Histology And Embryology Of Visual System

Question 1

Embryonic layers of eye

Answer 1

. Neuroectoderm forming retina, iris, and optic n.
. Surface ectoderm forming lens
. Mesoderm btw layers that forms vascular and fibrous coats of eye

Question 2

Optic cup and lens vesicle

Answer 2

. On day 22 optic sulci appear on each side of forebrain
. Sulci form outpouchings of forebrain (optic vesicles) when neural tube closes
. Optic vesicle projects away from prosencephalon into surrounding mesenchyme and connection to optic vesicle to prosencephalon narrows to become optic stalk
. Optic vesicle invaginated to form double layered optic cup

Question 3

Lens placode

Answer 3

. Formed by optic vesicle making contacts w/ surface ectoderm that cause them to elongate
. Lens placode envaginates and separates from surface ectoderm
. Lens vesicle is located at mouth of optic cup and continued differentiation forms the lens

Question 4

Invagination of optic vesicle characteristics

Answer 4

. Invagination more extensive inf.
. Cleft called choroid fissure forms
. Fissure allow hyaloid a. To reach inner chamber of eye
. During week 7 the choroid fissure closes and mouth of optic cup forms a round opening that will be future pupil

Question 5

Retina formation

Answer 5

. 2 layers of optic cup
. Outer layer is simple cuboidal pigmented epithelium (immediately adjacent to choroid of uvea post.)
. Inner layer: photoreceptive portion of retina in post. 4/5 of eye, an. 1/5 of layer is non-photoreceptive
. Intraretinal space btw 2 layers is obliterated and contains small amount of fluid, this is where retinal detachment occurs

Question 6

Ciliary body formation

Answer 6

. Post. Surface covered by epithelial layers of optic cup

. CT of the ciliary body and the ciliaris m. Derived from mesenchyme adjacent to optic cup

Question 7

Iris formation

Answer 7

. Post. Surface covered by epithelial layers of optic cup
. CT of iris is derived from adjacent mesenchyme
. Sphincter pupillae and dilator pupillae m.derived from neuroectoderm of optic cup

Question 8

Optic nerve formation

Answer 8

. Retinal epithelium in optic stalk proliferates when choroid fissure closes
. Hyaloid vessels become trapped w/in optic stalk and become central retinal a. And v.
. Ant. Portion of hyaloid vessels obliterate and become remnant in vitreous humor (hyaloid canal of vitreous body)
. Myelination of optic n. Is not complete until 10 week postnatally

Question 9

Choroid and sclera formation

Answer 9

. Formed from mesenchyme surrounding optic cup

. Sclera continuous w/ dura mater surrounding optic nerve

Question 10

Chambers of the eye formation

Answer 10

. Mesenchyme ant. To lens vesicles vacuolizes and these spaces become 1 large space as ant. Chamber of ye e
. Ant. Chamber splits mesenchyme into an inner later in front of lens and iris (iridopupillary membrane) and an outer layer continous w/sclera (substantia propria of cornea)
. Iridopupillary membrane obliterates andallows communication btw ant. And post. Chambers
. Post.chamber forms in mesenchyme btw future iris and lens

Question 11

Eyelid formation

Answer 11

. Develops in conjunction w/ ant. Chamber and cornea

. Form as sup. And inf. Folds of surface ectoderm that meet and fuse until eyelids open at week 26

Question 12

Cornea formation

Answer 12

. Surface ectoderm forms outer layer

. Mesenchyme forms substantia propria of cornea and an epithelial layer adjacent to ant. Chamber

Question 13

Coloboma iridis

Answer 13

. Choroid fissure fails to close and cleft persists giving pupil keyhole shape
. Can affect iris, ciliary body, retina, choroid, and optic n.

Question 14

Persistent iridopupillary membrane

Answer 14

.membrane fails toresorb completely and web of fibers will remain

Question 15

Congenital cataracts

Answer 15

. Lens becomes opaque during intrauterine life

. Genetic or cause of rubella infection during weeks 4-7

Question 16

Histology of cornea

Answer 16

. Outermost CT layer along w sclera
. Transparent and densely innervated w/ pain fibers
. Avascular
. Maintains transparency through constant removal of water from surfaces

Question 17

Sclera histology

Answer 17

. Outermost CT layer along. W/cornea
. Covers post. 5/6 of eye
. Collagen and elastic fibers arranged in irregular bundle
. Sclera is opaque and has higher water content than cornea

