Cornea Layers Flashcards
5 layers of the cornea outer to inner
Epithelium, bowman’s, stroma, (dua is controversial) descemet’s, endothelial
Thickness of the cornea (central and peripheral)
Central: 530-550 micrometers (0.53-0.55mm)
Peripheral: 710 micrometers (0.71mm)
If the center is too thick, worry about fluid inside.
If too thin, glaucoma risk factor.
How thick is the epithelium centrally?
50 micrometers thick centrally. Thicker in the peripheral.
What cells is the epithelium composed of?
Limal stem cells- basal- wing (transitional)- surface (apical)
Limbal stem cells of the epithelium in the cornea
Stem cells in the limbus that will replace basal cells as they migrate up in the epithelium. Damage the limbus, these stem cells will not replace basal cells as readily and this will damage the epithelia, causing constant corneal abrasions and pain.
Basal cells of the cornea epithelium
Single layer of columnar cells. Only these cells undergo mitosis and migrate outwards into layers. As they move up, they are replaced by limbal stem cells.
They secrete collagen type 4, which makes the basement membrane. The basal cells are connected to the underlying connective tissue by hemidesmosomes (protein) and anchoring fibrils (collagen type 7)
Wing cells of the cornea epithelium
2-3 layers of cells. This is a transition zone.
Surface (apical) cells of the cornea epithelium
2 layers of SSNK cells. Microvilli are present at the apex of the surface to bind ricin and secrete glycocalyx (MUC1, 4, and 16).
Tight junctions join the lateral sides of adjacent cells to form a complete barrier, preventing fluid and molecules from passing between cels. Fluid and molecules must pass THRU cells.
Undergo desquamation and are replaced by wing cells every 7-10 days
How fast can corneal epithelium heal?
Replaced within 7-10 days. Fast.
Hemidesmosomes
Composed of proteins and connect a cell to connective tissue with the help of anchoring fibrils (collagen type 7)
How thick is Bowman’s layer?
8-14 micrometers thick
Characteristics of Bowman’s layer
Acellular.
Composed of interwoven collagen fibrils in ground substance.
Mainly collagen type 1, but also contains 3, 5, and 6.
It is secreted by the corneal epithelial cells before birth, and does not regenerate. It is the transition layer to the stroma.
Characteristics of the stroma
500 micrometers thick (makes up 90% of the corneal thickness)
Composed of connective tissue (keratocytes, ground substance, and collagen) with WBC’s dispersed throughout.
Keratocytes of the stroma of the cornea
Corneal fibroblasts that synthesize collagen and ground substance. They are flattened cells with extensive branching, and lie between or within layers of the collagen. There is a higher density of them in the anterior stroma than the posterior stroma.
Collagen within the stroma of the cornea
- Predominantly type 1. (Also contains type 3, 5, and 6.)
- Fibrils are perfectly straight and spaced.
- Diameter of 30nm.
- They form 200 to 300 lamellae that lie parallel to the corneal surface and each fibril within the lamellae course in the same direction.
- Each lamellae extends from limbus to limbus.
Anteriorly (1/3), lamellae are thin and branch and interweave. This allows for more cross linking and increased rigidity.
Posteriorly (2/3), lamellae are thicker with less branching and interweaving
Collagen type 5 serves to maintain the uniform diameter of type 1.
Type 6 stabilizes keratocytes found between collagen layers.
Ground substance of the stroma of the cornea
Transparent gel composed of water, GAGs(v hydrophilic and help draw water into the stroma) and proteoglycans. Maintains the precise spacing between fibrils. Contribute to the relatively high stromal hydration.
Chondroitan sulfate and derma tan sulfate are abundant in the anterior stroma. If attached to protein, called decorin (proteoglycan)
Keratin sulfate is abundant in the posterior stroma. If attached to a protein, celled keratocan, lumina, or mimican.
What do collagen types 5 and 6 do in the stroma of the cornea?
5- maintains uniform diameter (30nm) of collagen type 1.
6- Stabilizes the keratocytes, which synthesize the collagen and ground substance.
Descemet’s membrane
Basement membrane of the endothelial layer.
Connects endothelial cells to the stroma, weakly.
Composed of mainly type 8 collagen, and sometimes type 4.
Arranged into 2 layers:
Anterior layer is secreted during embryonic development. V thin. 3 micrometers.
Posterior layer is secreted by the endothelium throughout life. By age 40, about 10 micrometers thick.
Thickened at its peripheral termination (called schwalbe’s line)
Schwalbe’s line
Thinking of Descemet’s membrane at peripheral termination before limbus,
Collagen types is descemet’s membrane
Mainly type 8, sometimes type 4
Corneal endothelium characteristics
Innermost layer of the cornea
5 micrometers thick
Composed of a single layer of flattened endothelial cells- basal surface adheres to Descemet’s membrane and the apical surface lines the anterior chamber. Microvilli extend from the apical surface.
70-80% of cells are hexagonal, which allows for minimal spacing between cells. This is the most efficient shape and is called the endothelial mosaic.
Desmosomes join the lateral sides of adjacent cells, so that fluid can leak between cells and pass into stroma from the anterior chamber.
Metabolic pumps are subundant here to pump ions out of the cornea and into the aqueous humor when necessary. Water then follows through aquaporins.
Endothelial mosaic
70-80% of the cells in the corneal endothelium are hexagonal, which allows coverage without gaps.
Cells in the corneal endothelial contain many what?
Since desmosomes between the endothelial cells provide an incomplete barrier, nutrients and water flow through to the cornea.
Therefore, Metabolic pumps (mainly Na+/K+ ATPases) must be present in the cells to get water out of the cornea. About 3 million per cell in order to pump ions into the aqueous humor. Water will exit the cornea and flow into the anterior chamber through aquaporins.
These cells have a large amount of mitochronidra (2nd most amount in the eye, before photoreceptor cells) which reflects the amount of ATP needed for the metabolic pumps.
Duas layer
Controversial layer located between the stroma and Descemet’s membrane.
15 micrometers thick.
Composed of dense connective tissue
Discovered in 2013 in donor corneas with mean age of 82. May be an artifact. Hasn’t been replicated in younger people.
