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20. eye

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20. eye

  1. 1. Sensory Systems The Eye
  2. 2. The Cornea
  3. 3. Histology of the front of the Eye
  4. 4. Ciliary Body and Processes
  5. 5. The Iris
  6. 6. The Lens
  7. 7. The Retina Ganglion Cells Bipolar Cells Rods and Cones Pigmented Retina
  8. 8. Rods and Cones Pigmented Retina
  9. 9. Rod Cone
  10. 10. EYE & SEEING/VISION The eyes are paired for security, a wider field of view, and to aid distance perception and other apects of seeing. However, ‘seeing double’ spells trouble! The eyes contain the receptors for light, provide mechanisms for influencing the light falling on the photoreceptors, and are constructed to hold their shape, and have transparent regions Although connected to the brain by the optic nerve, the eye has muscle attachments for movement, and needs blood vessels Other accessory structures, in and around the bony orbit housing the eye, comprise the ocular adnexa, e.g., tear glands & eyelids
  11. 11. GLOBE’S LAYERS & CHAMBERS outer tunic SCLERA & CORNEA sagittal view UVEA middle tunic RETINA inner LENS ANTERIOR CHAMBER POSTERIOR not CHAMBER Note: Posterior chamber isdoes posterior in the globe, but lie behind the anterior chamber VITREOUS CAVITY
  12. 12. GLOBE’S ANTERIOR STRUCTURES SCLERA CILIARY BODY CORNEA IRIS PUPIL LENS ZONULE ANTERIOR CHAMBER POSTERIOR CHAMBER Chambers filled with aqueous ZONULE suspends and pulls on the lens
  13. 13. GLOBE’S ANTERIOR STRUCTURES: LIMBUS SCLERA LIMBUS is where the cornea meets the sclera CILIARY BODY CORNEA PUPIL LENS ZONULE IRIS ANTERIOR CHAMBER POSTERIOR CHAMBER LIMBUS = corneoscleral junction LIMBAL epithelium is the sole source of cells, by mitosis & migration, for turnover & repair of the corneal epitheium
  14. 14. OPTICAL ASPECTS I CILIARY BODY includes the muscle to relax the lens shape for more focusing dark UVEA prevents light reflecting around inside & is vascular Transparent CORNEA’S extra curvature focuses light IRIS controls the amount of light reaching the retina PUPIL size reflects iris muscles’ activity Vitreous body (a jelly) lets light LENS through, and keeps the layers attached Chambers are filled with the aqueous humor which provides tranparency, and its pressure holds the eye in shape ‘White of the eye’ is dense scleral connective tissue for strength, and to keep light out, aided by dark inner uvea posterior/neural RETINA is sensitive to light
  15. 15. OPTICAL ASPECTS I (Tabular) Transparent CORNEA’S extra curvature focuses light Chambers are filled with the aqueous humor which provides tranparency, and its pressure holds the eye in shape ‘White of the eye’ is dense scleral connective tissue for strength, and to keep light out, aided by dark inner uvea Dark UVEA prevents light reflecting around inside & is vascular IRIS controls the PUPIL size amount of light reflects iris reaching the retina muscles’ activity LENS adds focusing Vitreous body (a jelly) lets light through, and keeps the layers attached Posterior/neural RETINA is sensitive to light CILIARY BODY includes the muscle to relax the lens shape for more focusing Anterior ‘RETINA’ is a double-layered epithelium unresponsive to light
  16. 16. OPTICAL ASPECTS II: Focusing CILIARY BODY includes the muscle to relax the lens shape for more focusing On neural RETINA is a tiny REAL INVERTED IMAGE of object Transparent CORNEA’S extra curvature focuses light OBJECT Visual axis Elongated lens focuses light further onto the retina LENS Vitreous jelly lets light through, and keeps the layers attached
  17. 17. OPTICAL ASPECTS III: Near-vision Focusing CILIARY MUSCLE contracts; tension in zonule decreases; the lens’ elasticity changes it into a rounder shape Close OBJECT Visual axis LENS Rounder lens focuses light more onto the retina The older lens loses this elastic ability to change shape, so that one cannot clearly see close objects - presbyopia NEURAL RETINA
