Choroid Flashcards
What is the choroid?
the posterior portion of the middle vascular coat of the eye, the uveal tract
What is the choroid homologous to in the brain?
the pia-arachnoid
What type of tissue is the choroid?
thin, highly pigmented, vascular, loose conntective tissue between the sclera and the retina
Between which structures does the choroid exist?
between the sclera and the retina
What are the primary functions of the choroid? Give 5
- nourish outer layers of the retina
- conduit for vessels travelling to other parts of the eye
- may have thermoregulatory role
- absorption of light by choroidal pigment aids vision by preventing unwanted light from reflecting back through the retina
- regulation of blood flow in the choroid may influence IOP
What is the benefit of the choroid pigment absorbing light?
prevents unwanted light from reflecting back through the retina
How may the regulation of blood flow in the choroid influence intraocular pressure?
by affecting perfusion rates of the ciliary processes
What is a tapetum?
some mammals (carnivores, ruminants, cetaceans, seals) and non-mammals (fish, crocodiles) have a reflective tapetum in choroid or RPA, serves to increase photoreception in low light conditions
What may be the tissue that the tapetum in some species is made of and where it is usually located?
may be cellular or fibrous and may occupy only part, usually the upper portion, of the globe
What are the boundaries of the choroid?
extends from the optic nerve margins to the ciliary body
What is the thickness of the choroid at posterior pole vs anteriorly?
- posterior pole: 220 μm
- anteriorly: 100 μm
What is the inner surface of the choroid continuous with and what is its texture like?
- forms part of Bruch’s membrane beneath the RPE
- inner surface is smooth
What is the name of the outer surface of the choroid?
suprachoroid
What is the outer surface of the choroid like?
the suprachoroid is irregular and firmly attached to the lamina fusca of the sclera
What structure is the outer choroid surface attached to?
lamina fusca of the sclera
What are the 4 histological layers of the choroid?
- Bruch’s membrane
- choriocapillaris
- vascular layer
- suprachoroid
What type of tissue is Bruch’s membrane formed of?
modified connective tissue
How thick is Bruch’s membrane?
2-4 μm thick
hat is the histological appearance of Bruch’s membrane?
acellular glassy membrane beneath the RPE
What are the 5 layers which Bruch’s membrane consists of?
- RPE basal lamina (0.3 μm thick)
- inner collagenous zone
- middle elastic layer
- outer collagenous zone - blends with stroma between choriocapillaris
- basement membrane of endothelial cells in choriocapillaris
Which of the layers of Bruch’s membrane is not truly part of the choroid?
the RPE basal lamina
What does the middle elastic layer of Bruch’s membrane consist of?
incomplete interwoven bands or perforated sheets of elastic ‘fibres’
Which structure does the outer collagenous zone of Bruch’s membrane blend with?
stroma between the choriocapillaris
What do age-related changes in Bruch’s membrane result in?
areas of diffuse or discrete thickening - ‘drusen’
What does the choriocapillaris consist of?
extraordinarily rich bed of wide-bore fenestrated capillaries
How far does the choriocapillaris extend?
only as far as the ora serrata
What is the function of the choriocapillaris?
functions to provide nutritional support for the outer retina, especially the photoreceptors
What structure are the capillaries of the choriocapillaris arranged like?
more akin to a perforated vascular ‘net’ than a network of capillaries
At what point in the retia are the bore of the capillaries and the density of the ‘net’ of the choriocapillaris greatest?
near the macula
In which areas are the capillaries of the choriocapillaris fenestrated?
on their retinal aspect
What is a) the bore of the capillaries of the choriocapillaris and b) the diameter of the fenestrations on the retinal aspect?
a) 20-40 μm
b) 75-85 nm
What is the density of the fenestrae of the choriocapillaris capillaries?
46 per μm2
Are smooth muscles present in the choriocapillaris layer?
no, not usually
What vessels feed the capillaries of the choriocapillaris?
arterioles, from the layer composed of arterioles and venules (Sattler’s)