Choroid Flashcards
(86 cards)
1
Q
What is the choroid?
A
the posterior portion of the middle vascular coat of the eye, the uveal tract
2
Q
What is the choroid homologous to in the brain?
A
the pia-arachnoid
3
Q
What type of tissue is the choroid?
A
thin, highly pigmented, vascular, loose conntective tissue between the sclera and the retina
4
Q
Between which structures does the choroid exist?
A
between the sclera and the retina
5
Q
What are the primary functions of the choroid? Give 5
A
- 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
6
Q
What is the benefit of the choroid pigment absorbing light?
A
prevents unwanted light from reflecting back through the retina
7
Q
How may the regulation of blood flow in the choroid influence intraocular pressure?
A
by affecting perfusion rates of the ciliary processes
8
Q
What is a tapetum?
A
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
9
Q
What may be the tissue that the tapetum in some species is made of and where it is usually located?
A
may be cellular or fibrous and may occupy only part, usually the upper portion, of the globe
10
Q
What are the boundaries of the choroid?
A
extends from the optic nerve margins to the ciliary body
11
Q
What is the thickness of the choroid at posterior pole vs anteriorly?
A
- posterior pole: 220 μm
- anteriorly: 100 μm
12
Q
What is the inner surface of the choroid continuous with and what is its texture like?
A
- forms part of Bruch’s membrane beneath the RPE
- inner surface is smooth
13
Q
What is the name of the outer surface of the choroid?
A
suprachoroid
14
Q
What is the outer surface of the choroid like?
A
the suprachoroid is irregular and firmly attached to the lamina fusca of the sclera
15
Q
What structure is the outer choroid surface attached to?
A
lamina fusca of the sclera
16
Q
What are the 4 histological layers of the choroid?
A
- Bruch’s membrane
- choriocapillaris
- vascular layer
- suprachoroid
17
Q
What type of tissue is Bruch’s membrane formed of?
A
modified connective tissue
18
Q
How thick is Bruch’s membrane?
A
2-4 μm thick
19
Q
hat is the histological appearance of Bruch’s membrane?
A
acellular glassy membrane beneath the RPE
20
Q
What are the 5 layers which Bruch’s membrane consists of?
A
- 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
21
Q
Which of the layers of Bruch’s membrane is not truly part of the choroid?
A
the RPE basal lamina
22
Q
What does the middle elastic layer of Bruch’s membrane consist of?
A
incomplete interwoven bands or perforated sheets of elastic ‘fibres’
23
Q
Which structure does the outer collagenous zone of Bruch’s membrane blend with?
A
stroma between the choriocapillaris
24
Q
What do age-related changes in Bruch’s membrane result in?
A
areas of diffuse or discrete thickening - ‘drusen’
25
What does the choriocapillaris consist of?
extraordinarily rich bed of wide-bore fenestrated capillaries
26
How far does the choriocapillaris extend?
only as far as the ora serrata
27
What is the function of the choriocapillaris?
functions to provide nutritional support for the outer retina, especially the photoreceptors
28
What structure are the capillaries of the choriocapillaris arranged like?
more akin to a perforated vascular 'net' than a network of capillaries
29
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
30
In which areas are the capillaries of the choriocapillaris fenestrated?
on their retinal aspect
31
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
32
What is the density of the fenestrae of the choriocapillaris capillaries?
46 per μm2
33
Are smooth muscles present in the choriocapillaris layer?
no, not usually
34
What vessels feed the capillaries of the choriocapillaris?
arterioles, from the layer composed of arterioles and venules (Sattler's)
35
What is the arrangement arterioles and venules that feed the choriocapillaris?
hexagonal patches or 'lobules' of choriocapillaris are fed by a central precapillary arteriole that runs perpendicular to the flat choriocapillaris
36
Why is the lobular pattern of choriocapillaris that is fed by central precapillary arterioles clinically significant?
choroidal ischaemia often occurs as pale hexagonal patches (mosaic pattern)
37
What is the drainage of the choriocapillaris?
venous channels drain the periphery of the hexagonal choriocapillaris lobules
38
What are the 2 layers into which the vascular layer of the choriocapillaris can be subdivided?
