The Eye Flashcards
What anchors the basal corneal epithelial cells to the Bowman layer.
Hemidesmosomes
What is the blood ocular barrier and where is it located?
prevents extravasation of intravascular contents into the eye. It consists of intercellular junctions of adjacent cells at various locations in the eye: the blood–aqueous barrier and the blood–retina barrier (inner and outer).
anteroposterior diameter of the eye
23-25mm
Average transverse diameter of the adult eye
24mm
What is the anterior chamber
space between iris and cornea
What is the posterior chamber
anatomical portion of the eye posterior to the iris and anterior to the lens and vitreous face
Average anterior chamber depth
3.11mm
Average volume of AC
220 microlitres
Average volume of posterior chamber
60 microlitres
Volume of vitreous cavity
5-6mls
Total volume of the average adult eye
6.5-7.0 mls
Layers of the cornea (outer to inner)
- epithelium
- Bowman layer
- stroma
- Descemet membrane
- endothelium
Refractive index of the cornea
1.376
Average radius of curvature of cornea
7.7mm
Power of the cornea
43.25 dioptres
Total power of the eye
58.60 dioptres
Cornea vertical diameter
11mm
Cornea horizontal diameter
12mm
Is the posterior surface of the cornea more curved than the anterior surface?
Yes
Is the central cornea thinner than the peripheral cornea?
Yes the central cornea is thinner (0.5 mm) than the peripheral cornea (1.0 mm)
Where is the cornea flatter?
The cornea flattens in the periphery, with more extensive flattening nasally and superiorly than temporally and inferiorly.
What epithelium covers the corneal epithelium?
The anterior surface of the cornea is covered by a lipophilic, nonkeratinized, stratified squamous epithelium that is composed of 4–6 cell layers and is typically 40–50 μm thick
Density of corneal epithelial cells?
6000 cells/mm squared
How thick is the Bowman layer/membrane?
8-15 microns thick. Unlike the Descemet membrane, it is not restored after injury but is replaced by scar tissue.
What percentage thickness of the cornea is the stroma
90%. It is composed of collagen-producing keratocytes, ground substance, and collagen lamellae.
What collagen type is predominantly found in the stroma?
Type 1
III, IV, V, VI, XII, and XIV
What do the keratocytes resemble ultrastructurally?
fibroblasts
How many keratocytes are found in the cornea?
The cornea has approximately 2.4 million keratocytes, which occupy about 5% of the stromal volume; the density is higher anteriorly (1058 cells/mm2) than posteriorly (771 cells/mm2). Keratocytes are highly active cells rich in mitochondria, rough endo- plasmic reticula, and Golgi apparatus
How thick is the Descemet membrane?
At birth, the Descemet membrane is 3–4 μm thick, increasing to 10–12 μm at adulthood. It is a true basement membrane, and its thickness increases with age.
It is composed of an anterior banded zone that develops in utero and a posterior nonbanded zone that is laid down by the corneal endothelium throughout life
Which collagen is Descemet membrane rich in?
Descemet membrane is rich in type IV collagen.
What are Hassal Henle Warts?
Peripheral excrescences of the Descemet membrane, known as Hassall-Henle warts, are common, especially among elderly people. Central excrescences (corneal guttae) also appear with increasing age.
Which embryological layer is corneal endothelium derived from?
Neural crest cells
Corneal endothelial cell density?
In young adult eyes, approximately 500,000 cells are present, at a density of about 3000/mm centrally and up to 8000/mm2 peripherally.
What is the corneal endothelial cell mitotic capability?
Endothelial cell dysfunction and loss—through surgical injury, inflammation, or dis- ease (eg, Fuchs endothelial corneal dystrophy)—may cause endothelial decompensation, stromal edema, and vision loss. Because endothelial mitosis is limited in humans, destruction of cells causes cell density to decrease and residual cells to spread and enlarge
What structures are found at the limbus
- conjunctiva and limbal palisades of Vogt, which house the corneal stem cells
- episclera
- junction of corneoscleral stroma
- aqueous outflow apparatus (collector channel)
What is the surgical limbus
The surgical limbus, an external landmark for incisions in cataract and glaucoma surgery, is sometimes referred to as the gray or blue zone. Its blue-gray appearance is due to the scattering of light through the oblique interface between cornea and sclera, which
According to pathologists, what are the limbal landmarks?
