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OPTHT801 > Overview and Embryology > Flashcards

Overview and Embryology Flashcards

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1
Q

What is the anterior segment of the eye?

A

From the lens and anterior

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2
Q

Where is the posterior capsule of the eye?

A

Everything behind the posterior capsule of the lens

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3
Q

Where is the anterior chamber?

A

The space between cornea and the iris

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4
Q

Where is the posterior chamber?

A

Space between iris and lens

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5
Q

What are the dimensions of the eye?

A

23x 23.5 x 24mm long

Slightly flattened horizontally, inferotemporal bulge

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6
Q

Where does the globe of the eye sit within the orbit?

A

Anterior in a fat cushion

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7
Q

What are the layers of the globe?

A

Corneoscleral coat (transparent cornea and white sclera. Continuous at the limbus. External attachment to EOM. Largely avascular)

Uvea: iris, ciliary body and choroid. Internally lines sclera. Vascular and pigmented

Retina: retinal pigment epithelium and neuroretina. Covers inner surface of the globe. Runs from optic nerve to ora serrata. High energy demand

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8
Q

Describe the topography of the orbits?

A

Two pear shaped boney caves
Parallel medial walls and perpendicular lateral walls
Deep part is apex

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9
Q

What are the key connections between the apex of the orbit and the intra/extra cranial space?

A

Optic canal
Superior orbital fissue
Inferior orbital fissure

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10
Q

Which orbital walls sit between the orbit and the maxillary, ethmoid, frontal sinus and temporalis muscle?

A

Orbital floor: maxillary sinus
Medial wall: ethmoid sinus
Orbital roof: frontal sinus and anterior cranial fossa
Lateral wall: temporalis muscle and middle cranial fossa

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11
Q

What is the normal degree of temporal rotation of the orbit?

A

22-23 degrees

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12
Q

What is the origin, course and insertion of the rectus muscles?

A

Origin: Tendinous ring that surrounds optic nerve at orbital apex

Course:Follows orbital walls, separates orbit into intra and extra conal

Inserts: anterior to equator, obliques posterior
Tendons fuses with sclera

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13
Q

What is the origin, course of the superior oblique?

A

Origin: near tendinous ring,

Course: pass underneath superior and inferior recti. Runs to trochlea then back to superior aspect of the globe

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14
Q

What is the origin, course of the inferior oblique?

A

Origin: near inferonasal aspect of orbital rim
Course: passes underneath superior and inferior recti
Inserts on inferior globe

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15
Q

What is the origin, course and insertion of the levator palpebrae superioris?

A

Origin: Just above tendinous ring

Runs above superior rectus
Inserts into tarsal plate and upper eyelid skin

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16
Q

What are the innervation of the extraocular muscles?

A

CN3: all extraocular muscles including levator palpebrae superioris
Except SO4 and LR6
CN7: supplies facial muscles including orbicularis oculi that closes the lid
Sensation is V1

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17
Q

Where is the lacrimal gland?

A

Located superotemporally behind orbital rim
Drains into conjunctival fornix
Innervated by parasympathetics

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18
Q

What is the drainage pathway of tears?

A

Flow from lateral to medial
Lacrimal puncta to lacrimal canaliculi, then lacrimal sac, then nasolacrimal duct
This opens into the nasal cavity

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19
Q

During which movements does your eyelid move?

A

Vertical

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20
Q

What are the layers of the eyelid?

A

Skin
Muscle (orbicularis oculi)
Orbital septum/tarsal plate (continuous with each other)
Palpebral conjunctiva (continuous with conjunctiva in the fornix)
CN3 sends fibres through the orbital septum to skin to elevate the lid

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21
Q

Describe the afferent visual pathway

A

Retina-> optic nerve-> optic chiasm-> optic tract-> thalamus -> Optic radiations-> primary visual cortex -> association visual cortex

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22
Q

How much decussation occurs in the fibres at the optic chiasm?

A

About 50% of fibres from left retina will decussate to right optic tract

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23
Q

Where is the upper hemifield (upper world of vision) represented in the brain?

