Flashcards in Chapter 2: The Eye Deck (163):
What is the radius of curvature of the cornea? What is the radius of curvature of the sclera? What effect do the differences in radius of curvature have on the shaper of the eye?
Therefore, shape of the globe is an oblate spheroid
What is the AP diameter of the eye? What is the average transverse diameter of the eye?
What are the borders of the anterior chamber? What is the volume of aqueous it holds? How deep is it?
Bordered by the cornea anteriorly and the iris posteriorly
200 micro L
3 mm deep
What are the borders of the posterior chamber? What is the volume?
posterior of the iris to the anterior of the lens/face of the vitreous
volume= 60 micro L
comprises 2/3 of the volume of the eye
What is the total volume of the average eye?
What are the layers of the eyeball?
outermost= cornea + sclera
Middle= Uvea (choroid, ciliary body, iris)
What are the three layers of the pre corneal tear film?
outer= superficial oily layer (produced by meibomian glands)
middle= aqueous layer (produced by lacrimal glands)
bottom= Deep mucin layer (produced by conjunctival goblet cells)
What are the functions of the pre corneal tear film?
provides nutrients and 02 via diffusion
provides a clear smooth optical surface
contains Ig's, lysozymes, and lactoferrin
What are the dimensions of the avg cornea? How much refractive power does the cornea provide?
dimensions: 12 mm wide x 11 mm tall x 0.5-1.0 mm thick (0.5 in center, 1.0 at periphery--due to posterior surface being more curved than anterior surface)
Cornea provides 2/3 of refractive power of the eye, or about +43 diopters
Note; center of cornea is nearly spherical, with a diameter of 4.0 mm and a thickness of 0.5 mm
Is the thinning of the cornea toward the periphery symmetrical in all quadrants?
No, it becomes more flat in the nasal and superior quadrants then in the inferior and temporal quadrants.
Embryologically, what is the anterior surface of the cornea derived from?
What kind of cells cover the anterior surface of the cornea?
non-keratinized stratified squamous epithelium
What cells cover the basal lamina of the outer layer of the cornea?
2-3 layers of polygonal "wing cells"
Are the surface of the wing cells smooth? If not, how does the body compensate for this?
The surfaces of the cells are covered with microplicae and microvilli, causing it to be irregular. The body compensates via the pre corneal tear film
Where do the corneal epithelial cells originate from?
They originate from limbal stem cells and migrate centripetally.
What non-epithelial cells are found in the outer layers of the cornea?
wandering histiocytes, macrophages, lymphocytes, and pigmented melanocytes.
antigen-presenting langerhans cells found peripherally (migrate centrally with keratitis or old age)
Where is Bowmans layer located? Does it regenerate after injury?
located beneath the basal lamina of the corneal epithelium, is a tough layer consisting of randomly dispersed collagen fibrils 8-12 micrometers thick
Is not restored after injury--becomes scar tissue
constitutes 90% of corneal thickness, composed of collagen-producing keratocytes, ground substance, and collagen lamellae
How are the collagen fibrils in the cornea oriented?
In the anterior 1/3 of the stroma, the fibers are oriented obliquely
In the posterior 2/3 of the stroma, they are oriented parallel
What factors about the collagen promotes corneal transparency?
uniform fibril size and separation
What types of collagen are found in the corneal stroma?
Type I, III, V, VII
What is the ground substance found in the cornea? How does it contribute to corneal edema?
Ground substance consists of proteoglycans, which are highly charged, and contribute to swelling during corneal edema.
How many keratocytes are found in the cornea? How are they distributed within the stroma?
There are 2.4 x 10^6 keratocytes in the average adult cornea. There is a higher density of keratocytes anteriorly (1058 cells/mm^2) compared to posteriorly (771 cells/mm^2)
How do the keratocytes contribute to corneal transparency?
They have a flat profile and an even distribution within the coronal plane that minimizes the disturbance of light transmission
What is Descemets Membrane?
a true basement membrane, the basal lamina of the corneal endothelium
Rich in Type IV Collagen
T/F: The thickness of descemets membrane increased with age?
True. At birth, it is between 3-4 mm thick, and increased in size until adulthood, where it is 10-12 mm thick.
What are the parts of Descemets Membrane?
Anterior banded zone that develops in utero, and a posterior non-banded zone that develops throughout life.