Pathologists consider the posterior limit of the limbus to be formed by another plane perpendicular to the surface of the eye, approximately 1.5 mm posterior to the termination of the Bowman layer in the horizontal meridian and 2.0 mm posterior in the vertical meridian, where there is greater scleral overlap
Where is the sclera thinnest?
The sclera is thinnest (0.3 mm) just behind the insertions of the rectus muscles
Where is the sclera thickest?
thickest (1.0 mm) at the posterior pole around the optic nerve head.
What are the most common sites of scleral rupture from blunt trauma?
- in the superonasal quadrant, near the limbus
- in a circumferential arc parallel to the corneal limbus opposite the site of
impact - behind the insertion of the rectus muscles
Is the sclera avascular?
Yes. The sclera, like the cornea, is essentially avascular except for the vessels of the intra- scleral vascular plexus, located just posterior to the limbus, and the episcleral vessels. The episcleral vessels have superficial and deep plexuses
In episcleritis which plexus is involved?
The superficial plexus runs beneath the Tenon capsule in a radial pattern; in episcleritis, it is this vascular plexus that is involved
In scleritis which plexus is involved?
The deep episcleral plexus rests on the surface of the sclera and is the layer involved in scleritis.
Which emissaria (channels) penetrate the sclera?
- anterior emissaria: penetration of the anterior ciliary arteries anterior to the rectus muscle insertions
- middle emissaria: exit of vortex veins
- posterior emissaria: lamina cribrosa, penetration of the short and long posterior
ciliary vessels and ciliary nerves
Through which emissaria does malignant melanoma of the choroid spread extraocularly?
Extraocular extension of malignant melanoma of the choroid occurs by way of the middle emissaria.
What are Axenfield loops?
Branches of the ciliary nerves that supply the cornea sometimes leave the sclera to form loops posterior to the nasal and temporal limbus. These nerve loops, called Axenfeld loops, are sometimes pigmented and, consequently, have been mistaken for uveal tissue or malignant melanoma
What structures do the anterior chamber angle contain?
- Schwalbe line
- Schlemm canal and trabecular meshwork (also see the section Trabecular
Meshwork) - scleral spur
- anterior border of the ciliary body (where its longitudinal fibers insert into the
scleral spur) - peripheral iris
What 3 structures make up the trabecular meshwork?
Layers of the trabecular meshwork:
uveal,
corneoscleral,
juxtacanalicular.
The point of highest resistance to outflow is at the juxtacanalicular layer. The outlet channel traverses the limbus and drains into an aqueous vein
What is Schwalbe line the peripheral termination of?
Descemet membrane
Which ciliary muscle does the scleral spur receive the insertion of?
longitudinal ciliary muscle, contraction of which opens up the trabecular spaces.
Do trabecular meshwork layer cells have contractile properties?
Yes which may influence outflow resistance.
They also have phagocytic properties.
Do trabecular meshwork layer cells have contractile properties?
Yes which may influence outflow resistance.
They also have phagocytic properties
Why is the resistance to drainage of aqueous highest in the juxtacannalicular meshwork?
partly because the pathway is narrow and tortuous and partly because of the resistance offered by extracellular proteoglycans and glycoproteins.
Properties of Schlemm canal
A circular tube that closely resembles a lymphatic vessel
Formed by a continuous monolayer of nonfenestrated endothelium and a thin connective tissue wall
Basement membrane of the endothelium is poorly defined
Lateral walls of the endothelial cells are joined by tight junctions
(MIGS), a microstent is implanted in the Schlemm canal to bypass the trabecular meshwork, the point of greatest outflow resistance, thereby increasing aqueous outflow.
How many collector channels arose from Schlemm canal?
Approximately 25–30 collector channels arise from the Schlemm canal and drain into the deep and midscleral venous plexuses
What does the uvea comprise of?
- iris
- ciliary body (located in the anterior uvea)
- choroid (located in the posterior uvea)
Where is the uvea most firmly attached to the sclera at?
- scleral spur
- exit points of the vortex veins
- optic nerve
These attachments account for the characteristic anterior dome-shaped choroidal detachment.
Classification of uveitis according to SUN criteria?
