Embryology and anatomy of orbit Flashcards
1
Q
What are the characteristics of Early development?
A
- Neural tube precursor to CNS
- Neural groove deepens
- Day 17 - eye develops from cell population in anterior neural plate - eye fields
- Rapid growth of prosencephalon, forebrain
2
Q
How is optic sulcus formed ?
A
week 4 - lateral shallow grooves
Cells of cephalic neural crest migrate - head mesoderm
Neural tube closure
Grooves from outpouchings - optic vesicle
3
Q
What are optic vesicles?
A
Outgrowings of brain which make contact with surface ectoderm and this contact induces changes necessary for further development of eye
4
Q
What is the optic disc?
A
White, cup-like area in centre of optic disc
vesicles contact surface ectoderm - important for lens formation
5
Q
What happens on day 22?
A
Two small grooves develop on either side of developing forebrain in neural folds –> optic sulci
6
Q
What happens in the optic cup?
A
Vesicles contact surface ectoderm - important for lens formation
Invagination of optic vesicle - double walled optic cup
7
Q
What develops after invagination of the inferior optic cup?
A
Choroid fissure for hyaloid artery
8
Q
What is optic vesicle?
A
As neural tube closes, groove become outpockings
9
Q
Where do optic vesicle extend from?
A
Forebrain toward surface ectoderm through adjacent mesenchyme
10
Q
What happens as the optic vesicles grow toward ectoderm?
A
Their connection to the forebrain become attenuated to form optic stalks
11
Q
What is the development of optic cup?
A
- The portion of each optic vesicle interacts with surface ectoderm induces that area of ectoderm to form a thickening called lens placode
- The lens placode invaginate to become a lens pit, soon forms a complete circle that pinches off from surface ectoderm to become a lens vesicle
12
Q
What is lens placode?
A
A precursor of lens
13
Q
What has optic or choroidal fissure?
A
The developping optic vesicle and stalk have a groove on their inner surface
14
Q
What is the function of choroidal fissure?
A
Blood vessels gain access to optic cup as well as lens vesicle
15
Q
What are branches of hyaloid artery?
A
A branch of the opthalmic artery and its accompanying vein
16
Q
What will the choroid fissure do?
A
Fuse
Completing eye wall inferiorly and enclosing vessels in a canal in optic stalk
17
Q
What happens during 5th week of optic cup development ?
A
Vesicle lose contact with ectoderm lying in the mouth of optic cup
18
Q
Lens vesicle
A
- surface ectoderm - lens placode
- Placode invaginates - lens vesicle
- 5th week vesicle loses contact with ectoderm lying in mouth of optic cup
19
Q
What do the cells of the posterior part of lens vesicle transform into?
A
Elongated, slender primary lens fibre
20
Q
What are the posterior cells for lens development?
A
Parallel fibres forming lens nucleus
21
Q
Lens development
A
Posterior cells
Crystalline proteins deposited intracellularly - refractive properties
Anterior layer forms cuboidal lens epithelium - regulated by fibroblast growth factors
22
Q
Optic stalk development
A
- Optic cup connected to the diencephalon - optic stalk
- Transient space - optic ventricle connects to 3rd ventricle via stalk
- Inferiorly have continuation of choroid fissure - fuses at 7-8 weeks
- Narrow tunnel in optic stalk
23
Q
What does hyaloid artery become?
A
Central retinal artery
24
Q
How are optic nerve fibres developed?
A
- In the framework of optic stalk
- Fibres from nerve fibre layer of retina grows into optic stalk by passing through choroidal fissure
- Inner and outer layers fuse, inner layer providing supporting neuroglial cells
25
What develops into glial system of nerve?
The neuroectodermal cells forming the walls of optic stalk
26
How are the fibrous septa of optic nerve developed?
Vascular layer of mesenchyme which invades nerve at 3rd fetal month
27
Inner wall of stalk
Nerve fibres from retina
28
What will the two layers of optic cup further differentiate into?
Retina of mature eye
29
Two layers are unequal in size
The outer one is thinner than inner one
30
What are the 2 portions that the optic cup be divided into?
1. Anterior 1/5 (rim)
| 2. Posterior 4/5 (rim)
31
What will the rim area in the retina form?
Iris and ciliary body
| The posterior 4/5 will form retina
32
What will the outer layer of posterior 4/5 become?
Layer of retina
33
What will the inner layer become?
Neural retina
| Separated by intraretinal space
34
What will the posterior 4/5 of inner layer differentiate into?
