The completely transparent cornea is located in front of the eye and has a horizontal oval shape. Together with the sclera, it constitutes one-sixth of the outer wall of the eye. The corneal tissue can be divided into five layers from front to back: the epithelium, Bowman's layer, stroma, Descemet’s membrane, and the endothelial layer. The cornea is transparent and lacks blood vessels but contains rich sensory nerve tissue. It has a specific shape, size, thickness, and radius of curvature and also possesses refractive power, playing an important role in the eye's refraction. Additionally, along with the scleral tissue, it provides special protection for the delicate contents of the eye. Due to its specific characteristics, the cornea has its own unique features.
Epithelial cell layer: This serves as the first important line of defense against corneal violations. It is 50μm thick, accounting for one-tenth of the total corneal thickness. It is non-keratinized squamous epithelium consisting of 4 to 6 layers of cells. The corneal epithelial cell layer grows rapidly and combines firmly, resisting most bacteria and toxins. After damage, it can regenerate within 24 hours without leaving scars. As this layer is rich in nerves, even minor injuries cause significant foreign body sensation. Since the cornea is exposed, the corneal epithelium is prone to injury, providing opportunities for pathogenic microorganisms. Common infections of the cornea occur due to improper handling of corneal foreign bodies, which may lead to corneal ulcers or even the risk of perforation.
Bowman layer: Also known as Bowman's membrane, it is located beneath the corneal epithelial cell layer and above the substrate layer with a thickness of 10~16um. It is a cell-free collagen fiber film that lacks flexibility. It serves as the basis for epithelial cell attachment and cannot regenerate after injury; instead, it is replaced by fibrous tissue. It resists trauma and infection with a certain degree of mechanical resistance.
Stroma: It accounts for about nine-tenths of the corneal thickness, is transparent and avascular, and has the same refractive index as the cornea. Composed of 200 to 250 layers of parallel arranged collagen fibers and bonding materials, each fiber layer is arranged in a cross, facilitating light passage and refraction. Due to the absence of direct vascular supply, metabolism is slow, making it difficult to remove pathological metabolites, often leading to prolonged inflammation. The transparency of the cornea depends on the uniformity and integrity of the corneal stroma. Any injury or inflammation affecting the corneal stroma, even if healed, will leave varying degrees of opacity (corneal nebula), affecting its curvature and transparency, causing visual impairment.
Descemet's membrane: Also known as Descemet’s membrane, it is located between the stroma and the endothelial cell layer, being the secretory product of the endothelial cells. It is a flexible, unstructured, highly resistant transparent film with great tenacity, strong mechanical tension, and microbial resistance. It can regenerate.
Endothelial layer: Composed of a single layer of hexagonal mosaic flat cells, these cells cannot regenerate from birth until death. Aging and dying cells leave their positions, which are covered by the expansion and migration of neighboring endothelial cells. After damage, the endothelial cells increase through neighboring expansion and migration to cover the area. The continuous layer of endothelial cells transports water molecules from the matrix layer into the anterior chamber, keeping the matrix in a dehydrated state to maintain transparency. Therefore, whether the entire cornea remains transparent and how to preserve material for corneal transplantation is an important research direction.
In immunology, the feature of the avascular cornea reduces contact with lymphocytes, thus excluding or minimizing allogeneic graft rejection, hence allowing easier masking of romantic sweetheart makeup transformations.
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