Histology of the Eye Flashcards
(85 cards)
1
Q
What are the 3 coats/tunics of the eye?
A
- Fibrous tunic
- Cornea
- Sclera - Vascular tunic
- Choroid
- Ciliary body
- Iris - Neural tunic
- Retina
2
Q
Describe the external white fibrous tunic?
A
Consists of sclera and cornea
1. Sclera - white fibrous layer covering
5/6 of the eye
2. Cornea - transparent structure forming the anterior 1/6 of the outer coat
3
Q
Describe the middle vascular pigmented tunic?
A
Consists of the choroid, ciliary body and iris
1. Choroid
- outer pigmented and inner vascular and invests 5/6 of the eye
- Nourishes the retina and darkens the eye
2. The ciliary body
- Thickened portion of the vascular coat between choroid and iris
- Consists of ciliary ring, ciliary processes and ciliary muscles
3. Iris
- Thin contractile circular pigmented diaphragm with central aperture the pupil
4
Q
Describe the internal nervous (neural) tunic?
A
Consists of the retina
- outer pigmented + inner nervous
- Posterior part - photosensitive
- Anterior part - not photosensitive
5
Q
What is the optic disc?
A
blind spot
- Consists of optic nerve fibers : formed by axons of ganglion cells which connect to rods and cones
- Has no receptors hence not photosensitive
6
Q
What is the cornea?
A
It is the transparent, avascular and highly innervated anterior portion of the fibrous coat
7
Q
Name the 5 layers of the cornea?
A
- Corneal epithelium.
- Bowman’s membrane.
- Stroma.
- Descemet’s membrane.
- Corneal endothelium.
8
Q
Describe corneal epithelium?
A
- Non-keratinized Stratified squamous epithelium
- Contains numerous free nerve endings
9
Q
Describe Bowman’s epithelium?
A
anterior limiting membrane
- separates the epithelium from the corneal stroma
- consists of densely packed collagen fibrils embedded in ground substance
- It is homogenous non-cellular layer containing type I collagen fibrils
10
Q
Describe stroma?
A
- It is the thickest layer (about 90%).
- It is composed of parallel lamellae of dense collagenous C.T.
11
Q
Stroma is composed of?
A
- Each lamella is composed mainly of parallel type I collagen fibers with long fibroblasts
*200 - 250 layers of collagen fibrils (mainly tropcollagen type I, III, V and VI.
*Flattened fibrocytes, ( keratocytes), are located between the layers of collagen fibres.
*The regular arrangement of the collagen fibres and their small diameter (20 - 60 nm) acount for the transparency of the cornea
12
Q
What is posterior endothelium?
A
Lines the posterior surface of the cornea
13
Q
Describe descemets membrane?
A
- thick basement membrane
- Descemet’s membrane (the posterior limiting lamina) separates the posterior endothelium and the corneal stroma
- The lateral margins of the cornea are continuous with the conjunctiva (anterior corneal epithelium) and sclera (corneal stroma)
14
Q
Describe the corneal endothelium?
A
Simple squamous epithelium
15
Q
What are the functions of corneal endothelium?
A
- Formation of Descemet’s membrane.
- Keeping the stroma relatively dehydrated (sod. pump > water withdrawal from the stroma).
16
Q
What is the corneoscleral junction (limbus)?
A
• It is the transition region between the cornea and sclera
• It is about 1.5 mm width
• it is highly vascular
17
Q
What does the limbus contain?
A
- Trabecular meshwork:
Endothelium-lined spaces. It leads to canal of Schlemm. - Canal of Schlemm:
It drains the aqueous humor into the venous system.
18
Q
What is a potential cause of glaucoma?
A
The aqueous humor is ultimately reabsorbed by small veins in the sclera.
A blockage of aqueous humor drainage will result in an increase of intraocular pressure (glaucoma) and eventually in neuronal degenration.
19
Q
Describe the histology of the sclera?
A
- tough layer of dense connective tissue
- collagenous fibres and networks of elastic fibres.
- Melanocytes are present in deep parts of the sclera
20
Q
What are the functions of the sclera?
A
- maintains the shape of the eyeball
- site of attachment of the ocular muscles.
- provides a point of insertion for part of the ciliary muscle.
Note: The sclerocorneal junction (limbus) houses the canal of Schlemm, through which the aqueous humor is drained into ciliary veins
21
Q
What is the sclera?
A
- The tough layer of dense connective tissue
- Distended by the intraocular pressure
- covers the posterior 5/6 of the fibrous tunic
22
Q
What is sclera proper?
