Cornea Flashcards
What is another name for the surface of the cornea?
air-tissue interface
Which structures of the eye are responsible for most of the refraction of the eye?
the surface of the cornea and associated tear film
What is the most important property of the cornea?
transparency
What is an additional important property of the cornea (as well as transparency)?
presents a tough physical barrier to trauma and infection
What are 4 important factors that are responsible for corneal transparency?
- avascularity
- regularity and smoothness of covering epithelium
- regular arrangment and spacing of the extracellular nad cellular components in the stroma
- the above is dependent on the state of hydration, metabolism and nutrition of the stromal elements
What are the dimensions of the cornea?
smaller in the vertical than in the horizontal
vertical: 10.6mm
horizontal: 11.7mm
(however viewed from behind, the circumference appears circular)
What is the central radius of the cornea?
7.8mm (peripheral corneal curvatuer is less marked)
What is the distribution of the thickness of the cornea like?
it is thicker at the periphery (0.67mm) than in the centre (0.52mm)
How can the shape of the cornea be visualised with imaging methods?
computerised video keratography, as a series of colour-coded topography maps
What condition is computerised video keratography (CVK) frequently used to diagnose?
astigmatism (also early keratoconus)
What do hotter (closer to red) colours represent in CVK?
higher dioptric values
How are images generated from CVK?
projection of over 6000 points of light onto the corneal surface in the form of placido rings. The images are analysed by the computer and complex colour-coded topographical/dioptric maps can be constructed
What is seen on CVK in astigmatism?
multiple higher points of the cornea:
How many layers is the cornea composed of?
5 layers
- corneal epithelium
- anterior limiting lamina: Bowman’s layer
- Substantia propria or corneal stroma
- Posterior limiting lamina (Desçement’s membrane)
- Corneal endothelium
How thick is the corneal epithelium?
50- 60 micrometres thick
What type of epithelium comprises the corneal epithelium?
stratified (5-6 layers), squamous, non-keratinised epithelial cells (superficial cells are flattened, nucleated and non-keratinised)
How do the superificial cells of the corneal epithelium form a barrier?
extensive tight junctions
How are adjacent cells in the corneal epithelium held together? And how are they held to the underlying basal lamina?
adjacent cells: desmosomes
to underlying basal lamina: hemidesmosomes and anchoring filaments
What are 2 features that characterised the anterior surface of the corneal epithelium?
numerous microvilli and microplicae (ridges) whose glycocalyx coat interacts with, and helps stabilise, the precorneal tear film
What is the function of the microvilli and microplicae (ridges) on the anterior surface of the cornea?
glucocalyx coat interacts with and helps stabilise the precorneal tear film
How are new cells in the corneal epithelium derived?
from mitotic activity in the limbal basal cell layer, whcih displace existing cells both superficially and centripetally in an X, Y, X migratory pattern
How does the corneal epithelium respond to repair disruptions in its integrity?
rapidly by amoeboid sliding movements of cells on the wound margin followed by cell replication
What layer do the basal epithelial cells of the corneal epithelium rest on? What 2 sub-layers is this form of?
basal lamina:
lamina lucida, 25nm
lamina densa 50nm
How is corneal epithelial cell adhesion maintained to the basal lamina?
basement membrane complex, which anchors the epithelium to the Bowman’s layer via a complex mesh of anchoring fibrils (type VII collagen) and anchoring plaques (type VI collagen)
these interact with the lamina densa and the collagen fibrils of Vowman’s layer
What are the 2 types of collagen involved in anchoring the corneal epithelium tohe Bowman’s layer?
- anchoring fibrils: type VII
- anchoring plaques: type VI
What type of cells is the cornela epithelium devoid of?
melanocytes
What type of cells are high in numer in the limal and peripheral corneal epithelium?
myeloid-derived major histocompatibility complex (MHC) class II antigen-positive dendritic cells (aka Langerhans cells)
In addition to the limbal and peripheral corneal epithelium, where else are myeloid-derived MHC class II antigen-positive dendritic cells present in low numbers?
in the central cornea (20-25 cells/ square mmm)
What is astigmatism usually the result of?
differences in the radius of curvature in the vertical and horizontal meridians
Are there MHC class-II negative dendritic cells in the corneal epithelium?
some studies have shown them in the mouse central corneal epithelium but these findings are difficult to verify
in vivo confocal microscopy (IVCM) suggests normal human central corneal epithelium contains dendritic cells but immunophenotype can’t be ascertained in the clinic
What are 2 features of the cornea considered crucial to the success of corneal grafting?
- paucity of antigen presenting cells such as dendritic cells in the human corneal epithelium
- avascular nature of the cornea
What is the Bowman’s layer (aka anterior limiting lamina)?
lies between corneal epithelium and corneal stroma
is a modified acellular region of the stroma
What is the thickness of the Bowman’s layer?
8 - 12 micrometres thick
What structures form the Bowman’s layer of the cornea?
fine, randomly arranged, collagen fibrils (20-30nm diameter), type I, III, V and VI
What are the 4 types of collagen within the corneal Bowman’s membrane?
- type I
- type III
- type V
- type VI
What separates the Bowman’s layer from the corneal epithelium?
thin basal lamina
the anterior surface of the Bowman’s layer is well delineated
What is the delineation like between the Bowman’s layer and the corneal stroma?
the posterior boundary of the Bowman’s layer merges with the stroma
Where does the edge of the Bowman’s layer terminate?
terminates abruptly at the limbus
What does the image show?
layers of the cornea
What is another name for the corneal stroma?
substantia propria
What is the structure of the corneal stroma?
dense connective tissue of remarkable regularity
2 micrometre thick, flattened, collagenous lamellae (200-250 layers) oriented parallel to the corneal surface and continuous with the sclera at the limbus
Which corneal layer makes up the vast majority of the cornea?
the corneal stroma (substantia propria)
What is the thickness of the collagenous lamellae which form the corneal stroma?
predominantly 2 micrometres thick
How many layers of collagenous lamellae are the in the corneal stroma?
200-250 layers
How does the corneal stroma connect with the sclera at the limbus?
continuous with the sclera at the limbus
How are the collagenous lamellae in the corneal stroma oriented?
parallel to the corneal surface and continuous with the sclera at the limbus