  18. 18. OPTICAL ASPECTS IV: Near-vision Focusing 2 CILIARY MUSCLE contracts; tension in zonule decreases; the lens’ elasticity changes it into a rounder shape At first hearing, these sound to be contradictory Under parasympathetic control the CILIARY MUSCLE contracts; tension in zonule decreases; the len capsule’s elasticity changes CILIARY MUSCLE lens into a rounder shape ZONULE LENS LENS LENS A-P view CILIARY MUSCLE - within the eye - is an intra-ocular muscle LENS
  19. 19. OPTICAL ASPECTS V: Central vision NEURAL RETINA The eyes are moved by the extra-ocular muscles to bring the object of attention onto the visual axis OBJECT Visual axis On the visual axis, the LENS MACULA/FOVEA is the region of retina for precise central vision
  20. 20. OPTICAL ASPECTS VI: Peripheral vision OBJECT Visual axis LENS This part of the anterior nonneural retina is the PARS PLANA Outside the macula, the remainder of the neural retina is responsible for peripheral vision - for things off the visual axis MACULA for central vision Can all of the neural retina see things? No
  21. 21. OPTICAL ASPECTS VII: NerveHead & Blind spot The neural retina contains not only the photoreceptors, but nerve and glial cells for the initial processing of the signals. The result is patterns of firing in retinal ganglion neurons whose axons leave the eye, at one place, to become the optic nerve LENS The place is the Nerve head/ optic disc/ optic papilla Also where vessels OPTIC NERVE The optic nerve leaves on the nasa/ medial side to the macula enter
  22. 22. OPTICAL ASPECTS VIII: Blind spot LENS The neural retina contains not only the photoreceptors, but nerve and glial cells for the initial processing of the signals. The result is patterns of firing in retinal ganglion neurons whose axons leave the eye, at one place, to become the optic nerve LENS LENS LENS OPTIC NERVE OPTIC UNSEEN OBJEC T CHIASMA Nerve head/ optic disc/papilla has no photoreceptorshence is blind The optic nerves leave on the nasa/ medial side to the macula, so that they can connect at the midline OPTIC CHIASMA: the starting device for binocular integration - seeing one object with two eyes and two sides to the brain
  23. 23. OPTICAL ASPECTS IX: Experiencing the Blind spot Place a small bright coin on a dark table, e.g., a quarter L Place another bright coin, e.g. a dime, to the right of the first coin LENS R LENS Cover your left eye with your left hand Fixate your right eye on the first coin Move the second coin to right & left with your right-hand forefinger, but not obscuring your view At about 17 cm separation, the second coin should disappear from view as its image falls on the right eye’s blind spot/ nerve head 10 25 [Reverse the side of second to first coin for testing the left eye]
  24. 24. OPTICAL ASPECTS X: Retinal layers LENS The neural retina contains not only the photoreceptors, but nerve and glial cells for the initial processing of the signals. These cells are arranged in layers with the photorecptors next to the uvea, and Vitreous the retinal ganglion output neurons innermost, close to the vitreous Thus, for peripheral vision, the light has to pass through all the retinal layers to reach the photoreceptors OPTIC NERVE For precise central vision, the nerve and glial cells lie as an annular hump around a depression - fovea - where the cone photoreceptors can respond to the unimpeded light
  25. 25. Retinal layers I: Cells simplified RETINAL GANGLION NEURONS : MULLER CELLS (glial) Between neurons and stretched across the layers BIPOLAR NEURONS PHOTORECEPTORS PIGMENT CELLS BRUCH’S MEMBRANE
  26. 26. Retinal layers II: Connection layers RETINAL GANGLION NEURONS Inner* Zone of Synapsing processes BIPOLAR NEURONS Outer Zone of Synapsing processes PHOTORECEPTORS : MULLER CELLS PIGMENT CELLS * Reference point for inner & outer is interior of the eye BRUCH’S MEMBRANE
  27. 27. Retinal layers III: Orientation * Reference point for inner & outer is interior of the eye GANGLION NEURONS Inner* Zone of Synapsing processes BIPOLAR NEURONS Outer Zone of Synapsing processes PHOTORECEPTORS PIGMENT CELLS The presence of a major basement membrane outside the pigment cells, here, is NOT the starting point for orientation. The reference point instead is opposite inside the eye , where there is an inconspicuous basal lamina around the vitreous BRUCH’S MEMBRANE is a substantial basement membrane