1. inner layer of intermediate-sized vessels - **arterioles and venules, Sattler's layer**
2. Outer component - **major arteries and veins, Haller's layer**
39
What does the inner layer of the vascular layer consist of and what is the alternative name for it?
* intermediate-sized vessels (arterioles and venules)
* Sattler's layer
40
What does the outer layer of the vascular layer consist of and what is the alternative name for it?
* major arteries and veins
* Haller's layer
41
What vessels chiefly supply the choroid?
* long and short posterior ciliary arteries
* recurrent branches from the anterior ciliary arteries anastomose with anterior choroidal vessels
42
How does venous drainage of the choroid occur?
via series of large vortex veins (venae vorticosae) - usually 4 (but may be up to 6)
43
How many large vortex veins (venae vorticosae) may drain the choroid?
usually 4 (but may be up to 6)
44
What path do the large vortex veins draining the choroid follow?
pierce the sclera through emissary canals and drain into the superior and inferior ophthalmic veins in the orbit
45
Which key veins do the vortex veins draining the choroid drain into?
superior and inferior ophthalmic veins in the orbit
46
What 4 things does the choroidal stroma consist of?
1. randomly arranged **collagen** **fibres** (type 1)
2. flattened ribbon-like **elastic** **fibres**
3. **fibrocytes**
4. numerous **melanocytes**
47
Which type of collagen fibres are found in the choroidal stroma?
type I
48
What characteristic is influenced by the extent of choroidal pigmentation?
the appearance of the fundus (inner lining of the eye) - highly pigmented choroid of darker-skinned people shows through more than in the fundus of a person with less skin pigmentation, in which the red-orange reflex is primarily the result of the choroidal vasculature
49
What 5 types of immunocompetent cells exist within the choroid, being a connective tissue?
1. Plasma cells
2. Lymphocytes
3. Perivascular mast cells
4. networks of resident tissue macrophages
5. dendritic cells
50
Where are the cell bodies of macrophages and dendritic cells in the stroma of the choroid? Where are their processes?
* **cell bodies:** external (sclerad) aspect of the choriocapillaris and deeper in the choroid
* **processes:** extend close to the basal aspect of the RPE
51
What is the role of choroidal immune cells and age-related macular degeneration?
* deposition of **lipid, complement and immunoglobulin G** that accompanies the senescent changes in **Bruch's membrane** may be augmented by **impaired macrophage recruitment and/or decreased homeostatic scavenging by the resident macrophages and dendritic cells in the choroid**
* thus may contribute to the accumulation of debris and formation of drusen in dry AMD
* and to eventual choroidal neovascularisation in wet AMD
52
What 3 substances are deposited in Bruch's membrane that accompany senscent changes that lead to AMD?
1. lipid
2. complement
3. immunoglobulin G
53
What exactly happens to choroidal immune cells contributing to AMD? 2 key things
* impaired **macrophage** **recruitment** and/or:
* decreased homeostatic **scavenging** by the resident **macrophages** and **dendritic** **cells** in the choroid
54
On what imaging do choroidal infarctions appear?
angiograms
55
How do choroidal infarctions appear on angiograms?
triangular areas near the equator, with the apex pointing towards the optic disc
56
What does peripheral retinal cobble or paving-stone degeneration represent?
chronic focal ischaemic changes in the anterior choroid
57
Where is the suprachoroid and what is its thickness?