What are Hansall-Henle Warts?
Peripheral excrescences of descemets membrane that are common findings in the elderly
Describe the make-up of the corneal endothelium? From what embryonic layer are they derived?
Consists of a single layer of mainly hexagonal cells
Derived from neural crest cells
Describe the orientation of the endothelial cells and their function
The cells are oriented that their apical surface faces away from descemets membrane. They function as an ion pump that helps maintain corneal deturgescence and transparency.
Can you replace corneal endothelial cells?
Endothelial mitosis is limited. Therefore, when injury occurs, residual cells typically spread and enlarge, decreasing cell density.
What is the sclera?
covers 4/5 of the posterior surface of the globe
is covered by Tenons capsule
Where is the sclera the thinnest? Where is it the thickest?
It is thinnest behind the insertions of the rectus muscles (0.3 mm)
It is thickest at the posterior pole around the ON head (1.0 mm)
When scleral rupture occurs due to blunt trauma, where is the most common location of rupture?
The superonasal quadrant near the limbus
What is the blood supply of the sclera?
It is essentially avascular, except for the superficial episcleral vessels and the interscleral vascular plexus
What are Emissaria?
they are numerous channels that penetrate the sclera allowing for passage of arteries/veins/nerves
What are Axenfeld Loops?
They are branches of the ciliary nerve that supply the cornea, that sometimes leave the sclera to form loops posterior to the limbus
What is the episclera?
consists of dense vascular CT that merges with the superficial sclera and tenons capsule/conjunctiva
What part of the sclera contains the fibers with the largest diameter?
The outer collagen fibers have a larger diameter (1600 A) compared to the inner fibers (1000 A)
What is the opaque appearance of the sclera attributed to?
1- increased variation in the collagen fibril separation and diameter
2- increased degree of fibril interweaving
What is the limbus?
the transition zone b/w the peripheral cornea and the sclera (occurs over 1.0-1.5 mm)
What structures comprise the limbus?
There are 5 structures in total:
1- conjunctival and limbal palisades
4- Corneoscleral stroma
5- Aqueous outflow apparatus
How is the surgical limbus divided?
Into 2 zones.
1- Anterior blue-gray zone: overrides the clear cornea and extends from bowman's membrane to schwalbes lines
2- Posterior White Zone: overlies the TM and extends from schwalbes line to the scleral spur (iris root)
What structures comprise the Anterior Chamber Angle?
1- Schwalbes Line (termination of Descemets)
2- Schlemm Canal and TM
3- Scleral Spur
4- Anterior border of Ciliary Body
Under what conditions is the angle deeper? More Shallow?
The angle is deeper in patients who are aphakic, pseudophakic, and myopic
The angle is more shallow in hyperopic patients
How deep is the angle at the center (typically)?
What is the volume of the anterior chamber in the emmetropic eye?
200 micro L
What are the paths that the aqueous can drain through?
Primary drainage via the TM
Seconary drainage via the uveoscleral pathway
In what population does 50% of the aqueous drain via the uveoscleral pathway?
Internal Scleral Sulcus
accommodates schlemms canal externally and TM internally
Insertion and action of the longitudinal ciliary muscle? Innervation?
Inserts onto the scleral spur. Contraction causes the TM to open and therefore increased drainage.
Nerve fibers mediated via sympathetic, sensory, and pterygopalatine pathways
What is the Trabecular Meshwork?
a circular sponge work of CT lined by trabeculocytes consisting of 3 laters. Its apex is located at Schwalbes line and its base at the scleral spur and ciliary body
What are the three layers of the TM?
The Uveal Portion, the Corneoscleral Meshwork, and the Juxtacanalicular Tissue.
Uveal Portion of the TM
Composed of cord-like trabeculae
Has fewer elastic fibers than corneroscleral meshwork
apertures are larger and less circular than that of the corneroscleral meshwork
Corneoscleral Portion of TM
a series of thin, flat, perforated CT sheets arranged in a laminar pattern
each beam is covered by a monolayer of thin trabecular cells
Invest the extent of Schlemms Canal, including endothelial meshwork
What is the Endothelial Meshwork, and what is its significance?