1. Photoreceptive rods and cones (pars optica retina)
2. Ganglion cells
3. Bipolar nerve cells
35
what gives rise to epithelium of iris and ciliary body?
Anterior rim of optic cup
36
What contributes to pigmented epithelial layer?
Outer layer of optic cup
37
What is the role of ciliary body?
1. Accomodation
2. Aqueous humour production
3. lens maintenance
38
What do stroma of iris and ciliary body develop from?
Neural crest cells that migrate into area
39
What develops from optic cup neuroectoderm?
Within stroma of iris
| Sphincter pupillae and dilator pupillae
40
What is the function of iris and ciliary body?
Controls light level via central aperture-pupil
41
What are Iris and Ciliary body?
Optic cup margins
42
What is RPE?
External pigment layer of iris
43
What is the inner layer of iris and part of ciliary body?
Anterior 1/5 inner layer of optic cup
44
What are sphincter and dilator pupillae muscles?
Neural ectoderm in vascular mesenchyme surrounding iris
45
What are the suspensory ligaments of lens?
Loose mesenchyme between lens and ciliary body
46
What does ciliary process fold secrete?
Aqueous humour
47
Where does the vitreous body form?
Centre of optic cup posterior to the lens
48
Where are the vitreous humor derived from?
Mesenchymal cells of neural crest origin
49
What does vitreous humor contain?
Phagocytic cells
| Remove blood and other debris
50
What does the mesenchyme *(mesoderm) invade?
Optic cup - choroid fissure
51
What happens during end of embryonic period, week 8?
Optic cup and lens are supplied by hyaloid artery
52
What are hyaloid vessels?
Vascular layer on inner surface of retina and delicate fibre network between lens and retina
53
What is vitreous body?
interstitial network spaces filled with transparent gelatinous
substance
54
vitreous blood supply–
from mesenchyme surrounding
pigment epithelium of optic cup to form richly vascularised
pigment containing choroid
55
What does the outer layer of mesenchyme form?
Outer tough collagen rich sclera
56
What does the sclera correspond to?
Dura mater of brain and is continuous with dura of optic nerve
57
Where are extrinsic ocular muscles derived from?
Pre-otic myotomes
58
What are the 2 layers that the external surface of optic cup condenses into?
1. Inner, pigmented, vascular layer = choroid
| 2. Outer, fibrous layer = sclera
59
What is cornea?
1. Anteriorly to sclera
| 2. Continuous with corneal stroma
60
What are the 3 layers of cornea?
1. External epithelial lauer (surface ectoderm)
2. Middle dense connective tissue (Substantia propria) - continuous with sclera
3. Inner flat epithelium - from mesenchyme, lining anterior chamber space
61
How are cornea richly vascularised?
loose connective tissue between anterior
chamber and lens - continuation of choroid- irido-pupillary
membrane
involutes, leaving pupil unobstructed
62
What are eyelids?
folds of ectoderm and associated
| mesoderm
63
What happens to the eyelid at end of embryonic period?
Still open
64
What are the undersides of eyelids lined by?
delicate ectodermal
epithelium with loose richly vascularised
connective tissue, continuing onto sclera as
conjunctiva
65
What are the master gene for eye development?
PAX6
66
Where is PAX6 initially expressed?
band in anterior neural ridge
| of neural plate before neuralation
67
PAX6 master gene
initially single eye field which separates into 2
| primordia
68
What is separation signal for molecular regulation?
Sonic-hedgehog expression expressed in prechordal plate
69
What is role of SHH?
up-regulates PAX2 expression in optic stalks
and down-regulates PAX6, restricting expression
to optic cup and lens
70
Globe and soft tissue
```
adult globe axial diameter
25mm
cornea & sclera medium to
low signal on T1 &T2
ciliary body, iris &
choroido-retinal layer T1
hyperintense
aqueous humour T1
hypointense & T2
hyperintense
posterior chamber
vitreous- 98% water, <2%
collagen
lens - 65% water, 35%
protein; intermediate T1
signal & T2 hypointense;
hyperdense on CT
```
71
What are the optic nerve?
white matter tract surrounded by pia, arachnoid
| and dura
72
Optic nerve
subarachnoid space (0.5-0.6mm) continuous
with intracranial CSF, and widest behind globe
sheath dura blends with sclera anteriorly and
periorbita of optic canal posteriorly
continuous with intracranial dura
intracanalicular segment, 5mm long; passes
above ophthalmic artery in optic canal and then
through annulus tendon of Zinn
73
How many muscles control movement of eye?
