Fig. 4.6  Corneal back growth of epithelium after a burn: epithelial back growth, absence of the Bowman’s membrane, and vascularization of the subjacent stroma

filaments supported by the basement membrane. They certainly play a part in the cellular migration. On its lateral and apical sides, their cytoplasmic membrane has desmosomes and gap junctions securing the cohesion of basal cells with each other and with the intermediate cells. Located on the basement membrane of the cell, the hemidesmosomes are part of the functional complex that enables the adhesion of epithelium to the subjacent structures.

4.3.1.6  The Basement Membrane

It separates the Bowman’s membrane from the corneal epithelium. A 80-Å (angstrom) thick membrane, it is synthesized by the basal epithelial cells. It is bound to the basal cells and, in case of epithelial lesions of destruction; it guides and supports the back growth of the epithelium. It is constituted of type IV collagen and proteoglycans. The basement membrane has various functions: it guides the cellular migration, especially during the morphogenesis; it maintains the architecture of tissues, supporting the adhesion of cells; it sends and transmits information; it is a semipermeable membrane.

4.3.2  Bowman’s Membrane

Bowman’s membrane is an acellular membrane, which is 12-mm thick and located at the basement membrane

and the stroma. During the fourth month of the embryonic period, the Bowman’s membrane is certainly secreted by the epithelial cells, but it cannot be regenerated by these in case of a lesion. Therefore, a hurt to the Bowman’s membrane results in a fibrous scar lesion. The Bowman’s membrane is made of collagen fibers, mostly type I fibers randomly spread in a ground substance similar to the one of the corneal stroma.

4.3.3  The Stroma

It counts for about 90% of the global thickness of the human cornea. It is mainly made of an extracellular matrix rich in ground substance and of collagen lamellae the orientation of which is specific and complex. There are only a few cellular entities; some fibrocytes also named keratocytes, some lymphocytes, monocytes, some Langerhans cells as well as Schwann cells surrounding the very numerous axons that innerve the cornea. These cells account for less than 5% of the volume of the corneal stroma, which is not vascularized in normal conditions. Fibrocytes or keratocytes are flattened cells parallel to the collagen lamellae and to the cornea. They represent the resting state of the cell, which is similar to the one of the fibrocytes in other parts of the body. In case of a lesion, fibrocytes can migrate, while it is well known that they play a part in the secretion of the collagen fibers and of the ground substance (glycoproteins and proteoglycans).

There are about 200 2-mm thick collagen lamellae, piled up on each other in a very organized arrangement, which differs not only according to the corneal location but also according to the individual.

These lamellae play a part in both the transparency and the resistance to internal pressure of the eye.

4.3.3.1  Keratocytes

Keratocytes are cells of conjunctive type or fibrocytes. They account for 3–5% of the stromal volume. They are star-shaped cells, arranged in parallel to the corneal surface and to the collagen lamellae. Their nucleus is voluminous and flattened and has even edges. Their granular cytoplasm contains a few organelles, showing a weak metabolic or synthetic activity. Multiple cytoplasmic expansions spread from the nucleic region