves the tension of the radiating fibres of the zonule -lens assumes more globular shape.  At the same time - sphincter pupillae muscle contracts,- pupil becomes smaller - light through the thickest, central part of the lens reaches the retina.  The lens, unlike the cornea, can change its dioptric power, allowing distant or near objects to be focussed on the retina. We focus by deforming the lens Under water the cornea has very little refractive power Why ? RI Air = 1.0002926, Water = 1.333 To get greater refractive power Need – higher RI How? Much higher protein concentration – up to 1200 mgs/ml Fish lens too rigid for accommodation by deformation NB Fish lens is spherical in shape Teleost (Boney) fish - retractor lentis -relaxed for near vision – moves away from retina for distance vision Cartilaginous fish (sharks, rays) - protractor lentis, relaxed for far vision – moves closer to retina for neat vision Avian eyes PECTEN Non-sensory comb-like structure - blood vessels belonging to the choroid Function - nourishes retina & controls vitreous body pH ????? Lens – lower protein content, more aqueous Why? Diving ducks – Hooded mergansers RI Air = 1.0002926, Water = 1.333, cornea = ? cornea = 1.376 Levy & Sivak (1980) J. Comp. Physiol. 137, 267-272  The lens is made up of three parts  an elastic capsule  a lens epithelium (confined to the anterior surface of the lens)  the lens fibres.  The capsule - elastic basal membrane - envelopes entire lens.  thickest, anteriorly and posteriorly, close to the equator, measuring 20 m. It is thinnest at the posterior pole (3 m).  electron microscope- lens capsule about 40 lamellae, with each lamella resembling a unit basal lamina. Lamella - collagen fibrils embedded in a glycosaminoglycan matrix. Capsule  lens epithelium lies beneath the capsule. It is found only on the anterior surface of the lens.  The lens epithelium lies beneath the capsule. It is found only on the anterior surface of the lens. Microstructure – Lens Epithelium Functions Homeostasis and osmolarity Synthesis of crystallin Production of lens fibres Lens fibres  At the equator, the epithelial cells elongate and form columnar cells, which become arranged in rows.  At the equator, the epithelial cells elongate and form columnar cells, which become arranged in rows.  The lens fibres constitute the main mass of the lens. The fibers are formed by the multiplication and differentiation of the lens epithelial cells at the equator.  At the equator, the lens cells elongate and turn, so that their long axis is parallel to the surface of the lens. Microstructure – Lens Fibres Adult Lens Fibre production Anterior surface Push anteriorly under epithelium 1. Cuboidal Epithelial cells elongate, forming columnar cells 2. Apical surface of the cell grows and pushes anteriorly 3. Basal surface grows and pushes posteriorally 4. Both processes continue to grow, meeting opposite fibres at irregular sutures Posterior Surface Push posteriorally under the capsule Microstructure – Lens Fibres Adult Lens Embryonic Lens Primary lens fibres of the embryonic lens Secondary lens fibres of the adult lens  This process continues throughout life. Alignment of the cell nuclei produces the nuclear pattern known as lens bow. Lens bow – aligning of newly produced lens fibre nuclei in the lens cortex Equatorial region  Each elongated lens cell is now called a lens fibre.  Each fibre 4 7m , hexagonal in cross-section up to 12mm in length  The older fibres (whose nuclei have fragmented and disappeared) form the nucleus of the lens, the younger fibres (which are nucleated) form the cortex.  Fibres do not meet at the poles- along irregular lines called sutures.  The lens fibres are tightly packed together and interlocked through their plasma membranes. Microstructure – Lens Fibres General Structure High concentration of Gap Junctions for metabolic communication Cortical and nuclear fibres differ in structure Cortical Fibres: Have nucleus and organelles Nuclear fibres: No nucleus or organelles  Lens nucleus – much more compact than cortex. Higher protein concentration.  Human cortex – 100 to 150 mgs/ml  Human nucleus – 200 to 300 mgs/ml  Change in protein density – refractive index gradient (RIGN)  RGIN – important in minimising aberrations x-ray interferometer SPring-8 synchrotron - Japan Figure 1. Images of a) porcine; b) ranine; c) murine; d) newt; e) piscine eyes in the sagittal plane Measured refractive index gradients of a) porcine; b) ranine; c) murine; d) newt; e) piscine lenses  lens grows throughout life  birth - diameter 6.5 mm   Adults - 10 mm. can reach 15 mm in older people.  The lens continues to grow and does not discard any of its cells. These are continually added to the central portion of the lens as lens fibres.  The central part of the lens becomes less pliable and more compact. The lens capsule increases in thickness with age. Lens growth 15 yrs 140mg 85 yrs 255mg i.e. 4.5 g/day Causes presbyopia – progressive loss of accommodation  With advancing age, the lens becomes denser and less elastic and the ability to accommodate is lessened (presbyopia).  The lens can be defective in the way it refracts:  Hypermetropia (Far Sightedness): This condition can arise if the eyeball is too short anteroposteriorly or the lens of the eye is not strong enough to bend the light rays sufficiently.  Myopia (Near-sightedness) in this condition the eyeball is too long anteroposteriorly or, rarely, the lens of the eye is too strong.  Presbyopia. This is due to loss of lens elasticity as a result of aging.  The individual cannot accomodate for near vision because the lens cannot assume a spherical shape.