A
• Sclera Proper: consists of interlacing bundles of type I collagen
(dense collagenous C. T., irregular type).
• Melanocytes are located in the deeper regions.
23
Q
What does the choroid consist of?
A
- loose connective tissue,
- A sense network of blood vessels
- Connective tissue cells and melanocytes(give the choroid its dark colour).
24
Q
What is the choriocapillary layer?
A
Small blood vessels in the innermost part of the choroid that supplies the retina with nutrients.
25
What is the choroid?
It is the vascular, pigmented posterior portion of the middle vascular tunic
26
What is the structure of the choroid?
It is composed mainly of loose C.T. with melanocytes.
It is separated from the retina by its Bruch's membrane.
27
What is Bruchs membrane?
- Bruch's membrane (glassy membrane) which consists of the basal laminae of the capillaries and of pigmented epithelium as well as elastic and collagen layers
- located between the choroid and the retina.
- It consists of two layers of collagen fibres and a network of elastic fibres between them
28
What is the ciliary body?
It is the anterior continuation of the choroid
- It surrounds the lens.
29
What does the ciliary body consist of?
- Smooth muscle cells form three bundles, the ciliary muscle.
- Ciliary processes are short extensions of the ciliary body towards the lens.
- The inner surface of the ciliary body and its processes are lined by the ciliary epithelium formed by the pars ciliaris of the retina
30
Describe the structure if the ciliary body?
- It is formed of loose vascular and pigmented C. T. that contains 3 bundles of smooth muscle cells (ciliary muscle).
- Its inner surface is lined by pars ciliaris retinae ( 2 rows of columnar cells; outer pigmented and inner non- pigmented layers).
- Its inner surface is highly folded forming the ciliary processes
- Suspensory ligament of the lens - Zonuel fibers, which consist of fibrillin, extend from the ciliary processes towards the lens.
31
Describe the layers of the ciliary body?
1. The outer cell layer is pigmented, whereas the inner cell layer, is nonpigmented
2. The cells of the inner layer of the ciliary epithelium produce the aqueous humor of the eye
32
Describe the structure of ciliary processes?
- Processes project from the inner surface of the anterior 1/3 of the ciliary body towards the lens.
- Are covered by pars ciliaris retinae (2 rows of columnar cells).
- They give attachment to the lens suspensory ligaments (zonule fibers).
33
What are zonule fibers?
The Zonule fibers, which anchor the lens, are attached to the ciliary processes.
These radially-oriented fibers form the Zonule of Zinn.
34
What is mechanism of accommodation according to the ciliary body?
- Two of the bundles of the ciliary muscles attach to the sclera and stretch the ciliary body when they contract, thereby regulating the tension of the zonule fibres.
- The reduced tension will result in a thickening of the lens which focusses the lens on close obiects
Note:
Relaxed ciliary muscles = far accommodation
Contracted ciliary muscles = near accommodation
35
What is the iris?
Continuation of the ciliary body
36
What are the 2 components of the iris?
1. anterior uveal or stroma face
- fibroblasts and melanocytes (nonepithelial)
2. The posterior neuroepithelial surface
- covered by the retina
37
Describe the anterior uveal (iridial stroma) face of the iris?
• consists of a vascularized loose connective tissue rich in melanocytes, macrophages and fibrocytes, and annular smooth muscle, sphincter pupillae muscle at the tip
• Myoepithelial cells in the outer (or anterior) layer is continuation of the retina, i.e. the layer adjacent to the stroma of the iris, have radially oriented muscular extensions.
• These extensions form a flat sheet immediately beneath the anterior layer of the retina, the dilator pupillae muscle.
38
Describe the posterior neuroepothelial surface of the iris?
- Has two pigmented sub-layers:
1. outer - pigmented layer of ciliary body : myoepithelial cells that forms the dilator pupilae muscle
2. inner highly pigmented inner layer continuation of the retina forms the posterior epithelium
39
Name and describe the 5 layers of the iris?
It is formed of 5 layers:
• 1- Anterior border layer:
Incomplete layer of fibroblasts and melanocytes.
• 2- Stroma:
Poorly vascularized C.T. with fibroblasts and melanocytes
• 3- Vessel layer
Well-vascularized loose C.T.
Centrally, it contains circularly arranged smooth muscle fibers (sphincter pupillae muscle).
• 4- Dilator pupillae muscle layer:
Contains radially arranged myoepithelial cells.
• 5- Posterior surface layer (pigmented epithelium layer):
It is composed of 2 rows of pigmented epithelial cells (pars iridis retinae).
They are the continuation of pars ciliaris retinae.
