The normal cornea is avascular and is composed of 5 layers (Fig 6-1):
The corneal epithelium is nonkeratinized stratified squamous epithelium without goblet cells; it ranges from 5 to 8 cell layers in thickness. The epithelial basement membrane is thin and indistinct and is best seen with periodic acid–Schiff (PAS) stain.
Located immediately beneath the epithelial basement membrane is the Bowman layer, an acellular layer of the anterior stroma that is composed of densely packed, randomly arranged collagen fibrils.
The corneal stroma makes up 90% of the total corneal thickness and consists of collagen-producing keratocytes (fibroblast-like cells), collagenous lamellae, and proteoglycan ground substance. The collagen lamellae are uniform in size and periodicity, resulting in corneal transparency. The posterior-most stroma is acellular and strongly adherent to the underlying Descemet membrane. This results in a surgical cleavage plane between the posterior-most stroma and the remainder of the stroma (the pre-Descemet Dua layer), which can be exploited in deep lamellar keratoplasty.
Figure 6-1 Normal cornea. A, The cornea is composed of epithelium (Ep), Bowman layer (B), stroma (S), Descemet membrane (D), and endothelium (En). B, On higher magnification, periodic acid–Schiff (PAS) stain highlights the epithelial basement membrane (EBM), distinguishing it from Bowman layer (B). Because of dehydration of the tissue during processing for paraffin embedding, multiple areas of separation (clefts) between the stromal lamellae are evident on normal histology (arrows). If the stromal clefts are absent, corneal edema or scarring is likely. This is an example of a meaningful artifact. C, Higher magnification (hematoxylin-eosin [H&E] stain) also delineates Descemet membrane (D); endothelium (En); and a thin acellular layer of pre-Descemet stroma (arrowheads). The keratocyte nuclei (arrow) are apparent. Note that the Descemet membrane is best visualized with PAS stain as this membrane is the basement membrane of the endothelium.
(Courtesy of George J. Harocopos, MD.)
Descemet membrane is a PAS-positive true basement membrane that is produced by the corneal endothelium. The anterior portion of Descemet membrane (known as the anterior banded layer ultrastructurally) is formed during embryogenesis. The membrane slowly thickens throughout life because of the production of additional basement membrane material by endothelial cells (posterior banded layer).
The corneal endothelium is composed of a single layer of cells. The cells appear hexagonal en face (eg, on confocal microscopy). In a histologic cross section of the cornea, the endothelial cells have a low cuboidal appearance. The endothelium is primarily responsible for corneal deturgescence. See BCSC Section 2, Fundamentals and Principles of Ophthalmology, and Section 8, External Disease and Cornea, for discussion of the embryology, structure, and physiology of the cornea.
DelMonte DW, Kim TK. Anatomy and physiology of the cornea. J Cataract Refract Surg. 2011; 37(3):588–598.
Dua HS, Faraj LA, Said DG, Gray T, Lowe J. Human corneal anatomy redefined: a novel pre-Descemet’s layer (Dua’s layer). Ophthalmology. 2013;120(9):1778–1785.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.