  28. 28. : Retinal layers IV: Terminology MULLER CELLS INNER LIMITING MEMBRANE NERVE FIBER LAYER RETINAL GANGLION NEURONS GANGLION CELL LAYER Inner* Zone of Synapsing processes INNER PLEXIFORM LAYER BIPOLAR NEURONS INNER NUCLEAR LAYER Outer Zone of Synapsing processes OUTER PLEXIFORM LAYER PHOTORECEPTORS OUTER NUCLEAR LAYER OUTER LIMITING MEMBRANE PHOTORECEPTOR LAYER PIGMENT CELLS BRUCH’S MEMBRANE PIGMENT CELL LAYER
  29. 29. INNER LIMITING MEMBRANE Glial cell layer NERVE FIBER LAYER GANGLION CELL LAYER INNER PLEXIFORM LAYER INNER NUCLEAR LAYER OUTER PLEXIFORM LAYER OUTER NUCLEAR LAYER PHOTORECEPTOR LAYER PIGMENT CELL LAYER OUTER LIMITING MEMBRANE RETINAL GANGLION NEURONS Inner* Zone of Synapsing processes BIPOLAR NEURONS’ BODIES Outer Zone of Synapsing processes PHOTORECEPTOR BODIES PHOTORECEPTOR CONES & RODS RETINAL PIGMENT CELLS
  30. 30. INNER LIMITING MEMBRANE NERVE FIBER LAYER GANGLION CELL LAYER INNER PLEXIFORM LAYER INNER NUCLEAR LAYER OUTER PLEXIFORM LAYER OUTER NUCLEAR LAYER PHOTORECEPTOR LAYER PIGMENT CELL LAYER Retinal layers V: H&E stained
  31. 31. Retinal layers V: H&E stained NERVE FIBER LAYER GANGLION CELL LAYER INNER PLEXIFORM LAYER INNER NUCLEAR LAYER OUTER PLEXIFORM LAYER OUTER NUCLEAR LAYER PHOTORECEPTOR LAYER PIGMENT CELL LAYER
  32. 32. Retinal layers VI: More cell types? GANGLION NEURONS Why not have the photoreceptors directly stimulate action potentials in the ganglion cells? BIPOLAR NEURONS The light causes changes in photoreceptor membrane potentials, but it takes STEPS to achieve actual ganglion-cell firing The synaptic arrangement shown transmits ‘signals’ just inwards Additional synapses and cell types provide for integrative influence and interactions across the retina PHOTORECEPTORS Two types of photoreceptor - rods & cones have somewhat different connection patterns & very different light sensitivities PIGMENT CELLS
  33. 33. Retinal layers VII: More cells 2 GANGLION NEURONS Both provide for crosswise connections, and need more investigation AMACRINE CELL BIPOLAR NEURONS HORIZONTAL CELL ROD For low light & black-grey perception CONE For daylight & color perception Provides the visual acuity of the fovea PIGMENT CELLS BRUCH’S MEMBRANE
  34. 34. PHOTORECEPTOR STRUCTURE I INNER FIBER CONE PEDICLE ROD : MULLER CELLS : Attachment of Muller cell INNER FIBER INNER SEGMENT INNER SEGMENT MITOCHONDRIA OUTER SEGMENT CILIUM connecting segments for transport OUTER SEGMENT Stacked BILAMINAR DISCS with photopigment Photopigment - iodopsin(s) absorbs in red, green or blue regions of light spectrum
  35. 35. CONE ROD PHOTORECEPTOR STRUCTURE II CONES: are larger than rods are far fewer, except in the fovea have a differently shaped outer segment have different photopigments - NOT rhodopsin - and responsiveness to light their synaptic end - pedicle - is much larger than the rod’s spherule do not shed discs for phagocytosis by pigment cells
  36. 36. Signal transduction & Electrical activity I GANGLION NEURONS L I G H T AMACRINE CELL BIPOLAR NEURONS HORIZONTAL CELL ROD CONE Outer segments Light passes through the retina to be absorbed by the photopigment stacked in the rod/cone outer segments The light has to alter electrical activity: in photoreceptors, the light stimulus counteracts an existing depolarized state from cyclic nucleotide-gated ion channels - so reduced, that a hyperpolarization occurs, causing the receptor to stop releasing + transmitter from vesicles in its spherule, so changing membrane potentials in the bipolar cells, which signal to the ganglion cell that it should produce an action potential for its optic-nerve fiber L I G H T
  37. 37. Signal transduction & Electrical activity II In the DARK SODIUM CHANNEL - held open by bound cGMP allows Na to leave, DEPOLARIZING the cell + Na+ Na+ cGMP L I G H T In the LIGHT SODIUM CHANNEL - closes because cGMP cGMP Na+ cGMP dissociates With rising intracellular Na+ a hyperpolarization occurs Why the dissociation? cGMPphosphodiesterase hydrolyzes cGMP, so lowering its intracellular level cGMP But what activates the enzyme?