= a multilayered network of cells b.w the outermost layers of the corneoscleral meshwork and the endothelial lining of schlemms canal
Significance: contributes most to outflow resistence
Why does the endothelial meshwork contribute most to outflow resistance?
thought to contribute due to it being a narrow and tortuous pathway that can also be blocked by glycosaminoglycans and proteoglycans
What is Schlemms Canal?
a circular tube resembling a lymphatic vessel formed by a continuous monolater of non-fenestrated endothelium with a thin CT wall
Endothelial cells are connected by tight junctions
What are the collector channels? What are they also known as?
arising from Schlemms canal, are 25-30 channels that drain into either the deep, midscleral, or episcleral venous plexuses.
Channels that drain into the episcleral venous plexuses are also known as "aqueous veins"
What is the uveal tract?
the main vascular component of the eye consisting of the iris, choroid, and ciliary body
What are the attachment sites of the uveal tract?
There are 3 attachment sites:
- Scleral Spur
-Exit points of the vortex veins
- Optic Nerve
What is the Iris?
The most anterior portion of the uveal tract
made up of blood vessels and CT
mobility allows the pupil to change size
What is the Iris Stroma?
composed of pigmented cells (melanocytes) and non-pigmented cells, collagen fibrils, and a matrix (containing hyaluronic acid)
What forms the bulk of the Iris Stroma?
Blood vessels and nerves of the Iris, most of which follow a radial course and arise from the major arterial circle
anastomoses occurs between the arteries and veins at the collarette (thickest part of the stroma) to form the minor vascular circle of the Iris.
Where is the Major Arterial Circle located?
In the apex of the Ciliary Body
Facts about the posterior pigmented Iris Epithelium
densely pigmented and continuous with non-pigmented epithelium of the ciliary body
polarity is maintained from embryogenesis
curves around the pupillary margin and extends for a short distance onto the anterior border layer of the Iris Stroma (aka a "Pigment Ruff")
What is the dilator muscle of the Iris embryologically derived from?
From the Optic Cup (therefore, neuroectoderm)
Where is the dilator muscle located in the Iris?
it lies parallel and anterior to the posterior pigmentary epithelium
Describe the innervation of the dilator muscle of the iris. What kind of stimulation leads to contraction?
has dual sympathetic and parasympathetic innervation, with alpha 1 adrenergic stimulation causing contraction
Describe the sympathetic chain
1st Order Neurons: originates in the posterior ipsilateral hypothalamus, travels inferiorly down the SC and synapses in the intermediolateral gray matter of T1.
2nd Order Neuron: The nerve fiber leaves the SC, travels over the pulmonary apex, through the stellate ganglion (without synapsing), and synapses on the Superior Cervical Ganglion
3rd Order Neuron: Post-Ganglionic fibers travel with ICA plexus, enter the cavernous sinus, travels with the ophthalmic division of CN V to the dilator muscle
What embryological tissue is the Iris sphincter muscle derived from?
What is the sphincter muscle composed of? Where is it located?
A circular band of smooth muscle. It is located near the pupillary margin in the deep stroma anterior to the pigmented posterior iris epithelium
What is the primary innervation of the sphincter muscle?
Myelinated parasympathetic fibers originating in CN III nucleus that responds to muscarinic stimulation.
What is the pathway of the parasympathetic fibers that innervate the Iris sphincter muscle?
Fibers leave the edinger-westphal nucleus and follow the inferior division of CN III after bifurcating in the cavernous sinus. They continue with the branch that supplies the IO muscle, exit, and synapse on the ciliary ganglion. Post-ganglionic fibers then travel with the short ciliary nerves to the iris sphincter
What is the Ciliary Body?
Bridges the anterior and posterior aspects uveal tract.
It is triangular in shape with its apex directed posteriorly toward the ora serrate
Its base gives rise to the Iris
Where does the ciliary body attach to the sclera?
It attaches at the scleral spur, where the longitudinal muscle fibers also insert
What are the two functions of the ciliary body?
aqueous humor production and lens accomodation
What is the size of the ciliary body? What makes up the ciliary body?
Pars Plana and Pars Plicata
What is the Pars Plana?
a relatively avascular smooth pigmented zone, 4 mm wide, extending from the ora serrate to the ciliary process
What is the Pars Plicata?
richly vascularized, consisting of 70 residual folds/ciliary processes
Where do the zonular fibers attach to the pars plicata and what is their blood supply?
