ICO Exam Flashcards
Innervation of the conjunctiva
Ophthalmic division of CN V
Blood supply of the conjunctiva + where is the vascular watershed between the anterior and posterior territories
- Anterior ciliary arteries supply blood to the bulbar conjunctiva.
- The tarsal conjunctiva is supplied by branches of the marginal arcades of the lids.
- The proximal arcade, running along the upper border of the lid, sends branches proximally to supply the fornical and then the bulbar conjunctiva as the posterior conjunctival arteries.
- The Iimbal blood supply derives from the ciliary arteries through the anterior conjunctival arteries.
vascular watershed between the anterior and posterior territories lies approximately 3 or 4 mOl from the limbus
Precorneal tear film layers + expression
- superficial oily layer produced predominantly by the meibomian glands
- middle aqueous layer produced by the main and accessory lacrimal glands
- deep mucin layer derived from the conjunctival goblet cells.
The surface cells of the cornea and conjunctiva also express a mucinous glycocalyx.
Corneal epithelium basal cells width and density
12 µm, 6000 cells/mm2
Wing cells cornea
Overlying the basal cell layer in the corneal epithelium, 2 or 3 layers, polygonal
Microplicae and microvilli make the apical surfaces of the wing cells highly irregular; however, the
precorneal tear film renders the surfaces optically smooth
Superficial corneal epithelial cells thickness
30 µm
What nonepothelial cells can appear within the corneal epithelium?
- Wandering histiocytes
- macrophages
- lymphocytes
- pigmented melanocytes
are usually components of the peripheral cornea. - Antigen-presenting Langerhans cells are found peripherally and move centrally with age or in response to keratitis.
Bowman layer thickness
8-12 µm
The stroma’s collagen types
I, III, V, VI
What type of collagen forms the anchoring fibril ofthe epithelium?
VII
How many keratocytes does the cornea have? %?
~ 2.4 million, ~ 5% of the stromal volume
Keratocyte density in the cornea
anteriorly 1058 cells/mm2
posteriorly 771 cells/mm2
Macroperiodicy of the fibrils in the corneal stroma
640 Å
Collagen fibril orientation in the cornea
- obliquely oriented lamellae in the anterior third of the stroma (with some interlacing)
- parallel lamellae in the posterior two-thirds
The corneal collagen fibrils extend across the entire diameter of the cornea, finally winding circumferentially around the limbus.
What is the corneal stroma composed of?
- collagen-producing keratocytes
- ground substance
- collagen lamellae
Where are the goblet cells in the conjuctiva?
The goblet cells (unicellular mucous glands) are concentrated in the inferior and medial portion of the conjunctiva, especially in the region of the caruncle and plica semilunaris. They are sparsely distributed throughout the remainder of the conjunctiva and are absent in the limbal region.
Ground substance of the cornea?
- consists of proteoglycans that run along and between the collagen fibrils.
- Their glycosaminoglycan components (eg, keratan sulfate) are highly charged and account for the swelling property of the stroma.
Where are the keratocytes in the cornea?
In the stroma, they lie between the corneal lamellae
Corneal keratocytes ?
- synthesize both collagen and proteoglycans.
- highly active cells rich in mitochondria, rough endoplasmic reticula, and Golgi apparatuse
- have attachment structures, communicate by gap junctions, and have unusual fenestrations in their plasma membranes
- Their flat profile and even distribution in the coronal plane ensure a minimum disturbance of light transmission
How many types of corneal keratocytes are there?
Studies with vital dyes suggest that there may be at least 3 different types
Descemet membrane thickness
- at birth 3-4 µm
- in adulthood 10-12 µm
Thickness increases with age
What is the Descemet membrane composed of?
- anterior banded zone that develops in utero
- and a posterior nonbanded zone that is laid down by the corneal endothelium throughout life
( These zones provide a historical record of the synthetic function of the endothelium. )
What type of collagen is in the Descemet membrane?
It is rich in type IV collagen
like other basal laminae
What are Hassall-Henle warts?
- Peripheral excrescences of the Descemet membrane
- they are common, especially among elderly people
How are the central excrescences of the cornea called?
cornea guttae
appear with increasing age?
What is the corneal endothelium composed of?
a single layer of mostly hexagonal cells derived from the neural crest
Corneal endothelium developmental origin?
neuroectodermal
Number and density of corneal endothelial cells?
- ~ 500,000 cells
- ~ 3000/mm2
How can the size, shape, and morphology of the endothelial cells be observed?
by specular microscopy at the slit lamp
Orientation of the corneal endothelial cells
The apical surfaces of these cells face the anterior chamber; their basal surfaces abut the Descemet membrane
Corneal endothelial cells - biology
- Typically, young endothelial cells have large nuclei and abundant mitochondria.