  38. 38. L I G H T In the LIGHT Signal transduction & Electrical activity III 1 RHODOPSIN L I G H T RHODOPSIN OPSIN 2 Photon isomerizes retinal 11-cis-RETINAL to Light is absorbed by the photopigment stacked in the rod outer segment OPSIN 3 all-transRETINAL TRANSDUCIN - a G protein Changed shape of retinal forces OPSIN molecule to alter its conformation 4 OPSIN 6 α 1 subunit activates 7 cGMP cGMPphosphodiesterase, which hydrolyzes cGMP, so lowering its intracellular level 8 5 Altered OPSIN binds TRANSDUCIN, releasing α 1 subunit resulting in a cGMP dissociation from the Sodium channel
  39. 39. Signal transduction IV: Recovery & adaptation In the DARK SODIUM CHANNEL - held open by bound cGMP Ca2+ also allows Ca2+ to enter cGMP In the LIGHT Ca2+ SODIUM CHANNEL - closes because cGMP dissociates Falling Ca2+ unbinds inactivating Ca2+ Ca 2+ from RECOVERIN which can then stimulate Guanylyl cyclase to make more cGMP Ca2+ entry is blocked Intracelllular Ca2+ falls cGMP Recovery? With cGMP restored, it can quickly associate again with the sodium channnels
  40. 40. RHODOPSIN OPSIN TRANSDUCIN - a G protein retinal The G-protein cascade allows amplification of the signal initially detected by the retinal
  41. 41. Signal transduction & Electrical activity V GANGLION NEURONS L I G H T AMACRINE CELL BIPOLAR NEURONS HORIZONTAL CELL Pattern of ganglion-cell firing alters Bipolar cells’ GABA or glycine then inhibits ganglion activity less In response, bipolar cells hyperpolarize Receptor reduces the release of glutamate + transmitter from vesicles in its spherule ROD CONE Outer segments Simplified sample sequence The light stimulus causes a hyperpolarization Light passes through the retina to be absorbed by the photopigment stacked in the rod/cone outer segments
  42. 42. Signal transduction & Electrical activity VI COMPLICATING aspects include: As in the CNS, inhibition is used extensively L I G H T There are many subtypes of ganglion, amacrine , horizontal & even bipolar cells The GABA interplexiform is an additional type Amacrine cells use electrical (nexus) synapses in addition to chemical, e.g., dopaminergic, ones ON cells respond to a stimulus brighter than background, OFF to one darker than surround Great convergence of connections characterizes the rod system Arrangements for color & movement signal processing are elaborate
  43. 43. OPTIC NERVE Nerve fibers acquire myelin as they leave the eye NERVE-FIBER LAYER LAMINA CRIBROSA un-myelinated Holes in the sclera for the nerve fibers A weak spot RETINA SCLERA DURA ARACHNOID & PIA DURA
  44. 44. RETINA in OPHTHALMOSCOPY All this transparency to let light in means that, when the interior of the eye is illuminated, one can look in, with magnification, at the inside of the back of the eye - the fundus NORMAL VIEW FUNDUS MACULA OPTIC DISK VESSELS Macula lies circa two Disk Diameters (2 DD) temporally to the optic disc
  45. 45. SOME RETINA QUESTIONS in OPHTHALMOSCOPY FUNDUS - Correct color for race? Any spots? No unevenness? NORMAL VIEW MACULA - Any vessels over it? Too red? OPTIC DISK - Not too pale? No bulge, or excessive excavation? VESSELS - Right size? Not bent? Correct course? Engorged veins?