The zonular fibers attach to the valleys of the ciliary processes, whose blood supply is capillary plexuses that are fed via the major arterial circle.
What kind of epithelium lines the ciliary body?
a double layer of pigmented and non-pigmented epithelium
Where is the non-pigmented ciliary body epithelium located? What kind of epithelium makes up the non-pigmented epithelium?
Non-Pigmented Layer: located b/w the aqueous humor and the pigmented layer of the CB
In the pars plana, the epithelium is cuboidal. In the pars plicata, the epithelium is columnar.
What are some characteristics of the pigmented ciliary body epithelium?
cells are relatively uniform, with each cuboidal cell having multiple basal infoldings, a large nucleus, mitochondria, an extensive endoplasmic reticulum, and many melanosomes
ER and golgi apparatus are important in the formation of the aqueous humor
What is the major blood supply of the Ciliary Body? Venous Drainage?
Anterior and Long Posterior Ciliary Arteries, which join together to form a multi-layered arterial plexus
Consists of the superficial episcleral plexus, a deeper intramuscular plexus, and an incomplete major arterial circle (located in CB, NOT IRIS)
Drains via the vortex system
How many muscle layers are there in the ciliary muscle and what are they?
There are 3 layers to the ciliary muscle, which function as a normal smooth muscle unit
Longitudinal Layer (outer), radial layer (middle), and circular layer (inner)
Note: Rich in type VI collagen
What is the largest muscle layer of the ciliary muscle, and where does it insert?
The longitudinal (outer) layer, which inserts into the scleral spur
What is the primary innervation for the ciliary muscle?
parasympathetic fibers of CN III via the short ciliary nerves. Cholinergic stimulation causes contraction
What is the Choroid?
the most posterior aspect of the uveal tract
provides nourishment to the outer retina
How many layers make up the choroid, and what are they?
Middle: layer of small vessels
Outer: layer of large vessels
What is the blood supply of the choroid? Venous drainage?
long and short posterior ciliary arteries and perforating anterior ciliary arteries
Therefore, has a higher 02 concentration than most of the eye, being 2-3 percent below normal arterial level
Venous drainage via the vortex system
What is Bruchs Membrane?
a seroes pf CT sheets resulting from fusion of the basal lamina of the RPE and the choriocapillaris
What are the components of Bruchs Membrane?
- RPE basal lamina
- inner collagenous zone
- thick porous band of elastic fibers
- outer collagenous zone
- basal lamina of choriocapillaris
What is the choriocapillaris? How does this differ from the middle and outer layers of the choroid?
a continuous fenestrated layer of large capillaries (40-60 micrometers in diameter) lying in a single plane beneath the RPE
Differs from the other layers of the choroid in that it is fenestrated, whereas the others are not.
What is the Lens?
a biconvex structure located directly behind the posterior chamber and pupil that contributes +20D to the total refractive power of the eye.
What are the dimensions of the lens?
Diamter: @ birth- 6.5 mm Adulthood- 9-10 mm
Width: @ Birth- 3 mm Adulthood- <=6 mm
What effect does ciliary muscle contraction have on the lens?
When contracting, the ciliary muscle moves forward and inward, easing tension on the zonular fibers and allowing the lens to assume a more globular shape, allowing for accommodation.
Why does the power of the lens decrease with age?
- increased size of the lens
- altered mechanical relationship
- increased stiffness of lens nucleus (due to changes in the crystalline lens fibers)
- alterations in the geometry of zonular attachments occurring with age
- changes in lens capsule elasticity
What is the blood supply and innervation of the lens?
Trick question--lens is avascular, lacks innervation, and receives all nutrients via the aqueous and vitreous.
When does the lens capsule develop?
During embryologic development in utero
What does the lens capsule consist of?
consists of fine filaments of type IV collagen arranged in lamellae arranged parallel to the surface.
What are the differences between the anterior and posterior lens capsule?
Anterior capsule- synthesis continues throughout life until an adult thickness of around 15.5 micrometers is reached; contains laminin (a fibrogranular material)
Posterior capsule: only synthesized during embryogenesis, and therefore maintains a constant thickness of around 2.8 micrometers
Where is the lens epithelium located?
epithelium is located between the anterior and equatorial capsule, but NOT the posterior capsule
central zone- represents a stable population of cells whose # decreased with age
Intermediate zone- show occasional mitosis
Peripheral zone- rows of cuboidal post-equatorial cells that form the germinative zone (undergo mitosis)
Note: if cells from the peripheral zone are left behind during phacoemulsification, PCO can occur
What part of the lens fibers are present at birth? What part develops post-natally?
