Choroid and vitreous Flashcards
What connects the lamina fusca (innermost scleral layer) to the choroid?
Fine collagen fibres- weak attachment
Potential space between choroid and lamina fusca =perichoroidal space
What runs within the perichoroidal space?
Long and short posterior ciliary arteries and nerves
Where does the choroid become continuous with the pia and arachnoid?
At optic nerve
What are the 3 layers of the choroid?
Suprachoroidea/lamina fusca
Vessel layer(Sattlers and Hallers)
Capillary layer/ Choriocapillaries
Bruch’s membrane (acellular)
What are the 5 layers of Bruch’s membrane?
2-4 microns thick (thickest near disc) BM of endothelium of capillaries Outer layer of collagen fibres Elastic fibre layer Inner layer of collagen fibres BM of pigment epithelium of retina
Where does the choroid run from?
Ora serrata anteriorly
Optic nerve posteriorly
Internally: RPE
Externally: Lamina fusca
What is the function of choroid?
Nourish outer layers of retina
Conduct blood vessels. nerves to anterior uvea
Thermoregulation
Absorbs excess light to prevent reflection (melanocytes within the choroidal stroma)
What is thickness of choroid?
Thickest at posterior pole 0.22mm
0.1mm at equator
What is the arterial supply of choroid?
Mainly long and short posterior ciliary arteries
Long from equator to orra serrata (anterior choroid)
Short from optic nerve to equator
Recurrent branches from anterior ciliary arteries
Describe the uveal vessels
Supply whole eye except neurosensory retina
Comes from ophthalmic artery
Becomes medial and lateral posterior ciliary arteries which divide into 10-20 branches
Most become short posterior ciliary arteries
2 become long posterior ciliary arteries
Some short posterior ciliary arteries anastomose around the optic nerve to form Circle of Zinn-Haller
Where do the long posterior ciliary arteries run?
Enter along horizontal meridian
Run anteriorly between sclera and choroid to ciliary body
Sends branches to ora serrata to supply anterior choroid up to equator
Also supplies iris, anterior part of ciliary body and ciliary processes
Where do the short posterior ciliary arteries run?
Run forward with optic nerve
Most enter globe temporally to optic nerve
Run in Haller’s layer of the choroid
Supply posterior choroid up to equator
Medial branches form Circle of Zinn/Haller to supply optic nerve head
Where do the anterior ciliary arteries run?
Run with recti (2 for each rectus except LR which only has 1)
Contribute to main arterial circle of iris
Contributes small amounts to supply choroid
What is the venous drainage of the choroid?
Number of vortex veins variable
2 inferior vortex veins which drain into inferior ophthalmic vein
Exit the globe at equator via posterior sclera
2 superior vortex veins -> superior ophthalmic vein
Ophthalmic veins drain into cavernous sinus
Where do drusen lie?
Between the retinal pigment epithelium and Bruch’s membrane
Describe the choriocapillaris/capillary layer of choroid
Continous vascular sheet that lies beneath Bruch’s membrane
A lobular structure
Architecture of the layer varies from area to area
Largest capillaries in the body
Describe the different variations in the choriocapillaris
Prepapillary/submacular regions: Honeycomb
Posterior pole equator: lobular with central postcapillary venules and precapillary arterioles
Peripheral region: Elongated
Describe the capillaries of the choriocapillaris
Fenestrated-more fenestrated on Bruch's membrane side Wide lumen Meshwork spaces vary 1. posterior-small +oval 2. equator- long+wide 3. peripheral-irregular
What gets more blood, the choroid or retina?
Choroid blood flow 20x that of retinal blood flow
Exceeds nutritional deman
May play thermoregulatory role and buffer for rises in IOP
What are the two layers of the stroma/vessel layer of choroid?
Hallers: outer-large arterioles
Sattlers: deeper- intermediate arterioles
Why is infarction of the choroid uncommon?
Extensive anastomosis between vessels
Which nerves run in the stroma of the choroid?
20 short posterior ciliary nerves from ciliary ganglion
2 long posterior ciliary nerves from nasociliary nerve V1
Nerves branch at ora serrata
Which layer of the choroid contains melanocytes?
vessel/stroma layer
Lamina fusca
Why are suprachoroidal infusions usually anterior to the equator?
