toward multiple directions and then reach some adjacent expansions of keratocytes via the tight junctions. Unlike ordinary fibrocytes, keratocytes can migrate during the scarring process. They gather on the edges of the lesion where they mute into fibroblast. Thus, they secure the synthesis of proteoglycans forming the extracellular matrix and of glycoproteins including collagen.

4.3.3.2  The Collagen Lamellae

There are about 200 to 250 collagen lamellae in the human corneal stroma. Each lamella is 2-mm thick and 9–260-mm wide. They are piled up on each other and have a very organized orientation. In the two thirds of the back of the stroma, the main axis of the collagen fibers of a lamella is perpendicular to the fibers of the lamellae that are above or below it. In the one third at the front of the stroma, the arrangement is less regular and the lamellae are set in an oblique arrangement to each other. Inside each lamella, the collagen fibrillae are parallel to each other, with even size and interspace. They are covered and thus separated from each other by some ground substance. Most of the stromal collagen (90%) is type I collagen, but a smaller part of it is types III and V. These lamellae play a double part:

•They secure and maintain the transparency of the cornea, mainly thanks to type V collagen, which influences the diameter of the collagen fibers and thus plays a part in the corneal transparency [3].

•They secure and maintain the mechanical resistance to the intraocular pressure, mainly thanks to type I and III collagen.

4.3.3.3  Ground Substance

The ground substance wraps the collagen lamellae and the cellular entities. It is constituted of glycoproteins and proteoglycans synthesized by the keratocytes. The main proteoglycans are made of glycosaminoglycans and are keratan sulfates – mostly type I – for 60% and chondroitin sulfates for 40%. These glycosaminoglycans are one of the factors of the corneal transparency, via the regulation of the stromal hydration. Thus, during endothelial disorders, the rise of the water content

of the stroma causes an increase of the volume of the glycosaminoglycans. This makes the cornea thicker and breaks the even arrangement of the collage lamellae resulting in an alteration of the light transmission. At last, there is also some type VI collagen that might have a support function by separating the different constituents of the stroma.

4.3.3.4  Other Cells

The corneal stroma also contains Schwann cells, surrounding the corneal axons, and immunocompetent cells(TandBlymphocytes,monocytes,andLangerhans cells). These latter cells are very numerous at the level of the limbus close to small vessels, but there are also a few of them at the level of the one third at the front of the central corneal stroma [4].

4.3.4  Descemet’s Membrane

It constitutes the basement membrane of the corneal endothelium, separating it from the stroma. It is a strong, amorphous, and elastic membrane. It is about 10 mm in adulthood. It is synthesized by the endothelial cells from the fourth month of embryonic life. The Descemet’s membrane has got two distinct sides:

•A 3-mm thick front side corresponding to the striated embryonic portion secreted by the endothelial cells during gestation.

•A back granulous side secreted by the endothelial cells after birth, justifying the increase of thickness to 8–12 mm in adulthood.

A specificity of the Descemet’s membrane is the presence of types IV, V, VI, and VIII collagen, all arranged in an ordered structure.

4.3.5  The Endothelium

It is the layer at the back end of the cornea, in direct contact with the aqueous humor and limited at the front by the Descemet’s membrane.

It is made up of a unique layer of flat, even, and hexagonal cells, which are 5–6-mm high, 15–20-mm

wide, and not thicker than 6 mm. These cells have a hexagonal shape doting them with a honeycombed arrangement. The endothelial cells have almost no partition capacity; therefore, their number depends on their function. A corneal transplant requires 2,000 endothelial cells/mm2 at least in order to obtain a good transparency.

These cells derive from the neural crests and then probably have the same weak partition capacity as the nervous cells (Figs. 4.7 and 4.8).

These cells have three functions: synthesis, inner barrier, and active transport necessary to the properties of corneal deturgescence. Their nucleus is big and their cytoplasm rich in organelles showing the metabolic activity (mitochondria, Golgi’s apparatus, and numerous vacuoles and small granules at the apical level). The cytoplasmic membrane, to be found on the apical

side of the endothelium, touches the aqueous humor. Some short microvillosities increase the surface of the membrane touching the aqueous humor. Some apical prolongations on the edge fill in the empty intercellular spaces. Some apical intercellular junctions form a discontinuous barrier permeable to the migration of small molecules from the inner chamber to the intercellular spaces. At last, there are some ciliated structures. The lateral side of the cytoplasmic membrane supports the main intercellular junctions. The basal side has a tortuous outline, which increases the contact surface with the neighboring cells.

4.3.6  The Limbus

Located on the edge of the transparent cornea, it maintains the junction with the opaque sclera. On the level of the epithelium, it is the transition between a multilayer scale-like corneal epithelium and a cylindrical conjunctival epithelium with two cellular bases, with continuity of the basement membranes. At the epithelial level, the cells of the corneal epithelium are gradually replaced by a conjunctival epithelium made of two layers of cylindrical cells accompanied by calyciform cells.

Located in the limbal epithelium, the basal cells act as stem cells for the basal cells of the corneal epithelium (Fig. 4.9).

Some melanocytes and Langerhans cells penetrate between the limbal epithelial cells. The subepithelial level