L21 - Fibrous Layer (the Limbus) Flashcards
(25 cards)
What is the limbus?
A band that encircles the periphery of the cornea at the corneo-scleral junction; approximately 1.5–2.0 mm wide.
What anatomical changes occur at the limbus?
Abrupt change of radius of curvature, presence of external and internal scleral sulcus.
What are the anterior and posterior boundaries of the limbus (histologically)?
Anterior: plane connecting the termination of Bowman’s and Descemet’s layers; Posterior: plane perpendicular to the globe surface passing through the posterior edge of the scleral spur.
What epithelial change occurs at the limbus?
Corneal epithelium becomes conjunctival epithelium.
How does the stroma change at the limbus?
Regular corneal stroma becomes irregular scleral stroma.
What happens to the corneal endothelium at the limbus?
It becomes discontinuous to wrap around the strands of the trabecular meshwork.
Where do Bowman’s and Descemet’s membranes terminate?
At the anterior border of the limbus.
Which structures begin within the limbal area?
Conjunctival stroma, episclera, and Tenon’s capsule.
How thick is the limbal epithelium?
10–15 cell layers thick.
What types of cells may be present in the basal layer of the limbal epithelium?
Melanocytes.
Is pigmentation evident in the limbus?
Yes, pigmentation may be evident in the limbus and conjunctiva.
How does the organization of collagen bundles change at the limbus?
Transition from regular corneal lamellae to irregular, random scleral collagen bundles.
Is there a line of demarcation at the limbus?
No, the transition is gradual and no line of demarcation may be identified.
How do scleral fibrils extend at the limbus?
They extend further anteriorly on the external than on the internal side.
What are the main tissue components identified in a cross-section of the limbus?
Epithelium, loose connective tissue stroma (limbal conjunctiva, Tenon’s capsule, episclera, limbal stroma), conjunctival stromal vessels, peripheral corneal arcades, episcleral vessels, intrascleral plexus, deep scleral plexus, scleral spur, anterior part of ciliary muscle, Schlemm’s canal, trabecular meshwork, uveal meshwork, iris process, Bowman’s layer, Descemet’s membrane.
What is the scleral spur?
A group of collagen fibrils within the stroma, lying circumferentially, continuous with the collagen of the trabeculae, forming an annulus that may help maintain corneal curvature.
What happens to Descemet’s membrane at the limbus?
It tapers at the anterior limbal boundary and its posterior portion becomes interlaced with connective tissue for the anterior sheets of the trabecular meshwork.
What happens to the endothelium at the limbus?
It continues into the anterior chamber angle as the endothelial covering.
Where do Tenon’s capsule and the episclera begin and end?
They begin in the limbus but do not continue into the cornea.
What is the palisades of Vogt?
Radial projections of limbal epithelium and stroma that extend into the cornea in a spoke-like fashion; 0.5 mm wide and 2–4 mm long; suspected site of corneal epithelial stem cell origin.
What is the suspected site of corneal epithelial stem cell origin?
The epithelium in the area of the palisades of Vogt.
What is the X, Y, Z hypothesis of corneal epithelial maintenance?
X: proliferation of basal epithelial cells; Y: centripetal movement of peripheral cells; Z: epithelial cell loss from the surface.
What is the main source of limbal blood supply?
Capillary loops from conjunctival and episcleral vessels that surround the cornea.
How is blood drained from the limbus?
Limbal veins collect blood from anterior conjunctival veins; 25–30 collector channels drain into radial episcleral veins, which empty into anterior ciliary veins.
