Embryology and anatomy of orbit Flashcards
What are the characteristics of Early development?
- 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
How is optic sulcus formed ?
week 4 - lateral shallow grooves
Cells of cephalic neural crest migrate - head mesoderm
Neural tube closure
Grooves from outpouchings - optic vesicle
What are optic vesicles?
Outgrowings of brain which make contact with surface ectoderm and this contact induces changes necessary for further development of eye
What is the optic disc?
White, cup-like area in centre of optic disc
vesicles contact surface ectoderm - important for lens formation
What happens on day 22?
Two small grooves develop on either side of developing forebrain in neural folds –> optic sulci
What happens in the optic cup?
Vesicles contact surface ectoderm - important for lens formation
Invagination of optic vesicle - double walled optic cup
What develops after invagination of the inferior optic cup?
Choroid fissure for hyaloid artery
What is optic vesicle?
As neural tube closes, groove become outpockings
Where do optic vesicle extend from?
Forebrain toward surface ectoderm through adjacent mesenchyme
What happens as the optic vesicles grow toward ectoderm?
Their connection to the forebrain become attenuated to form optic stalks
What is the development of optic cup?
- 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
What is lens placode?
A precursor of lens
What has optic or choroidal fissure?
The developping optic vesicle and stalk have a groove on their inner surface
What is the function of choroidal fissure?
Blood vessels gain access to optic cup as well as lens vesicle
What are branches of hyaloid artery?
A branch of the opthalmic artery and its accompanying vein
What will the choroid fissure do?
Fuse
Completing eye wall inferiorly and enclosing vessels in a canal in optic stalk
What happens during 5th week of optic cup development ?
Vesicle lose contact with ectoderm lying in the mouth of optic cup
Lens vesicle
- surface ectoderm - lens placode
- Placode invaginates - lens vesicle
- 5th week vesicle loses contact with ectoderm lying in mouth of optic cup
What do the cells of the posterior part of lens vesicle transform into?
Elongated, slender primary lens fibre
What are the posterior cells for lens development?
Parallel fibres forming lens nucleus
Lens development
Posterior cells
Crystalline proteins deposited intracellularly - refractive properties
Anterior layer forms cuboidal lens epithelium - regulated by fibroblast growth factors
Optic stalk development
- 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
What does hyaloid artery become?
Central retinal artery
How are optic nerve fibres developed?
- 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
What develops into glial system of nerve?
The neuroectodermal cells forming the walls of optic stalk
How are the fibrous septa of optic nerve developed?
Vascular layer of mesenchyme which invades nerve at 3rd fetal month
Inner wall of stalk
Nerve fibres from retina
What will the two layers of optic cup further differentiate into?
Retina of mature eye
Two layers are unequal in size
The outer one is thinner than inner one
What are the 2 portions that the optic cup be divided into?
- Anterior 1/5 (rim)
2. Posterior 4/5 (rim)
What will the rim area in the retina form?
Iris and ciliary body
The posterior 4/5 will form retina
What will the outer layer of posterior 4/5 become?
Layer of retina
What will the inner layer become?
Neural retina
Separated by intraretinal space
What will the posterior 4/5 of inner layer differentiate into?
- Photoreceptive rods and cones (pars optica retina)
- Ganglion cells
- Bipolar nerve cells
what gives rise to epithelium of iris and ciliary body?
Anterior rim of optic cup
What contributes to pigmented epithelial layer?
Outer layer of optic cup
What is the role of ciliary body?
- Accomodation
- Aqueous humour production
- lens maintenance
What do stroma of iris and ciliary body develop from?
Neural crest cells that migrate into area
What develops from optic cup neuroectoderm?
Within stroma of iris
Sphincter pupillae and dilator pupillae
What is the function of iris and ciliary body?
Controls light level via central aperture-pupil
What are Iris and Ciliary body?
Optic cup margins
What is RPE?
External pigment layer of iris
What is the inner layer of iris and part of ciliary body?
Anterior 1/5 inner layer of optic cup
What are sphincter and dilator pupillae muscles?