  46. 46. SCLERA & regional specializations LENS Dense irregular connective tissue Some vessels by limbus & ciliary body Insertions of extraocular muscles Vitreous RETINA Lamina fusca LAMINA CRIBROSA OPTIC NERVE exits Melanocytes Loose episcleral CT SCLERA proper
  47. 47. UVEA Components outer sagittal view UVEA middle tunic SCLERA RETINA inner LENS 1 IRIS 2 CILIARY BODY 3 CHOROID
  48. 48. Sphincter constrictor muscle Anterior IRIS Posterior No epithelium LENS Melanocytes More stromal melanocytes, browner eyes Pigmented cuboidal epithelium Myoepithelial dilator cells modified deeper epithelial cells IRIDIAL STROMA of loose connective tissue
  49. 49. INTRAOCULAR MUSCLES Separate parasympathetic IIIrd cranial nerve controls CILIARY MUSCLE contracts; tension in zonule decreases; the lens’ elasticity changes it into a rounder shape Sphincter constrictor muscle IRIS Pupil Weak dilator effect from LENS sympathetics A-P views IRIS constrictor & dilator and CILIARY MUSCLES are intra-ocular muscles
  50. 50. ZONULE or SUSPENSORY LIGAMENT OF LENS CILIARY MUSCLE contracts; tension in zonule decreases; the lens’ elasticity changes it into a rounder shape LENS A-P view ZONULE comprsises many coated fibers, running from the ciliary body to the lens capsule COMPOSITION of the zonule shares many characteristics with basal-lamina materials, e.g.
  51. 51. ORA SERRATA PARS PLANA Posterior-to-Ant. view CILIARY MUSCLE LENS The junction between the neural retina and the double cuboidal epithelium on the plars plana and the ciliary body is very irregular - creating a serrated ‘mouth’ NEURAL RETINA
  52. 52. UVEA: Choroid The structure of the iris conveys much of the ROD structure of the choroid CONE IRIS PIGMENT CELLS BRUCH’S CHOROID loose vascular connective tissue MEMBRANE CHORIOCAPILLARIS Wide fenestrated capillaries to nourish the retina Melanocytes
  53. 53. ANGLE OF ANTERIOR CHAMBER & Aqueous Humor Corner of ant chamber between cornea & iris, where sclera starts ANTERIOR CHAMBER PUPIL LENS POSTERIOR CHAMBER Chambers filled with aqueous humor SCLERAL ANGLE is another name Epithelium of CILIARY PROCESSES makes AH
  54. 54. ANGLE of ANTERIOR CHAMBER Corner of ant chamber between cornea & iris, where sclera starts CORNEA Canal of Schlemm Trabecular meshwork ANTERIOR CHAMBER Spaces of Fontana in the meshwork SCLERA IRIS POSTERIOR CHAMBER CILIARY PROCESSES make aqueous humor CILIARY MUSCLE Uveoscleral outflow is another drainage route
  55. 55. AQUEOUS HUMOR: Production & Flow I Canal of Schlemm Trabecular meshwork ANTERIOR CHAMBER Chambers filled with aqueous humor PUPIL LENS POSTERIOR CHAMBER SCLERAL ANGLE Corner of ant chamber between cornea & iris, where sclera starts epithelium of CILIARY PROCESSES makes AH
  56. 56. AQUEOUS HUMOR: Production & Flow II Epithelium of CILIARY PROCESSES makes Aqueous Humor 6 POSTERIOR CHAMBER PUPIL 3 PUPIL SCLERAL ANGLE with Canal of Schlemm 2 4 ANTERIOR CHAMBER Trabecular meshwork 1 5 Uveoscleral outflow LENS
  57. 57. AQUEOUS HUMOR: Glaucoma Epithelium of CILIARY PROCESSES makes AH POSTERIOR CHAMBER PUPIL 6 5 4 1 2 3 ANTERIOR CHAMBER PUPIL LENS SCLERAL ANGLE with Trabecular meshwork Canal of Schlemm Blocked drainage/venous return of AH raises intra-ocular pressure, damaging vessels & the retina
  58. 58. CORNEA I: Layers CORNEAL EPITHELIUM Bowman’s membrane No vessels STROMA Keratocyte Descemet’s membrane Anterior chamber ENDOTHELIUM
  59. 59. CORNEA II: Layer constituents CORNEAL EPITHELIUM EPITHELIUM is stratified squamous, with nerve fibers Thin basal lamina Bowman’s membrane of dense fibrillar collagen STROMA No vessels anywhere of collagen fibers in very orderly lamellae, with regular alternating fiber orientations & much special proteoglycan Keratocytes are fibroblasts of the corneal stroma Descemet’s membrane - a thick basal lamina ENDOTHELIUM Not a vascular endothelium, but pumps water out of stroma Transparency factors Bowman’s membrane is modified stroma, not the basal lamina