- The active transport of ions by these cells leads to the transfer of water from the corneal stroma and the maintenance of stromal deturgescence and transparency
- Mitosis of the endothelium is rare in humans, and the overall number of endothelial cells decreases with age
How do the adjacent corneal endothelial cells interdigitate?
- in a complex way and form a variety of adherent junctions, but desmosomes are never observed between normal cells
- In cross section, pinocytotic vesicles and a terminal web (a meshwork of fine fibrils that increases the density of the cytoplasm) can be observed toward the apical surface of the cells.
- Junctional complexes are present at the overlapping apicolateral boundaries of contiguous cells.
- They form a significant but lesser barrier to ion and water flow than do the tight junctions of the epithelium.
What does corneal endothelial dysfunction and loss cause?
It may cause endothelial decompensation, stromal edema, and vision failure
In humans, endothelial mitosis is limited, and destruction of cells causes cell density to decrease and residual cells to spread and enlarge.
Causes of corneal endothelial dysfunction and loss
- surgical injury,
- inflammation,
- inherited disease (eg, Fuchs endothelial dystrophy
How are the superficial corneal epithelial cells attached to eachother?
occlusion of the zonular fibers (sometimes referred to as zonules).
These zonular fibers confer the properties of a semipermeable membrane to the epithelium
How are the deeper corneal epithelial cells attached to eachother?
desmosomes
How do the corneal keratocytes communicate?
by gap juctions
How much of the globe does the sclera cover?
posterior 4/5
Where do the tendons of the rectus muscles insert?
superficial scleral collagen
Sclera thickness
- thinnest (0.3 mm) just behind the insertions of the rectus muscles
- thickest (1.0 mm) at the posterior pole around the optic nerve head.
- 0.4-0.5 mm thick at the equator
- 0.6 mm thick anterior to the muscle insertions
Where can scleral rupture occur following a blunt trauma?
- in a circumferential arc parallel to the corneal limbus opposite the site of impact
- at the insertion of the rectus muscles
- at the equator of the globe
The most common site is the superonasal quadrant near the limbus
Scleral vessels
the sclera is essentially avascular except for the superficial vessels of the episclera and the intrascleral vascular plexus located just posterior to the limbus.
What are emissaria (sclera)?
Numerous channels that penetrate the sclera, allowing for the passage of arteries, veins, and nerves.
Extraocular extension of malignant melanoma of the choroid often occurs by way of the emissaria.
What are Axenfeld loops?
Branches of the ciliary nerves that supply the cornea sometimes leave the sclera to form loops posterior to the nasal and temporal limbus.
They are sometimes pigmented and, consequently, have been mistaken for uveal tissue or malignant melanoma.
They are typically found a constant distance from the corneoscleral junction, usually within 3-4 (?) mm of the limbus.
DD: Nevi Malignant Melanoma Cysts Scleral Foreign Bodies
What does the episclera consist of?
A dense vascular connective tissue that merges deeply with the superficial sclera and superficially with the Tenon capsule and the conjunctiva.
What does the scleral stroma consist of?
- bundles of collagen
- fibroblasts
- a moderate amount of ground substance
Diameter of the scleral collagen fibers
- outer scleral collagen fibers 1600 Å
- inner collagen fibers 1000 Å
What do the bundles of scleral collagen fibers contain?
- electron-dense bodies
- fibroblasts
- melanocytes
Why is the sclera not transparent like the cornea?
- greater variation in fibril separation and diameter
- the greater degree of fibril interweaving in the sclera
In addition, the lack of vascular elements, such as the scleral emissaria, contributes to corneal clarity.
Collagen fibrils of the sclera - orientation, buildup
- vary in size and shape and taper at their ends, indicating that they are not continuous fibers as in the cornea
- The inner layer of the sclera (lamina fusca) blends imperceptibly with the suprachoroidal and supraciliary lamellae of the uveal tract. The collagen fibers in this portion of the sclera branch and intermingle with the outer ciliary body and choroid.
Limbal structures
l. conjunctiva and limbal palisades
2. Tenon capsule
3. episclera
4. corneoscleral stroma
5. aqueous outflow apparatus
What is the Schwalbe line?
termination of the Descemet membrane
Zones of the surgical limbus
2 equal zones:
(1) an anterior bluish-gray zone overlying the clear cornea and extending from the Bowman layer to the Schwalbe line and
(2) a posterior white zone overlying the trabecular meshwork and extending from the Schwalbe line to the scleral spur, or iris root.
What is the limbus histologically?
- The transition from opaque sclera to clear cornea occurs gradually over 1.0-1.5 mm and is difficult to define histologically.
- The corneoscleral junction begins centrally in a plane connecting the end of the Bowman layer and the Schwalbe line
- Internally, its posterior limit is the anterior tip of the scleral spur
- 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