Question 18

Lens histology

Answer 18

. Avascular, biconvex structure post. Toiris
. Suspended from ciliary body by zonunlar fibers (suspensory ligaments) that help maintain its shape
. Thickness of lens in controlled by action of ciliary body m.
. Becomes less elastic w/ age, ability to accommodate is lost at 50-55 y/o

Question 19

Uvea histology

Answer 19

.middle layer of eye
. Vascular
. Contains choroid, ciliary body, and iris

Question 20

Choroid

Answer 20

Thin layer of CT w/specialized network of large capillaries (choriocapillaris)

Question 21

Ciliary body

Answer 21

. Circumferential structure attached to lens by suspensory ligaments
. Contains ciliary m. For lens accommodation

Question 22

Iris

Answer 22

Pigments disc w/ central aperture
. Contains smooth. Fibers that regulate size ofpupil
. Sphincter pupillae m. Surround pupillary m. And controlled by PNS
. Dilator pupillae m. Is collection of radially arranged smooth m. Fibers innervated by SNS

Question 23

Retina

Answer 23

. Transparent tissue that covers inner surface of post. Wall of globe
. It is outgrowth of diencephalon
. Small ant. Portion lies on post. Surface of ciliary body and iris and is not photocreceptive
. Larger post. Portion is photoreceptive

Question 24

Retinal pigment epithelium (RPE)

Answer 24

. Single layer of cells whose basal surface is in contact w/ choriocapillaris
. Apical surface in contact w/ rod and cone outer segments
. Functions: absorption of excess light, storage of vit. A for visual transduction, maintenance of nutrition for photoreceptors, phagocytosis of discs shed by photoreceptors during visual transduction

Question 25

Rods

Answer 25

. Outnumber cones 20:1
. Sensitive to low levels of light
. Responsible for night vision
. During bright light the rod is saturated and does not function
. Visual pigment is regenerated under low illumination but takes 5-30 minutes
. Reason why we can’t see at first going from brightly lit to dimly lit area

Question 26

Cones

Answer 26

. Specialized for acuity at expense of sensitivity
. Individual cones contain 1 of 3 visual pigments (blue, green, red) and differential stimulation of all cone types produces white light

Question 27

Bipolar cells

Answer 27

Dendrites receive impulse transmissions from rods and cones
. Axon of cell synapses w/ ganglion cell
. 10-100 rods may synapse on one bipolar cell but. Only 1 cone will synapse there
. Rods have high convergence and cones have low convergence

Question 28

Horizontal and amacrine cells

Answer 28

. Retinal assoc. neurons that modify visual data from photoreceptors to enhance borders and contours and inc. contrast

Question 29

Muller cells

Answer 29

. Cell bodies located adjacent to bipolar cells
. Cytoplasmic processes extend through retina to form outer and inner limiting membranes
. Functions like oligodendrocytes: provide metabolic support for retinal cells

Question 30

Ganglion cells

Answer 30

. Last link in retinal component of visual pathway
. Ganglion cell dendrites receive impulse transmissions from bipolar cells and each ganglion axons joint other 1 million ganglion axons o form optic n.

Question 31

Regional distribution of rods and cones

Answer 31

. Rods absent in central fovea and inc. in number towards the peripheral retina
. Cones are densely packed in central retina and dec. rapidly in all peripheral directions

Question 32

Macula lutea

Answer 32

. Small, pale yellow circular area at post. Pole of globe in direct line w/ visual axis
. Represents area for central vision
. Center has shallow depression (fovea centralis) that is area for sharpest vision and most acute color discrimination
. All retinal layers from ganglion layer to outer nuclear layer are displaced laterally so light passes directly to photoreceptors
. Fovea only has cones w/ 1:1:1 synapse w/ bipolar and ganglion cells

Question 33

Optic disc/papilla

Answer 33

. Located slightly med. to post. Pole of retina
. Site of exit for optic n. Fibers
. Lacks photoreceptors so it is blind spot
. Has pale pink color from blood vessels entering and exiting through it

Question 34

Papilledema

Answer 34

. Bulge of fluid in optic disc from inc. ICP that wastes CSF to compress the ophthalmic vessels and impede venous drainage

Question 35

compartments and Chambers of eye

Answer 35

. Lens, suspensory ligaments, and ciliary bodies partition eye into large post. Compartment and smaller ant. Compartment
. Iris further subdivides the ant. Compartment into ant. And post. Chambers
. Post. Compartment contains gelatinous vitreous humor that is not replenished
. Ant. Compartment filled w/ watery aqueous humor that is continually replaced
. Aqueous humor drains from corneoscleral junction, blockage causes glaucoma