Neural ectoderm in vascular mesenchyme surrounding iris
What are the suspensory ligaments of lens?
Loose mesenchyme between lens and ciliary body
What does ciliary process fold secrete?
Aqueous humour
Where does the vitreous body form?
Centre of optic cup posterior to the lens
Where are the vitreous humor derived from?
Mesenchymal cells of neural crest origin
What does vitreous humor contain?
Phagocytic cells
Remove blood and other debris
What does the mesenchyme *(mesoderm) invade?
Optic cup - choroid fissure
What happens during end of embryonic period, week 8?
Optic cup and lens are supplied by hyaloid artery
What are hyaloid vessels?
Vascular layer on inner surface of retina and delicate fibre network between lens and retina
What is vitreous body?
interstitial network spaces filled with transparent gelatinous
substance
vitreous blood supply–
from mesenchyme surrounding
pigment epithelium of optic cup to form richly vascularised
pigment containing choroid
What does the outer layer of mesenchyme form?
Outer tough collagen rich sclera
What does the sclera correspond to?
Dura mater of brain and is continuous with dura of optic nerve
Where are extrinsic ocular muscles derived from?
Pre-otic myotomes
What are the 2 layers that the external surface of optic cup condenses into?
- Inner, pigmented, vascular layer = choroid
2. Outer, fibrous layer = sclera
What is cornea?
- Anteriorly to sclera
2. Continuous with corneal stroma
What are the 3 layers of cornea?
- External epithelial lauer (surface ectoderm)
- Middle dense connective tissue (Substantia propria) - continuous with sclera
- Inner flat epithelium - from mesenchyme, lining anterior chamber space
How are cornea richly vascularised?
loose connective tissue between anterior
chamber and lens - continuation of choroid- irido-pupillary
membrane
involutes, leaving pupil unobstructed
What are eyelids?
folds of ectoderm and associated
mesoderm
What happens to the eyelid at end of embryonic period?
Still open
What are the undersides of eyelids lined by?
delicate ectodermal
epithelium with loose richly vascularised
connective tissue, continuing onto sclera as
conjunctiva
What are the master gene for eye development?
PAX6
Where is PAX6 initially expressed?
band in anterior neural ridge
of neural plate before neuralation
PAX6 master gene
initially single eye field which separates into 2
primordia
What is separation signal for molecular regulation?
Sonic-hedgehog expression expressed in prechordal plate
What is role of SHH?
up-regulates PAX2 expression in optic stalks
and down-regulates PAX6, restricting expression
to optic cup and lens
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
What are the optic nerve?
white matter tract surrounded by pia, arachnoid
and dura
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
How many muscles control movement of eye?
6
How many muscles control eyelid elevation?
1
What are extra-ocular muscles 1?
- 4 recti
- 2 oblique
- Levator palpebrae superioris (striated)
Where does recti originate from?
tendinous annulus of Zinn -
orbital apex
Medial rectus largest
inferomedial annulus &
optic nerve sheath
smaller lateral rectus
- common tendon above &
below SOF
Inferior rectus
Below orifice of optic canal
Superior rectus
Annulus above optic nerve and dural sheath
Levator palpebrae superioris
from lesser
sphenoid wing & superior rectus fibrous origin
superior oblique
e longest & thinnest muscle
courses anteromedially; tendon reflects around
fibrocartilaginous trochlea then posteromedially
at 45° degrees inferior to superior rectus
inferior oblique
from orbital plate of maxilla
lateral to opening of nasolacrimal canal
Osseous anatomy 1
orbit pyramidal cavity; complex bony anatomy
medial & lateral walls vertical
roof convex upwards but horizontal
floor slopes downwards-medial to lateral
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
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
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)
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
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
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
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
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
Maternal Infection
Viral- rubella, herpes simplex, cytomegalovirus cataracts, microphthalmos, necrosis Toxoplasmosis inflammation &/or retinal detachment Syphilis chorioretinal atrophy
Maternal Teratogens
Alcohol FAS abnormalities of face/eye Hydantoinmicrophthalmos, colobomas LSD severe eye abnormalities Thalidomide absence of eye