  60. 60. CORNEA II: Layer constituents CORNEAL EPITHELIUM is stratified squamous, with nerve fibers Thin basal lamina Bowman’s membrane of dense fibrillar collagen STROMA of collagen fibers in very orderly lamellae, with regular alternating fiber orientations & much special proteoglycan Keratocytes are fibroblasts of the corneal stroma No vessels are present Bowman’s membrane is modified stroma, not the basal lamina Descemet’s membrane - a thick basal lamina ENDOTHELIUM Not a vascular endothelium, but pumps water out of the stroma Transparency factors, & not present in the sclera
  61. 61. CORNEA III: Tear-film constituents oily/lipid layer - eyelid glands aqueous phase - Lacrimal Mucin layer From conjunctival & tearduct goblet cells CORNEAL EPITHELIUM TEARS: Protect the conjunctival & corneal surfaces Nourish the avascular cornea Wash out discrepancies to ‘corner’ of the eye Kill & restrain microorganisms Smooth corneal-surface optics
  62. 62. LACRIMAL/LACHRYMAL GLAND & PASSAGES LACHRYMAL GLAND LACHRYMAL DUCTS LACHRYMAL SAC NASOLACRIMAL DUCT Gland is superior and temporal to the eye facilitating the spread of tears across the eye to the collection points - the lacrimal puncta medially at the eyelids’ margin
  63. 63. LACRIMAL GLAND II Gland is superior and temporal to the eye facilitating the spread of tears across the eye to the collection points the lacrimal puncta medially at the eyelids’ medial/nasal margins evaporation is slowed by surface film of lipid from Meibomian glands LACRIMAL GLAND compound tubulo-alveolar gland with myoepithelial cells LACRIMAL DUCTS From eyelids LACRYMAL SAC with valves NASOLACRIMAL DUCT continuation of the sac to drain into lower nasal cavity
  64. 64. LACRIMAL GLAND III LACRIMAL GLAND Compound tubulo-alveolar gland Alveoli lined by pale columnar/cuboidal serous cells with myoepithelial cells Secretion - tears - comprises water antimicrobials - lysozyme, defensins, antibodies electrolytes - plasma-like (tears taste salty) Innervation - Parasympathetic in CN VII via Pterygopalatine ganglion Blinking - eyelid movement - is necessary to spread tears
  65. 65. UPPER EYELID I Palpebral part of Orbicularis oculi Muscle EYELID SKIN Dense connective-tissue TARSAL PLATE with Meibomian glands EYELASH LID MARGIN PALPEBRAL CONJUNCTIVA Levator palpebrae superior. Muscle BULBAR CONJUNCTIVA
  66. 66. UPPER EYELID II Palpebral part of Orbicularis oculi Muscle Levator palpebrae superior. Muscle Inserts into Tarsus, etc EYELID SKIN BULBAR CONJUNCTIVA Dense connective-tissue TARSAL PLATE with embedded Meibomian glands By the eyelash follicle are other small glands EYELASH fornix PALPEBRAL CONJUNCTIVA Stratified cuboidal epithelium with some goblet cells on loose CT LID MARGIN Where secretion of Meibomian modified sebaceous glands emerges
  67. 67. LENS EQUATOR & AXIS EQUATOR Anterior AXIS LENS Posterior pole Lateral view Posterior--Anterior view Lens shape is not quite as depicted: the anterior part is an ellipsoid; the posterior bulges back more as a parabyloid
  68. 68. LENS PARTS LENS CAPSULE Subcapsular epithelium (cuboidal) becomes elongated LENS FIBERS (cells) at the LENS BOW by filling themselves with crystallins the proteins that confer long-lasting transparency ZONULE FIBERS
  69. 69. CATARACT - Lens becomes opaque LENS CAPSULE Common in old age UV radiation is an accelerating factor Naphthalene (in mothballs) is another agent, as is Overheating with infrared radiation from furnaces e.g., in glassblowers, & Traumatic damage to the lens capsule and epithelium Lentectomy, and replacement with an artificial lens usually cure Posterior-capsule opacification is one risk ZONULE FIBERS
  70. 70. EYE DEVELOPMENT I: Some specifications The eye comprises many tissues, structures, and layers that require contributions from three main sources Using multiple sources needs tight coordination of signals and controls The body’s covering has to have a transparent region For optics, the lens needs to be roundish, the eye almost spherical, with the retina precisely hemispherical Spaces - chambers and cavity - have to be created inside Blood vessels have to be introduced early into the soonto-be-enclosed round eye Nerves (afferent & efferent) to & from the brain are needed External & internal muscles & other auxilliary structueres are needed