30 μm thick transition zone between the choroid and the sclera
58
What is the structure of the suprachoroid?
thin, interconnected lamellae of melanocytes, fibroblasts and connective tissue fibres separated by a thin 'potential' (supra- or perichoroidal) space
59
What 3 key structures are present in the suprachoroid?
melanocytes, fibroblasts, connective tissue fibres
60
What can happen to the potential supra- or perichoroidal space in the suprachoroid in pathological conditions?
may become separated by fluid and blood
61
When is the supra- or perichoroidal space frequently artefactually enlarged?
in histological preparations
62
Which vessels exist in the suprachoroid?
it is an avascular layer; only vessels are those that traverse the suprachoroid entering or leaving the choroid
63
What do the lamellae of the suprachoroid blend with?
the choroid and the lamina fusca of the sclera
64
Which space is the suprachoroidal space continuous with anteriorly?
supraciliary space
65
What type of smooth muscle cells are present in the suprachoroid?
highly organised network of non-vascular smooth muscle cells
66
Where are the highly organised networks of non-vascular smooth muscle cells in the suprachoroid particularly evident? 3 places
1. behind the **fovea**
2. around the entry points of the **posterior ciliary arteries and nerves**
3. in bundles running **parallel to vessels travelling anteriorly** from the posterior pole as far as the exit points of the vortex veins
67
What is the function of the network of non-vascular smooth muscle cells in the human choroid?
remains speculative, may include regulation of blood flow
68
What are the 2 key nerves which innervate the choroid?
long and short ciliary nerves
69
What is the origin of the long ciliary nerves?
from nasociliary branch of V1
70
What is the pathway of the long ciliary nerves supplying the choroid?
* from nasociliary branch of V1
* pass through choroid and transmit sensory fibres to the cornea, iris and ciliary body
71
What are 3 structures in addition to the choroid that are innervated by the long ciliary nerves?
1. cornea
2. iris
3. ciliary body
72
What fibres are also carried in the long ciliary nerves and what do they supply?
sympathetic fibres; carry nerves to the dilator pupillae
73
What structure do the short ciliary nerves arise from?
ciliary ganglion
74
What type of fibres are carried in the short ciliary nerves?
* sensory fibres (from nasociliary nerve)
* sympathetic
* parasympathetic fibres (from nerve III, but also VII)
75
What nerve do sensory fibres in the short ciliary nerves arise from?
nasociliary nerve
76
Which 2 nerves do sympathetic and parasympathetic fibres in the short ciliary nerves arise from?
1. CNIII (predominantly)
2. CNVII
77
What are 2 types of fibres carried in the long ciliary nerves?
1. sensory fibres to cornea, iris and ciliary body
2. sympathetic fibres to dilator pupillae
78
Where do parasympathetic fibres carried in the short ciliary nerves synapse?
pterygopalatine ganglion
79
How do both long and short ciliary nerves pierce the sclera?
in the form of a ring 2-3mm anterior to the optic nerve sheath, along with long and short posterior ciliary arteries
80
What structures pierce the sclera with the long and short ciliary nerves?
long and short posterior ciliary arteries
81
Where do the nerve terminals from long and short ciliary nerves travel following piercing the sclera?
the nerve terminals branch extensively and form **plexi of unmyelinated fibres** in the **choroid and suprachoroid** adjacent to vascular smooth muscle cells
82
Are the fibres of long and short ciliary nerve terminals myelinated or unmyelinated?
unmyelinated
83
Do the nerve terminals of the long and short ciliary nerves extend to the choriocapillaris?
no
84
What substances have been found to be released by fibres in the choroid? 2 types
Vasoactive intestinal peptide (VIP) and neuropeptide Y
85
What is thought to be the function of VIP and neuropeptide Y in the choroid?
VIP =vasodilator agent, and neuropeptide Y =vasoconstrictor agents
86
What may the axons of recently discovered multipolar and bipolar ganglion cell bodies in the choroid which are immunoreactive for nitric oxide synthase (NOS) and VIP supply?
What may be the role of these axons?
the choroidal vasculature (vasodilatory) or non-vascular smooth muscle cells
may have a mechanosensory role (due to structure and immunohistochemical characteristics)