- anterior: anterior chamber
- intermediate: vitreous
- posterior: choroid (primary or secondary from the retina)
- panuveitis: anterior chamber, vitreous, and retina or choroid
Major structures of the iris
- stroma
- vessels and nerves
- dilator muscle and anterior pigmented epithelium
- sphincter muscle
- posterior pigmented epithelium
What does the iris stroma consist of?
pigmented cells (melanocytes),
nonpigmented cells,
collagen fibers,
matrix containing hyaluronic acid.
Where is the major arterial circle located?
ciliary body
Where does the dilator pupillae muscle originate from embryologically?
The dilator muscle develops from the anterior pigmented epithelium and is derived from the neuroectoderm.
It lies parallel and anterior to the posterior pigmented epithelium
Where does the sphincter pupillae muscle originate from embryologically?
Like the dilator muscle, the sphincter muscle is derived from neuroectoderm.
Where are the dilator and sphincter pupillae muscles located on the iris
Sphincter pupillae- near the pupillary margin in the deep stroma, anterior to the posterior pigmented epithelium of the iris
What is the posterior pigmented epithelium of the iris continuous with?
nonpigmented epithelium of the ciliary body and thence with the neurosensory portion of the retina.
What do the apex and base of the ciliary body form?
apex of the ciliary body is directed posteriorly toward the ora serrata. The base of the ciliary body gives rise to the iris.
What is the only attachment point of the ciliary body onto the sclera?
at its base, via its longitudinal muscle fibers, where they insert into the scleral spur.
What are the functions of the ciliary body?
2 principal functions:
1) aqueous humor formation and lens accommodation.
2) It also plays a role in the trabecular and uveoscleral outflow of aqueous humor.
2 parts of the ciliary body?
pars plana
pars plicata
Anatomy of the pars plana?
avascular, smooth, pigmented zone that is 4 mm wide and extends from the ora serrata to the ciliary processes.
The safest posterior surgical approach to the vitreous cavity is through the pars plana, located 3–4 mm from the corneal limbus.
Anatomy of the pars plicata?
richly vascularized and consists of approximately 70 radial folds, or ciliary processes. The zonular fibers of the lens attach primarily in the valleys of the ciliary processes but also along the pars plana
How many layers of epithelial cells are found in the ciliary body?
lined by a double layer of epithelial cells: the inner, nonpigmented ciliary epithelium and the outer, pigmented ciliary epithelium.
The basal lamina of the nonpigmented epithelium faces the posterior chamber, and the basal lamina of the outer pigmented epithelium is attached to the ciliary stroma and blood vessels
What type of tight junctions in the ciliary body maintain the blood-aqueous barrier?
Zonulae occludentes
What retinal layer is the posterior non pigmented ciliary epithelium continuous with?
ILM retina
What is the main arterial supply to the ciliary body?
the anterior and long posterior ciliary arteries, which join to form a multilayered arterial plexus consisting of a superficial episcleral plexus; a deeper intramuscular plexus; and an incomplete major arterial circle
What is the venous drainage of the ciliary body?
The major veins drain posteriorly through the vortex system, although some drainage also occurs through the intrascleral venous plexus and the episcleral veins into the limbal region.
What 3 layers of muscle fibres are found in the ciliary body?
- longitudinal
- radial
- circular
Most of the ciliary muscle is made up of the outer layer of longitudinal fibers that attach to the scleral spur.
Innervation of ciliary muscle?
Innervation is derived mainly from parasympathetic fibers of the third cranial nerve via the short ciliary nerves.
Parasympathetic vs sympathetic innervation to the ciliary body?
Sympathetic fibers have also been observed and may play a role in relaxing the muscle. Cholinergic drugs contract the ciliary muscle.
How thick is the choroid?
0.25 mm in thickness and consists of 3 layers of vessels
Choroid layers of vessels?
- the choriocapillaris, the innermost layer
- a middle layer of small vessels (Sattler layer)
- an outer layer of large vessels (Haller layer)
Blood supply to the choroid?
long and short posterior ciliary arteries and from the perforating anterior ciliary arteries
Blood drainage from the choroid?
Venous blood drains through the vortex system. Blood flow through the choroid is high compared with that through other tissues. As a result, the oxygen content of choroidal venous blood is only 2%–3% lower than that of arterial blood.
Does the choriocapillaris have fenestrations?
Yes.
However The middle and outer layers of choroidal vessels are not fenestrated.