Where do fibers differentiate into the adult cortex in the lens?
In the superficial and deep regions
What are characteristics of lens fibers?
hexagonal in cross section
possess numerous finger-like projections (interdigitation of these form the lens sutures)
What attribute of the lens fibers allows for high refractive index?
Increased concentration of lens crystallins (alpha, bravo, and gamma crystallins)
What do the zonular fibers do? Where do they originate from? Where do they attach to?
They hold the lens in place
Originate from the ciliary body (valleys of the ciliary processes of the pars plana and pars plicate)
attach to the lens capsule anterior and posterior to the equator
What are the zonular fibers made of?
multiple filaments of fibrillin
What happens to the zonules in distance vision? Near vision?
Distance vision- zonules are held under tension (ciliary muscle is relaxed) and lens is flat
Near Vision- zonules are relaxed due to ciliary muscle contraction and the lens assumes a globular shape
What is the Retina?
a thin transparent structure that developes from the inner and outer layers of the optic cup
How many layers are there in the retina? What are they?
10 layers: ILM, NFL, GCL, IPL, INL, MLM, OPL, ONL, ELM, rod and cone outer segments
what does the term "fundus oculi" mean?
refers to the part of the posterior eye visible via ophthalmoscopy, consisting of the retina, vessels, and ON head.
What is the macula?
5-6 mm in diameter, is the area of the retina that is rich in cones and responsible for color vision/high acuity vision.
Located b/w the temporal arcades
What is the Ora Serrata?
the junction b/w the pars plana and the retina
Why does the ocular fundus have a reddish color?
The red color is due to the transmission of light reflected from the posterior sclera through the capillary bed of the choroid
What is the RPE?
Retinal pigment epithelium, is a monolayer of hexagonal cells that extends anteriorly from the optic disc to the ora serrata
Note: at the ora, it merges with pigmented epithelium of ciliary body)
What are the functions of the RPE?
- vitamin A metabolism - blood/retina barrier
-phagocytosis of photoreceptor outer segments
- production of mucopolysaccharaide matrix surrounding photoreceptor outer segments
- active transport of materials into and out of RPE
- heat exchange - formation of basal lamina of bruchs membrane - absorption of light (decreases scatter)
How are cells of the RPE attached to one another?
Zona occludentes and Zona adherentes, which provide structural stability and help to maintain the blood-retina barrier
How do RPE cells differ in the fovea compared to the retinal periphery?
Foveal cells are taller, thin, and contain an increased number of larger melanosaomes
At the periphery, RPE cells are shorter, broader, and less pigmented
What are lipofuscin granules?
thought to arise from the discs of photorecepter outer segments that are residual s/p phagocytosis by RPE
What are phagosomes?
membrane-enclosed packets of disc outer segments that have been engulfed by RPE
What are Drusen?
accumulation of incompletely digested residual bodies, lipofuscin granules, phagosome, and other material excreted beneath the RPE basal lamina
typically located b/w basement membrane of RPE and inner collagenous zone of Bruchs membrane
What are the neuronal elements of the neurosensory retina?
Consists of highly specialized rods and cones, horizontal cells, bipolar cells, and amacrine cells
Description of a Rod
consists of an outer segment that contains multiple laminated discs resembling a "stack of coins" and a central connecting cilium
microtubules have a 9+0 configuration
What are the elements of the rod inner segment?
1- an outer ellipsoid containing numerous mitochondria
2- an inner myoid containing a large amount of glycogen
Extrafoveal cones v Foveal cones
Extrafoveal Cones: have conical ellipsoids and myoids, and their nucleri are closer to the ELM than rod nuclei
Foveal Cones: have cylindrical inner segments, but are otherwise identical to extrafoveal cones
What are horizontal cells?
neuron that makes synaptic connections with many rod spherules and cone pedicles; also spread processes throughout the OPL
What are bipolar cells?
vertically oriented neurons whose dendrites synapse with either rod or cone synaptic bodies, and their axons make contact with ganglion/amacrine cells in the IPL
How many nerve fibers are in the optic nerve?