  71. 71. DEVELOPMENT of the EYE I from CNS 35 days pc 3 brain ‘vesicles’ are subdividing Mesencephalon Rhombencephalon BRAIN Diencephalon now four, then Rhombencephalon divides into Met- & Melencephalons Cephalic flexure/bend Cervical flexure start the folding Telencephalon Surface ECTODERM MESENCHYM E Neural RETINA ECTODERM Already before 35d pc, on each side of the ‘head’, interactions have started between surface ECTODERM, a bulge of the FOREBRAIN & the MESENCHYME
  72. 72. EYE PARTS’ EMBRYONIC SOURCES Surface ECTODERM MESENCHYME UVEA LENS SCLERA CORNEAL EPITHELIUM CORNEAL STROMA Connective tissue & muscle (& vessels) come from cranial mesenchyme LENS Neural RETINA ECTODERM RETINA OPTIC NERVE VITREOUS Two ectoderms drive events and shaping
  73. 73. ANTERIOR EYE PARTS’ EMBRYONIC SOURCES Surface ECTODERM LENS CORNEAL EPITHELIUM How does a surface layer produce two separate structures? In much the same way as an endocrine gland is produced: by a downgrowth of cells that then break off the surface connection Here the downgrowth makes the lens vesicle, conferring a roundish shape from early on Mesenchyme To have enough cells for the future cornea and for the lens vesicle, the surface ectoderm first thickens to form a lens placode over the brain-derived optic vesicle
  74. 74. LENS & OPTIC CUP DEVELOPMENT I While still growing, both placode and end of the optic vesicle invaginate optic vesicle Mesenchyme Intraretinal space lens placode Double wall of optic cup is starting to form Optic vesicle precedes the lens vesicle and is a distinct structure
  75. 75. OPTIC CUP DEVELOPMENT II: Choroid fissure Mesenchyme Blood vessels have to be introduced early into the soon to be enclosed round eye Together with the invagination centrally at the end of the optic cup, an invagination along the cup & stalk’s inferior surface occurs, to create the choroid fissure in which runs the hyaloid artery
  76. 76. OPTIC CUP DEVELOPMENT II: Coloboma Mesenchyme Blood vessels have to be introduced early into the soon to be enclosed round eye Together with the invagination centrally at the end of the optic cup, an invagination along the cup & stalk’s inferior surface occurs, to create the choroid fissure in which runs the hyaloid artery Also, an annular vessel runs around the outside of the Imagine a penis in which the urethra near & into the optic cup glans is still open on its underside - the condition of hypospadias - (but now contains an artery) Defects in the eye from failure of the choroid fissure to close are colobomas
  77. 77. OPTIC DEVELOPMENT III: Lens vesicle Mesenchyme LENS VESICLE Mesenchyme l e n s Inner wall thickens p l a c o d e Deeper part of Placode sinks into mesenchyme & makes a vesicle Optic cup becomes deeper Attachment to surface ectoderm will be broken so that surface ectoderm can become corneal epithelium & intervening mesenchyme can form the corneal stroma
  78. 78. OPTIC DEVELOPMENT IV: Lens differentiation Mesenchyme Attachment to surface ectoderm lost Mesenchyme Anterior vesicle cells become subcapsular epithelium Basal lamina becomes lens capsule Posterior vesicle cells become elongated lens cells Posterior vesicle cells form the nucleus of the lens. Subsequent lens cells derive from the subcapsular epithelium
  79. 79. OPTIC DEVELOPMENT IV: Lens differentiation Mesenchyme Anterior-vesicle cells become subcapsular epithelium Basal lamina becomes lens capsule Lumen obliterated Posterior-vesicle cells elongate to lens cells LENS
  80. 80. OPTIC DEVELOPMENT V: Retina differentiation I Mesenchyme Outer layer of cup stays thin and beomes pigment cell layer Intra-retinal space occluded Inner layer of cup thickens and becomes Neural layer Hyaloid artery reaches inside cup After a while, the lens and vitreous no longer need it, and it atrophies. Only the neural retina continues to depend on it, but under another name - central artery of the retina
  81. 81. OPTIC DEVELOPMENT VI: Retina differentiation II Mesenchyme Inner layer of cup thickens and becomes Neural layer Where cells multiply, form layers and differentiate to the several cell types of the neural retina Outer layer of cup stays thin and beomes pigment cell layer

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