Does fluorescein diffuse across choriocapillaris?
small molecules such as fluorescein, which diffuse across the endothelium of the choriocapillaris, do not leak through medium and large choroidal vessels.
What cells are found in the choroidal stroma?
Abundant melanocytes, as well as occasional macrophages, lymphocytes, mast cells, and plasma cells, appear throughout the choroidal stroma
Impact on lack of melanin in albinism on patients receiving retinal laser
Loss of melanin production within the rPe and melanocytes within the choroid and iris occurs in patients with ocular and oculocutaneous albinism. Lack of pigmentation within the posterior segment can impair uptake during laser photocoagulation.
How much refractive power does the lens contribute to the refractive power of the eye
20D
Equatorial Diameter of the lens at birth and at adult?
equatorial diameter is 6.5 mm at birth; it increases in the first 2–3 decades of life and remains approximately 9–10 mm in diameter in adults.
Anteroposterior diameter of the lens at birth and at adult?
The anteroposterior width of the lens is approximately 3 mm at birth and increases after the second decade of life to approximately 6 mm by age 80 years.
Describe accommodation
achieved by ciliary muscle contraction, which moves the ciliary muscle mass forward and inward. This contraction relaxes zonular tension and allows the lens to assume a globular shape, causing its anterior radius of curvature to shorten
Vascular and nervous supply of the lens?
The lens lacks innervation and is avascular.
The lens depends solely on the aqueous and vitreous for its nourishment
Which collagen type is the lens capsule rich in?
Type 4
Does the thickness of the anterior lens capsule increase throughout life?
Yes
Why does the posterior lens capsule not increase in thickness?
Because there are no lens epithelial cells posteriorly, the thickness of the posterior capsule remains constant.
What is the thickness of the anterior and posterior lens capsule?
Anterior-15.5 μm
Posterior- 2.8 μm
What material does the anterior lens capsule consist of?
a fibrogranular material, identified as laminin, which is absent from the posterior capsule at the ultrastructural level.
Where is the germinative zone of the lens found?
Peripherally in the equatorial lens bow area, there are meridional rows of cuboidal preequatorial cells that form the germinative zone of the lens
What is responsible for the refractive index of the lens?
the high concentration of lens crystallins (α, β, and γ) in the fiber cytoplasm.
What is the Y orientation of lens sutures?
the anterior Y-shaped suture and the posterior inverted Y–shaped suture
Where do the lens zonular fibres originate from?
the basal laminae of the nonpigmented epithelium of the pars plana and pars plicata of the ciliary body
Where do lens fibres attach on the lens?
These fibers attach chiefly to the lens capsule anterior and posterior to the equator
What protein are lens fibrils made up of?
Fibrillin
Why is the fundus reddish colour?
due to the transmission of light reflected from the posterior sclera through the capillary bed of the choroid.
Which structure does the retina and its underlying epithelial layer embryologically come from?
Optic vesicle
Neurosensory retina layers
- internal limiting membrane
- nerve fiber layer
- ganglion cell layer
- inner plexiform layer
- inner nuclear layer
- middle limiting membrane
- outer plexiform layer (referred to as Henle fiber layer in the foveal region)
- outer nuclear layer
- external limiting membrane
- rod and cone inner segments
- rod and cone outer segments
How many rods are there in the eye?
approximately 100–125 million rods
How many cones are there in the eye?
6–7 million cones in the human retina
Ratio between rods and cones
approximate ratio of 20:1
Do tight junctions exist between outer segment of photoreceptors and RPE apical processes?
No
The factors responsible for keeping these layers in apposition are poorly understood but probably involve active transport and other mechanisms, including van der Waals forces, oncotic pressure, and electro- static forces.
What does the rod outer segment structure look like?
contains multiple laminated discs resembling a stack of coins and a central connecting cilium
What are the 2 parts of the rod inner segment?
2 additional elements: an outer ellipsoid containing numerous mitochondria, and an inner myoid containing a large amount of glycogen
Are rod or cone discs attached to the cell membrane?
Rod discs are not attached to cell membrane
Cone discs are attached to the cell membrane
What are rod terminals that synapse called?
Spherules
What are cone terminals that synapse called?