1x10^6, or a little over 1 million
What glial elements are found in the neurosensory retina?
Mullers cells, astrocytes, etc....that provide structural support and nutrition to the retina
What are muller cells?
glial cells that extend vertically from the ELM inward to the ILM; nuclei are located in the INL
What is the blood supply of the inner portion of the retina?
branches of the central retinal artery
macula is supplied by a cilioretinal artery, a branch from ciliary circulation
How do retinal vessels help maintain the blood-retinal barrier?
Retinal vessels contain a single layer of non-fenestrated endothelial cells covered by a basal lamina that is attached to mullers cells and other glial elements
What is the difference between rod and cone nuclei?
Rod discs do not attach to the cell membrane (therefore, are discrete structures)
Cone discs are attached to the cell membrane, and thought to be renewed by membranous replacement
How do retinal vessels differ from other vessels throughout the body?
Retinal vessels lack an IEL and a continuous layer of smooth muscle cells
How far into the retina do the retinal vessels typically extend?
Up until the middle limiting membrane
What part of retinal vessels do vaso-occlusive phenomena occur, and why?
They typically occur at AV junctions, as the vessels share a basement membrane
What is the External Limiting Membrane (ELM)
the outermost layer of the neurosensory retina, and not considered a true membrane.
Is formed by attachments of adjacent photoreceptors and muller cells
Is highly fenestrated is
What is the outer plexiform layers?
second outermost layer of neurosensory retina, composed of interconnections b/w the photoreceptor synaptic bodies and horizontal/bipolar cells
What is the Fiber Layer of Henle?
the OPL in the macula, where it is thicker and contains more fibers (because axons or rods and cones become longer and more oblique as they deviate from the fovea)
What is the Inner Nuclear Layer?
contains the nuclei of muller cells as well as bipolar/amacrine/horizontal cells
What is the middle limiting membrane?
a zone of desomosome-like attachments in the region of the synaptic bodies of photoreceptors
What is the Inner Plexiform Layer?
consits of axons of bipolar/amacrine cells, dendrites of ganglion cells, and their synapses
What is the ganglion cell layer?
consists of bodies of ganglion cells that lie near the inner surface of the retina
What is the nerve fiber layer?
formed by axons of ganglion cells (typically not myelinated until they exit via the lamina cribosa)
What is the internal limiting membrane?
formed by the footplates of muller cells and their attachments to the basal lamina
not a true membrane
How are the cells/layers of the retina oriented?
In the outer/middle layers of the retina, the cells are oriented perpendicularly to the retina.
In the inner layer, the cells are oriented parallel to the retinal surface
What the Macula?
an area of the retina responsible for high acuity vision that histologically contains >1 layer of ganglion cell nuclei
typically yellow in color
Why is the macula yellow in color?
the presence of carotenoid pigments, lutein and zeaxanthin
high proportion of lutein corresponds to more rod-dense areas, which higher proportions of zeaxanthin correspond to more cone-dense areas.
What is the fovea?
a concave central retinal depression around 1.5 mm in diameter
What is the parafovea?
the area around the fovea, around 0.5 mm wide
where the GCL/INL, OPL are the thickest
What is the perifovea?
surrounds the parafovea
1.5 mm wide
What is the foveola?
a central depression within the fovea consisting only of cones, muller cells, and other glial elements
0.35 mm wide x 0.1 mm thick
foveal cones are shaped more like rods, but are cytologically the same as extrafoveal cones
Why is the foveal avascular zone important?
it is an important clinical landmark in the treatment of subretinal neovascular membranes by laser photocoagulation
What is the Ora Serrata?
the bounday b/w the pars plana and retina
a watershed zone b/w the anterior and posterior ciliary vascular systems
What is the volume of the vitreous?
makes up 4/5 of ocular volume, or 4 mL
What is the vitreous made of? What percentage of the vitreous is water?
Fine collagen fibrils (mainly type II) and hyalocytes
Why is the vitreous important?
important to the metabolism of ocular tissues, as it provides a route for their metabolites
What are the sites of attachment of the vitreous?
- the retina peripherally at the vitreous base
- at the disc margin
- at the perimacular region
- along retinal vessels
- at the periphery of the posterior lens capsule