Histology and Physiology of the Cornea

The cornea enables the penetration of rays of light inside the eye, thanks to completely particular optical capacities allied to its great fragility largely due to its distant origin within the aquatic species. We will see how the histology of the cornea helps to understand this permanent miracle.

Doted of a refractive capacity of about 43 dioptres, the cornea is a sphere-cylinder like lens and counts for two thirds of the refractive power of an eye at resting state.

4.2  Anatomy Reminder

With a size of about 1.3 cm2 and an average diameter of 11.5 mm in adulthood, the cornea has an ovoid shape with a horizontal axis on its front side and is circular on its back side. It is 0.5-mm thick at the center and 1-mm thick on the edge. As shown by modern measurements, the shape of the cornea can differ for different individuals. On its edge there is the limbus that partly has the same characteristics and ensures the junction with the totally opaque sclera, which surrounds the entire eyeball.

M. Gérard (*)

Medical director of Head and Neck Unit, Ophtalmologist, Cayenne Hospital, Cayenne, French Guiana

e-mail: gerardmax@caramail.com

P. Josset

In charge of courses on the history of the medicine,

Curator of the Dupuytren museum Paris, Anatomopathologist, “Armand Trousseau” Children’s Hospital, Paris, France e-mail: patrice.josset@trs.ap-hop-paris.fr

The cornea is made up of five layers [1]:

1.The epithelium and its basement membrane

2.Bowman’s membrane

3.The corneal stroma

4.Descemet’s membrane

5.The endothelial cells

4.3.1  The Epithelium and Its

Basement Membrane

The corneal epithelium is made of stratified and keratinless scale-like and junction-like cells. These cells are arranged on 4–6 layers at the central part and 4–8 layers on the edge of the cornea. Their combined thickness is about 50–60 mm representing about 10% of the whole thickness of the cornea. The structure and function of the epithelium strictly depends on the lacrymal secretion that covers it and plays a big part in the preservation of the cells and in their transparency.

We will now study, from edge to center, the lacrymal secretion and the layers of the corneal epithelium.

4.3.1.1  The Lacrymal Secretion

It cannot be dissociated from the epithelium because they share optic and metabolic functions. The lacrymal secretion is made up of three layers, from outside to inside:

•A 0.1-mm thick superficial lipidic layer with a main function of reducing the speed of evaporation of the lacrymal secretion. This lipidic film can only be

secreted by the conjunctival tarsal cells located on the palpebral edge.

• A middle 7-mm thick aqueous layer secreted by the lacrymal glands, with an average debit of

1.2 mL/min.

•A deep layer rich in mucus and the origin of which is to be found in the conjunctival calyciform cells. This mucus is in relation with the apical epithelial cells via the glycocalyx. This interface between the lacrymal secretion and the epithelium influences the quality of the corneal surface. Indeed, the stability of the lacrymal secretion and the epithelial absorption of its metabolites both depend on the mucin and the apical expansions of the epithelial cells [2].

4.3.1.2  The Corneal Epithelium

It is formed very early in the ocular embryogenesis, at first as the simple epithelium, which is gradually replaced by the final form of epithelium: this latter is mostly made of malpighian epithelial cells but also contains non-epithelial lymphocytes (mostly T lymphocytes) and Langerhans cells (cells of presentation of the antigen), both playing a part in immunity. There are also melanocytes at the level of the basal layer. These non-epithelial cells can mainly be found at the level of the corneal edge, close to the limbus (Figs. 4.1 and 4.2).

Fig. 4.1  Aspect of the cornea at the fourth month of pregnancy: the epithelium is constituted of one or two layers, the Bowman’s membrane appears

Fig. 4.2  Aspect of the cornea at the fourth month of pregnancy: the endothelial cells are dense and cubical

4.3.1.3  The Superficial Cells

At the level of the superficial layer, itself made of two or three layers, the cells become more and more flattened and are 45-mm long and 4-mm thick. Their nucleus is stretched out along the long axis and is 25-mm long. Their cytoplasm contains three specific structures: contractile proteins, glycogen granules, and a very developed Golgi’s apparatus.

Thanks to specular microscopy and electron scanning microscopy, some cells with various aspects have been identified: it could be different stages of differentiation and maturation of the superficial cells preceding their desquamation. Along their evolution, the metabolic activity of the superficial cells slows down until it stops. They are then evacuated into the lacrymal secretion.

The most superficial cells are the most mature ones, with a low metabolic activity, and they are going to desquamate. In electron scanning microscopy, the corneal surface looks like a mosaic of flattened and polygonal cells, with uneven sizes. Their nucleus is reduced to a few condensed chromatin clods.

Their cytoplasmic membrane is bristled up with microvillosities and micropilis. They increase the exchange surface with the lacrymal secretion and enable its fixing. Their lateral and basal cytoplasmic membranes have got junctional complexes uniting these superficial cells together. There are three types of junctional complexes:

•The desmosomes are very strong sticky junctions maintaining the mechanical cohesion of the

superficial cells. They are located along the lateral and basal membranes.

•The tight junctions are impermeable junctions maintaining the cellular cohesion and mostly represent a stopping system that prevents the passage of molecules and liquids from extracellular environment. They are located at the level of the apical part of the lateral cytoplasmic membranes.

•The gap junctions are communicating junctions, kinds of tunnels enabling the molecular exchanges between neighboring cells. They are spread in an anarchic way on the lateral and basal faces.

These junctional complexes all disappear during desquamation.

4.3.1.4  The Intermediate Cells

These are cells of transition between the basal and superficial cells. They constitute the thickest and biggest layer. They are polygonal cells, with a convex front side and a concave back side. They are arranged on two or three layers at the center and five to six layers on the edge. Their nucleus is active and stretched out along the big axis of the cell. Their cytoplasm contains a very developed Golgi’s apparatus as well as tonofilaments (microtubules and keratin filaments) connected to the desmosomes. Their cytoplasmic membranes are only united desmosomes and gap junctions that enable both the unity of intermediate cells and the union of intermediate and basal cells (Figs. 4.3–4.5).

Fig. 4.3  Central part of corneal epithelium in adulthood 1

Fig. 4.4  Central part of corneal epithelium in adulthood 2

Fig. 4.5  Edge of corneal epithelium in adulthood

4.3.1.5  Basal Cells

Basal cells are the germinative cells of the corneal epithelium. They are 18-mm long cylindrical cells with a 10-mm diameter. They are arranged in one layer set on the basement membrane. Their nucleus is oval and oriented along the big axis of the cell (Fig. 4.6).

The stem cells of the basal cells are located at the level of the limbus, which come from centripetal migration. The daughter cells migrate to form the intermediate cells. Their cytoplasm, which is rich in glycogen and mitochondria, shows their high metabolic activity. It also contains a Golgi’s apparatus, some microtubules, and some keratin filaments connected to each other by desmosomes and hemidesmosomes. Most of all this cytoplasm contains some actin