6
74
How many muscles control eyelid elevation?
1
75
What are extra-ocular muscles 1?
1. 4 recti
2. 2 oblique
3. Levator palpebrae superioris (striated)
76
Where does recti originate from?
tendinous annulus of Zinn -
| orbital apex
77
Medial rectus largest
inferomedial annulus &
| optic nerve sheath
78
smaller lateral rectus
- common tendon above &
| below SOF
79
Inferior rectus
Below orifice of optic canal
80
Superior rectus
Annulus above optic nerve and dural sheath
81
Levator palpebrae superioris
from lesser
| sphenoid wing & superior rectus fibrous origin
82
superior oblique
e longest & thinnest muscle
courses anteromedially; tendon reflects around
fibrocartilaginous trochlea then posteromedially
at 45° degrees inferior to superior rectus
83
inferior oblique
from orbital plate of maxilla
| lateral to opening of nasolacrimal canal
84
Osseous anatomy 1
orbit pyramidal cavity; complex bony anatomy
medial & lateral walls vertical
roof convex upwards but horizontal
floor slopes downwards-medial to lateral
85
Medial orbital wall
```
frontal process of maxilla
lacrimal bone with anterior
& posterior lacrimal crests
orbital plate of maxilla with
ethmoid & frontal bone
above
major component – lamina
papyracea
sphenoid at apex
```
86
Orbital floor
```
triangular shape
orbital plate of maxilla
medially
zygomatic bone laterally
palatine bone near apex
separated by inferior
orbital fissure from lateral
wall (CN5-V2, IOV)
grooved by infra-orbital
foramen and canal
```
87
Lateral orbital wall
```
mainly zygomatic bone
with anterior condensation
forming lower half
frontal bone above frontozygomatic suture
greater wing sphenoid- lies
postero-superior to inferior
orbital fissure, & separated
by superior orbital fissure
from lesser wing sphenoid
and optic canal (CN3,4,6 ,
V1 & SOV)
```
88
Roof and optic canal
```
majority of roof-orbital plate
of frontal bone
lesser wing sphenoid -small
posteromedial component
optic canal- wall of ethmoid
and sphenoid sinuses
medially, lesser sphenoid wing
cranially, anterior clinoid
process laterally & optic strut
caudally
strut of bone separates it
from superior orbital fissure
mean diameter- 5mm; roof
length10-12mm
```
89
Arteries 1
dual supply-ophthalmic artery & ECA branches
OA from ICA; below optic nerve - enters optic
canal within the dural sheath
pierces dura near orbital apex, lies lateral to the
nerve giving rise to central retinal artery
after genu crosses over nerve lying below
superior rectus and running anteromedially
alongside superior oblique and medial rectus
central retinal artery -inferior to optic nerve,
entering dural sheath 1cm posterior to globe
90
Arteries 2
other OA branches -long & short posterior ciliary,
muscular, lacrimal, anterior & posterior ethmoid,
supra-orbital & palpebral arteries
transosseous branches to lateral orbital wall -
small branches of superior temporal / anterior
temporal branch of ECA
anastamoses around orbital floor between
muscular branches of OA,anterior deep temporal
and infra-orbital artery, a branch of internal
maxillary
lids have rich blood supply from both OA,
transverse facial, zygomaticomalar, facial &
infraorbital branches
91
Veins
major vein - superior ophthalmic vein, to
cavernous sinus via SOF
anastamosis of angular and nasofrontal veins
runs anteromedially to posterolaterally over optic
nerve
inferior ophthalmic vein runs posteriorly in
relation to inferior rectus and either drains to
SOV or cavernous sinus
remaining orbital veins small and variable
vortex veins –anastamosing veins between SOV
and IOV
92
Congenital abnormalities
```
Trisomy 13,18,21
Cat-eye syndrome (chr 22) –
vertical orientated iris coloboma
Congenital retinoblastoma – AD
Congenital cataracts, glaucoma
Persistent
hyaloid artery, pupillary
membrane, hyperplastic vitreous
```
93
Maternal Infection
```
Viral- rubella, herpes simplex,
cytomegalovirus
cataracts, microphthalmos, necrosis
Toxoplasmosis
inflammation &/or retinal
detachment
Syphilis
chorioretinal atrophy
```
94
Maternal Teratogens
```
Alcohol
FAS abnormalities of face/eye
Hydantoinmicrophthalmos, colobomas
LSD
severe eye abnormalities
Thalidomide
absence of eye
```