40
Describe the colour of the iris?
• Posterior part of iris always brown in color
• People with brown/black eyes with pigment throughout iris
• People with blue eyes-rest of iris clear, brown pigment at back appears blue after passing through iris/cornea
41
What determines eye colour?
the variations in the amount of melanin pigment in each melanocyte determine eye color
- This is due to genetic variabilities in the expression of melanocyte proteins, such as tyrosinase, melanocortin receptor and others.
42
Describe the nervous system control of the muscles of the eye?
The sphincter or constrictor pupillae muscle forms a circle at the pupillary margin and is under parasympathetic control.
The dilator pupillae muscle is under sympathetic system control.
43
What are the 2 muscles that regulate the size of the pupil?
1. Pupillary constriction is mediated by the annular sphincter pupillae muscle, is clinically referred to as miosis
2. Pupillary dilation is mediated by constriction of the dilator pupillae muscle, as mydriasis
Note:
The sphincter/constrictor pupillae muscle is formed by a ring of smooth muscle.
The dilator pupillae muscle is made of a
myoepithelium, adjacent to the pigmented double epithelium.
44
Describe the mechanism behind focus?
• Ciliary muscles in ciliary body pull on lens to focus far away
• Elasticity of lens brings back to close focus
• Thus, with age, less elasticity, no close focus-›far-sighted
45
What is the path of light in the eye?
Cornea -› Anterior segment -› Pupil -> Lens ->
Posterior segment - > Neural layer of retina -› Pigmented retina
46
What is the location of the lens?
The lens is positioned behind the iris and between the ciliary processes
47
What is the function of ciliary smooth muscles?
The ciliary smooth muscles regulate the thickness of the lens
- a process called accomodation.
Note: The ciliary muscles are mainly under parasympathtic control.
48
What is the lens?
transparent, biconvex, elastic and avascular structure.
49
The lens consists of?
1. a lens capsule,
2. the subcapsular epithelium and
lens substance (cortical and nuclear lens cell fibres)
- It does not contain blood vessels or nerves
50
What are the cells of the lens?
1. The anterior side of the lens is covered by a simple epithelium
2. The basal side is facing anterior and is covered by a basement membrane/
capsule
3. the apical side is anchoring the posterior lens fibers
51
What is the lens capsule?
Lens
The lens capsule:
• thick, elastic basal lamina.
• formed by the mitotically active cells of the subcapsular epithelium
• The zonule fibres insert into the lens capsule.
Cells of the subcapsular epithelium (or anterior lens cells) that are mitotically active only cover the anterior
"hemisphere" of the lens.
• As they divide, cells gradually move towards the equator of the lens where they transform into lens fibres.
52
What are the cells of the lens?
Few organelles are scattered in a cytoplasm filled with filensin and cystallin (a, B and y) proteins.
- These proteins are responsible for the transparency and refractive properties of the lens and account for up to 60% of the mass of lens fibres.
53
What is the crystalline lens?
• Transparent, avascular, biconvex,
• Lens capsule - type IV collagen,
• New lens fibers are produced through out the life
54
What is presbyopia?
decreased elasticity and power of accommodation of lens with age
55
What is a cataract?
loss of transparency of lens, causes can be infections, metabolic, hereditary, trauma, UV light
56
What are the 2 regions of the retina?
1. The outer nonsensory retinal pigmented epithelium. - single layer of cuboidal cells from optic disc to ora serrata
2. The inner sensory layer of the retina - contains the photoreceptors, the first neurones which process the sensory information, and the neurones which convey the pre-processed sensory information to the central nervous system.
57
Describe the histology of the outer non sensory retinal pigmented epithelium?
The outer pigment epithelium is derived from the outer layer of the optic cup and
constitutes a simple columnar epithelium
The pigment epithelium cells contain many melanin granules.
58
Describe the cells of the pigmented epithelium of the retina?
• Cuboidal to columnar cells (single layer).
• Apical microvilli.
• Abundance of melanin granules.
59
What are the functions of the retina?
1- Absorb light.
2- Phagocytosis of membranous discs from tips of rods.
3- Esterification of Vitamin A (in SER).
60
What are the cell layers of the retina?
It is composed of 10 distinct layers (from outside to inside):
• 1- Pigmented epithelium
• 2- Rods and cones layer.
3- Outer limiting membrane.
4- Outer nuclear layer.
5- Outer plexiform layer.
6- Inner nuclear layer.
7- Inner plexiform layer.
8- Ganglion cell layer.
9- Optic nerve fiber layer.
10- Inner limiting layer.
61
What is the relationship between the retina and brain?