A

Beyond the lateral geniclate nucleus of the thalamus, it is represented in the lower brain

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24
Q

Where is the left hemifield (left world of vision) represented in the brain?

A

Beyond the optic chiasm, images on the left are represented in the right of the brain

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25
How much of the superior corneal limbus is usually covered by eyelid?
1-2mm
26
What are the lengths of the insertions of the rectus muscles?
Medial rectus: 5.5mm Inferior rectus: 6.5mm Lateral rectus: 6.9mm Superior rectus: 7.7mm
27
How thick is the sclera?
1mm
28
What is normal corneal thickness?
10.5mm (h) x 11.5mm (w) Corneal curve using keratometry: 7.2-8.6mm Central corneal thickness: 550nm
29
What are the two parts of the ciliary body?
Pars plicata and pars plana
30
What is the length of the ciliary body?
6mm | Pars plana 4mm (avascular and does not contain photoreceptors, therefore good place to access posterior segment)
31
Where is the ciliary body located
Temporally 1.5mm posterior to corneal limbus (7.5-8mm long) | Nasally 1.5mm posterior to corneal limbus (6.5mm long)
32
Where is the injection site into the posterior segment?
3.5cm posterior to limbus
33
How big should pupils be?
Light: 2-4mm diameter Dark: 4-8mm diameter Physiological anisocoria present in 20% of population
34
What is the normal cup to disc ratio?
0.3
35
What are the two main components of the blastocyst? Which will give rise to the embryo?
Inner cell mass-> becomes embryo | Trophoblast
36
What forms the embryonic ectoderm, endoderm and mesoderm?
Embryonic epiblast splits into embryonic ectoderm and primitive streak Primitive streak splits into embryonic endoderm and embryonic mesoderm
37
What is gastrulation?
The formation of the 3 germ layers Ectoderm (skin and nervous system) Mesoderm (bone, kidnye, RBC, muscle) Endoderm (pancreatic cell, thyroid cell)
38
What is neurulation of ectoderm?
It forms epidermal ectoderm and neuroectoderm
39
What does epidermal ectoderm give rise to?
Lens, cornea, skin, inner ear, hair, nails, mammary glands
40
What does neuroectoderm give rise to?
Retina, brain, nervous system
41
How does neurulation form neural crest cells?
Neural fold and the rim of the neuroectoderm dissociates from the neural tube as it invaginates and from each other Neural crest cells give rise to components of the cornea
42
How does the neural tube form the retina?
Neural tube develops out pockets of neuroectoderm called optic vesicles and these become the retina
43
Describe how the optic vesicles become primitive retina and lens
Optic vesicles become very closely associated with ectoderm which forms lens placode and then invaginate to form the lens pit, then round out to form the lens vesicle Lens vesicle separates from overlying ectoderm (that ectoderm will form cornea) Optic vesicles will become optic cups then retina
44
What are the two layers of the optic cup?
Inner: thicker, becomes neuroretina Outer: thinner, becomes retinal pigment epithelium
45
At what stage does vascular development of the eye begin?
Lens vesicle cells begin to elongate and the space between the lens vesicle and the optic cup increases Small blood vessels begin to fill this space and become the hyaloid vasculature
46
What do the posterior/anterior cells of the lens vesicle form?
Posterior: Primary lens fibres Anterior: forms lens epithelium
47
When does vitreous humour appear in the space between the lens vesicle and the optic cup?
Once the lens has differentiated into primary fibres and lens epithelium The space between lens and optic cup is full of hyaloid vessels and vitreous humour.
48
What do the cells at the rim of the optic cup become?
Ciliary body and iris
49
How does the lens reach its adult ovoid shape?
Cells are added equatoriarly
50
What is the tunica vasculosis lentis?
Hyaloid blood vessels associated with the lens
51
How is the anterior chamber formed?
Len vesicle separates from overlying corneal ectoderm and space fills with aqueous humor
52
Where do the mesodermal cells of neurocrest origin invade?
The space between the lens and the ectoderm
53
What do the mesodermal cells do in the anterior chamber?