Collagen lamellae packing in lamina fusca is not as compact anteriorly
What is the embryological origin of the choroid?
MESENCHYMAL
When during gestation is the choroidal vasculature developed?
6th week- choriocapillary network has basal lamina
and vortex veins develop
11th week posterior ciliary arteries have branches throughout the choroid
3rd-4th month-choroidal vasculature is mature
What is the definition of the vitreous?
Transparent gel, fills posterior globe Roughly spherical with anterior concavity 4ml 4g 99% water Denser at surface Viscosity is 2x H2O (varies with age)
Describe the embryology of the vitreous
ECTODERM +NEUROECTODERM +MESENCHYMAL
3 stages
4-5 weeks-Primary: network of cytoplasmic proteins between lens and optic cup derived partly from ectoderm (developing lens) and neuroectoderm (retinal layer of optic cup)-vascular supply hyaloid artery
Secondary: between primary vitreous and retina. Homogenous gel that pushes primary vitreous anteriorly. Basically ECM made of type 2 collagen+hyalocytes- avascular
3-4 months-Tertiary: large numbers of collagen fibres develop with formation of zonular fibres
What are the relations of the vitreous?
Anteriorly: ciliary body, lens and zonules
Posteriorly: retina
What is the patella fossa?
Anterior concavity in vitreous to accomodate lens
Separated from the lens by a potential space (Space of Berger)
What is the ligamentum hyaloideocapsure of Wieger?
Circular lining of vitreous delineating the patella fossa
Adherent to lens capsule but weakens with age
Adherent to hyaloid zonule
What is the anterior hyaloid surface?
Cortex of vitreous anterior to the ora serrata
Can be seen on slight lamp
What is the vitreous base?
Denser cortex attached to the posterior 2mm of pars plana and anterior 4mm of retina
Often site of retinal tears
What is the posterior hyaloid surface?
Cortex of vitrous posterior to ora serrata
Opposed to Internal limiting membrane
Firm adhesions to vessels
What is cloquet’s canal?
S- shaped canal within vitreous from space of Berger to space of Martegioni
Represents the site of the primary vitreous and hyaloid artery
How does the vitreous change with age?
Birth: homogenous
Infancy: cortex develops anteriorly. Central vitreous decreases in density
Adolescence: Vitreous tracts form anteriorly
Adult: Vitreous tracts reach posterior vitreous
Elderly: central vitreous liquifies and tracts are gradually broken down
Why does posterior vitreous detachment occur?
The liquid central vitreous enters subhyaloid space and acts like a wedge with inertia to strip vitreous cortex off retina with each eye movement
Which microscopic structures make up the vitreous?
Vitreous cells
Vitreous cortex (anterior hyaloid, vitreous base, posterior hyaloid, peripapillary and macular attachments)
Central vitreous
What makes up the vitreous cortex?
Dense zone enveloping vitreous
Inserts anteriorly onto basal lamina of ciliary epithelium and posteriorly onto ILM of retina
Made up of hyaluronic acid, GAGs and type 2 collagen
Basal lamina is 20-100nm thick
Thickens posteriorly and with age
Acts as a barrier to large molecules
Interrupted at pars plana/ciliary
What is the premacular cortex?
May become apparent with abnormal vitreoretinal adhesions
What is the prepapillary cortex?
1micro wide circular zone at dic margin where Muller cells end and optic nerve glia start
How does the central vitreous differ form the cortex?
Similar to cortex but less cells/collagen
GAGs separate collagen to maintain transparency
In elderly breakdown means collagen forms coarse strands (visible clinically)
Acellular
Which cells are present in the vitreous?
Found in vitreous cortex
Phagocytic cells called hyalocytes which make hyaluronic acid
Increase in number with inflammation
A few fibrocytes and glial cells
What is the composition of the vitreous
Mostly water Hyaluronic acid(HMW proteoglycan->highly viscous) Fine fibrillar collagen type II
What is the neurovasculature of the vitreous?
Not physiologically present
May have persistent hyaloid artery
Neovascularisation may occur secondary to retinal ischaemia
When does the hyaloid artery regress?
Tunica vasculosa lentis and hyaloid artery regress at the end of the 3rd month, leading to retraction of primary vitreous and secondary avascular vitreous develops. By month 7, blood flow in hyaloid has ceased