Retina is the outgrowth of the brain
62
Describe the photoreceptors of the retina?
• Neurons have specialized receptors at end with "photo pigment" proteins (rhodopsins)
- Rod cells function in dim light, not color-tuned
- Cone cells have three types: blue, red, green
- In color blindness, gene for one type of rhodopsin is deficient, usually red or green
• Photoreceptors sit on pigmented layer of choroid. Pigment from melanocytes--melanoma possible in retina!!
• Axons of photoreceptors pass on top or superficial to photoreceptor region
• Axons congregate and leave retina at optic disc (blind spot)
• Fovea centralis is in direct line with lens, where light is focused most directly, and has intense cone cell population (low light night vision best from side of eye)
• Blood vessels superficial to photoreceptors (retina is good sight to check for small vessel disease in diabetes)
63
What are rods and cones?
• Rods - more in # (12 million), more sensitive to light, used in dim or night light), have maximum absorption at 496 nm of light ( black and white pictures)
• Cones - less in # (7million), three classes (L,M,S), less sensitive to light (for day vision), have absorption at 420 (blue), 531(green) and 588 nm (red) of light, for color vision
64
Describe structure of photoreceptor cells?
1. Dendrite formed of:
-Outer segment (OS): contains membranous discs containing rhodopsin (in rods) and iodopsin (in cones).
- Connecting Stalk: with modified cilium.
-Inner segment (IS).
2. Cell body.
3. Axon: synapses with dendrite of bipolar neuron of inner nuclear layer.
65
What are the functions of photoreceptor cells?
Rods are receptors for dim light ( low intensity light).
Cones are receptors for bright light and color vision (red, green & blue).
66
Outer limiting membrane of retina?
A region of zonulae adherents junctions between Muller cells and the photoreceptors.
67
Outer nuclear layer of the retina?
Contains nuclei of the rods & cones.
68
Outer plexiform layer of the retina?
Contains axodendritic synapses between the photoreceptor cells and dendrites of bipolar and horizontal cells.
69
Inner nuclear layer of the retina?
• Inner nuclear layer:
- Contains the nuclei of:
1- Bipolar neurons.
2- Horizontal neurons.
3- Amacrine neurons (unipolar neurons):
4- Neuroglial cells (Muller cells) that extend between the vitreous body and the inner segments of rods and cones.
70
Inner plexiform layer of retina?
Contains axodendritic synapses between axons of bipolar neurons and dendrites of ganglion cells and amacrine cells.
71
Ganglion cell layer of retina?
Contains cell bodies of large multipolar neurons of the ganglion cells.
72
Optic nerve fiber layer of retina?
Contains unmyelinated axons of the ganglion cells.
N.B. These axons become myelinated as the nerve pierces the sclera.
73
The inner limiting membrane of the retina?
It is formed by the basal laminar of the Muller cells
74
What is the fovea centralis?
• It lies in the center of macula lutea.
• Cones are highly concentrated in the fovea.
• It is responsible for visual acuity.
75
What are the types of the cells in the retina?
• Types of cells in the retina:
1- Pigmented epithelium.
2- Nerve cells:
- Photoreceptor cells (rods & cones)
- Bipolar neurons.
- Ganglion cells.
- Association neurons:
i. Horizontal cells.
i. Amacrine cells.
3- Neuroglial cells:
- Muller's cells.
- Astrocytes.
76
4 cell groups in sensory retina are?
Four cell groups in sensory
retina a:
1. Photorecepter neurons - 1.
rods and cones
2. Conducting neurons - bipolar and ganglion cells
3. Association neurons - horizontal and amacrine cells
4. Supporting neuroglia cells
-Muller cells
77
What is the blind spot of the retina?
At the optic papilla the optic nerve penetrates the retinal layer and leaves the eye, and the retinal blood supply enters and exits.
This creates a blind spot in the retina.
78
What is the blood supply of the eye?
The second blood supply system of the retina is the retinal artery and vein system, which enters and exits at the optic papilla.
79
What is the conjunctiva?
• It is the transparent mucous membrane lining the inner surfaces of the eyelids (palpebral conjunctiva) and reflecting onto the sclera of the anterior surface of the eye (bulbar conjunctiva).
80
Describe the histology of the conjunctiva?
• 1- Epithelium:
Stratified columnar epithelium with numerous goblet cells.
• 2- Lamina propria:
Loose C.T.
81
What is retinal detachment?
82
What is iridodialysis?
83
What is opacified cornea?
84
What is corneal abrasion?
• Fluorescsein reveals corneal abrasion
85
What is a dislocated lens?
