Corneal diseases represent a major cause of blindness worldwide. Corneal transplantation advanced rapidly in the last decade with the development of new procedures. The cornea can be transplanted in full (penetrating keratoplasty (PK)), or only partially (lamellar keratoplasty). With the increasing demand for corneal tissue due to an aging population, there is a shortage of donor corneas suitable for transplantation. Progress in TE may thus offer new therapeutic solutions for the replacement of a diseased cornea.
Chambers of Fluid in the Eye
The anterior and posterior chambers are filled with aqueous humor.
The vitreous chamber is filled with the vitreous humor.
Secreted from the ciliary epithelium;
Maintains intraocular pressure.
Contains many phagocytes to remove cellular debris from visual field;
Avascular (Forrester et al., 2016).
|Anterior chamber||(Between cornea and iris)|
|Posterior chamber||(Between iris and lens)|
|Vitreous chamber||(Between the lens and the retina)|
The Corneal Epithelium
The corneal epithelium is attached to the conjunctival epithelium.
The narrow zone between the cornea and the conjunctiva is known as the limbus. The limbus is the source of corneal epithelial stem cells in humans.
Limbal stem cell proliferation maintains the cornea.
Limbal stem cells prevent conjunctival epithelial cells from migrating onto the corneal surface.
Damage to the limbus leads to conjunctivalization of the cornea (Ezhkova and Fuchs, 2010; Forrester et al., 2016).
The cornea comprises five layers: the epithelium, Bowman’s layer, the stroma, Descemet’s membrane, and the endothelium.
The corneal endothelium is the innermost layer of the cornea. The main function of these cells is to pump fluid out of the corneal stroma, allowing the cornea to remain optically clear. Loss of functionality of the endothelial layer results in stromal edema and vision loss. Descemet’s stripping automated endothelial keratoplasty (DSAEK) is the gold standard for the surgical treatment of corneal endothelial diseases. It involves removing the endothelium and its underlying basement membrane (Descemet’s membrane) and replacing it with a layer of posterior cornea cut from a donor eye. Compared to PK, DSAEK offers many advantages. This procedure promotes faster visual recovery and more predictable refractive outcomes. The main concern regarding the use of DSAEK is a high rate of postoperative endothelial cell loss and a risk of postoperative graft dislocation (Gorovoy, 2006).
Describe one TE technique to replace diseased corneal endothelium
Fibroblasts can be cultivated and induced to secrete their own extracellular matrix and form sheets to reconstruct stromal tissue. Corneal endothelial cells can then be seeded on each side of the reconstructed stroma to create a tissue-engineered cornea similar to the native cornea. This method is called the self-assembly approach (Carrier et al., 2009; Jay et al., 2015).
What are keratoprosthetic devices?
These are acellular artificial implants consisting of a central optic held in a cylindrical frame (Fig. 5.2). The keratoprosthesis replaces the section of cornea that has been removed. The development of a keratoprosthesis has encountered numerous difficulties, and many of the early corneal transplants had high infection rates and extrusion. In the late 1980s, the concept of a “core-and-skirt” device, in which a biointegrable “skirt” surrounds a central optic, became the most common design. The importance of size and contiguity of pores in the porous skirt were also fully appreciated (Avadhanam et al., 2015; Crawford et al., 2002). Keratoprosthesis such as the AlphaCor, previously known as the Chirila keratoprosthesis, consists of a polymethylmethacrylate device with a central optic region fused with a surrounding sponge skirt. The procedure to implant a keratoprosthesis is, however, complicated, and to date, clinical use of keratoprostheses has been infrequent.
What are the challenges of corneal TE?
The structure of the cornea is unique and the collagen architecture is difficult to replicate.
What are the properties of scaffolds required for corneal TE?
When constructing a tissue-engineered cornea, the choice of scaffold material is vital to enable the cells to form the same complex arrangement as they would in vivo. Scaffold materials available for corneal TE include acellular corneal stroma and collagen (Table 5.1).
Strong enough to withstand handling during surgery;
Flexible enough to take the shape of the eye and lay flat on the surface;
Easily produced with high speed and low cost;
Controllable biodegradability or bioresorbability.
There are two types of donor tissues which are used in surgery. The sclera, or white portion of the eye; and the cornea, which is the clear lens at the front of the eye.
The cornea is the clear lens at the front of the eye which focuses light onto the retina. Damage or disease to the cornea may cause it to change shape or become cloudy. This can cause pain, distorted or blurry vision. Corneal transplant replaces the damaged or diseased with a healthy donor cornea.
Penetrating Keratoplasty (PK)
The full thickness of the cornea is transplanted in this procedure.
Descemet’s Stripping Endothelial Keratoplasty (DSAEK), Descemet’s Membrane Endothelial Keratoplasty (DMEK)
These procedures are done to replace the Endothelial Layer of the cornea. The endothelial layer is a single cell layer at the back of the cornea. The endothelial cells act as pumps to maintain the pressure in the cornea and help keep it clear. If these cells do not function properly the pressure in the cornea will increase and it can become cloudy
Deep Anterior Lamellar Keratoplasty (DALK)
This procedure involved replacing the front layers of the cornea, the epithelium and stroma.
The sclera, or white portion of the eye, may be used in a number of different surgical procedures. Portions of the sclera, or grafts, are used. There can be up to eight six sclera grafts from an eye donation.
A graft, or patch, of sclera may be used over the tip of a pressure release valve (Ahmed Valve) during surgery for glaucoma.
A prosthetic (artificial) eye may be implanted using donor sclera. The prosthetic is wrapped with sclera and the four muscles which move the eye are reattached to the donor sclera. This allows the eye to move in unison with the natural eye.
Donor sclera may be used in eye lid reconstruction.