They become several flat layers, condense and secrete extracellular matrix
54
What forms the corneal endothelium?
The layer of mesodermal cells which lie closest to the lens, forms a monolayer of endothelium
55
Which layers give rise to the corneal stroma?
The mesodermal cells lying between the endoderm and corneal epithelial layer
56
Which cells form the ciliary body and the iris?
Mesodermal cells invading the rim of the optic cup
57
Which cells cause a build up of corneal stroma?
Keratinocytes secrete extracellular matric which increases the stroma
58
In which week of gestation does the eye develop?
4th week
59
What are the 3 primitive embryological tissues which give rise to the brain?
Prosencephalon (forebrain) Mesencephalon (midbrain) Diencephalon (hindbrain)
60
What is the most important gene in determining that the region of the neural plate that will become the eyes?
Pax 6 - eyes will not develop without it
61
What does the optic stalk become?
Optic nerve
62
How does the surface ectoderm become the lens placode?
As surface neuroectoderm touches optic vesicles, it induces overlying ectoderm to form the lens placode
63
How does the lens placode become the lens?
Lens placode invaginates into the optic cup, becoming the lens vesicle, then it separates from the overlying ectoderm to form the primitive lens
64
What is the blood supply to the primitive eye?
Hyaloid artery which runs in choriod fissure of optic stalk
65
What happens to the hyaloid vessels?
The choroid fissure of the optic stalk closes, capturing the hyaloid vessel within it and removing the intraretinal space Covered by glial sheath which usually regresses Hyaloid vessels then become the central artery and vein, running within the optic nerve Regress from lens and vitreous during 7th month gestation
66
What are some complication if the choroidal fissure fails to close?
coloboma of the iris (key hole pupil) retinochoroidal coloboma (retina not full formed-> hyperplasia of RPE) Coloboma of optic nerve head (large diameter) Non rhegmatogenous retinal detachment Not clear which genes are involved
67
What does the retina look like at 5-6 weeks gestation?
Outer nuclear zone with mitotic cells Inner marginal zone, no cells RPE
68
When does the inner half of the retina get some cells
7 weeks Cells migrate from outer zone into inner zone forming inner neuroblastic layer An acellular zone remains between inner and outer though (transient layer of Chievitiz)
69
When does the macula start to form?`
10-12 weeks Mitotic cells in this area stop dividing Cells mature and differentiate
70
Which cell types differentiate first?
Ganglion cells Horizontal cells Cone photoreceptors Then amacrine, Muller, bipolar and rods
71
When does the transient layer of Chievitz disappear?
10-12 weeks | Replaced by inner plexiform layer
72
When do photoreceptor pigments which detect light first get expressed?
10-15 weeks gestation
73
How does proliferation vary throughout the retina?
Starts in the centre at works outwards to periphery | As periphery is still dividing, the fovea stops dividing and starts differentiating
74
What pattern does melanin form in RPE cells?
Centeroperipherally
75
When do blood vessels appear at the optic nerve head?
14-15 weeks As fovea stops proliferating Led by astrocytes migrating into periphery responding to VEGF
76
How do the ganglion cells form the foveal avascular area?
Ganglion cells migrate inwards creating a layer that is 9 cells thick dome Peak number occur around 17 weeks Then some cells undergo apoptosis Astrocytes invading inwards encouraging blood vessel development do not get into the incipient foveal region where the ganglion cells are Therefore the foveal area remains avascular
77
What causes the fovea to form?
Possibly increased IOP
78
How does the fovea form?
Central ganglion layer dome indents Ganglion cells migrate horizontally away from fovea 30 weeks astrocytes are migrating to periphery, away from fovea along with ganglion cells and inner nuclear layer Cone photoreceptors migrate towards the fovea, becoming more elongated Eventually fovea has ++cone photoreceptors, no rods, no INL and no blood vessels and thin

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