Pedicles
What do Muller cells do?
glial cells that extend vertically from the external limiting membrane in- ward to the internal limiting membrane
Müller cells, along with the other glial elements (the fibrous and protoplasmic astrocytes and microglia), provide structural support and nutrition to the retina and are crucial to normal physiology. In addition, they contribute to the inner blood– retina barrier.
How do retinal blood vessels maintain the inner blood retinal layer?
This physiologic barrier is due to the single layer of nonfenestrated endothelial cells, whose intercellular junctions, under physiologic conditions, are impervious to tracer substances such as fluorescein and horseradish peroxidase
What is the outer blood retinal barrier made up of?
consists of tight junctions between adjacent RPE cells.
What retinal layers are supplied by the choroidal vessels?
The outer nuclear layer and remaining layers of the outer retina are perfused by the choroid. The outer plexiform layer represents a watershed zone in regard to perfusion
At what retinal layer does retinal detachment occur?
A potential space exists between this outermost layer of the neurosensory retina and the RPE and is the plane of separation in retinal detachment.
Which retinal layer has the photoreceptor nuclei layer?
ONL
What is found in the OPL of the retina?
composed of synapses between the photoreceptors and bipolar cells.
Which layer of the retina contains the superficial vascular plexus?
GCL
Which layer of the retina contains the deep vascular plexus?
INL
What structures are found in the INL
contains nuclei of bipolar, Müller, horizontal, and amacrine cells.
What does IPL contain?
consists of axons of the bipolar and amacrine cells and dendrites of the ganglion cells and their synapses.
Where is the retina thinnest and thickest?
The retina is thickest in the papillomacular bundle near the optic nerve (0.23 mm) and thinnest in the foveola (0.10 mm) and ora serrata (0.11 mm).
What area is defined as the macula clinically?
the area between the temporal vascular arcades
Histologically what is the area defined as the macula?
the region with more than 1 layer of ganglion cell nuclei
What is the diameter of the fovea?
1.5mm diameter approximately
Foveola diameter?
0.35 mm
Where in the retina is the GCL, INL, OPL the thickest?
Parafovea which is 0.5mm wide
What is the blood supply to the FAZ
entirely perfused by the choriocapillaris
Functions of the RPE
- vitamin A metabolism
- maintenance of the outer blood–ocular barrier
- phagocytosis of the photoreceptor outer segments
- absorption of light (reduction of scatter)
- formation of the basal lamina of the Bruch membrane
- production of the mucopolysaccharide matrix surrounding the outer segments
- maintenance of retinal adhesion
- active transport of materials into and out of the RPE
Where do lipofuscin granules arise from?
Lipofuscin granules probably arise from the discs of photoreceptor outer segments and represent residual bodies from phagosomal activity.
Clinically, these lipofuscin granules are responsible for the signal observed with fundus autofluorescence.
How are drusen formed?
hroughout life, incompletely digested residual bodies, lipofuscin, phagosomes, and other material are excreted beneath the basal lamina of the RPE.these contribute to the formation of drusen, which are accumulations of this extracellular material. Drusen can vary in size and are commonly classified by their ophthalmoscopic appearance as hard or soft.they are typically located between the basement mem- brane of rPe cells and the inner collagenous zone of Bruch membrane. Large soft drusen are associated with intermediate-stage age-related macular degeneration.
What 5 layers does the Bruch membrane compose of?
- basal lamina of the RPE
- inner collagenous zone
- relatively thick, porous band of elastic fibers
- outer collagenous zone
- basal lamina of the choriocapillaris
Is the bruch membrane permeable to fluorescein?
Yes
What does the ora serrata separate?
serrata separates the retina from the pars plana
Where is the margin of the ora serrata smooth and serrated?
Smooth- temporally
Serrated- nasally
What percentage of water is found in the vitreous?
99%
Why is the viscosity of the vitreous almost twice that of water?
Contains hyaluronic acid
Which collagen type is found in the vitreous?
fine collagen fibrils (chiefly type II)
Where is the vitreous most firmly adhered to the retina at?
Vitreous base
Where are additional strong adherence points of the vitreous to the retina at?
posterior lens capsule (hyaloideocapsular ligament; also known as ligament of Weiger), along the retinal vessels,
at the peri- macular region,
and at the optic nerve margin
What is the prominent area of liquifaction of premacular vitreous gel called?
premacular bursa, or precortical vitreous pocket.
