The corneal stroma makes up roughly 90% of the total corneal thickness (see Fig 1-4). The regular arrangement of stromal cells (keratocytes), fibers, and extracellular matrix is necessary for a clear cornea. Keratocytes vary in size and density throughout the stroma and form a 3-dimensional network throughout the cornea. They are flattened fibroblasts, located between the stromal collagen lamellae (Fig 1-5), and they continually digest and manufacture stromal molecules. Keratocyte density declines with age, by 0.9% per year for anterior density and by 0.3% per year for posterior density. It can also decline with refractive laser surgery and may not be completely restored.
Figure 1-5 Keratocytes (A) are flattened fibroblasts (B) situated between the stromal collagen lamellae.
(Reproduced with permission from Oyster CW. The Human Eye: Structure and Function. Sunderland, MA: Sinauer Associates; 1999:331.)
The corneal stroma is composed of an extracellular matrix formed of collagens and proteoglycans. Type I and type V fibrillar collagens are intertwined with filaments of type VI collagen. The major corneal proteoglycans are decorin (associated with dermatan sulfate) and lumican (associated with keratan sulfate).
Corneal transparency depends on maintaining the water content of the corneal stroma at 78%. Corneal hydration is largely controlled by intact epithelial and endothelial barriers and the functioning of the endothelial pump, which is linked to an ion-transport system controlled by temperature-dependent enzymes such as Na+,K+-ATPase. In addition, negatively charged stromal glycosaminoglycans tend to repel each other, producing a swelling pressure (SP). Because the intraocular pressure (IOP) tends to compress the cornea, the overall imbibition pressure of the corneal stroma is given as IOP – SP. The total transendothelial osmotic force is calculated by adding the imbibition pressure and the various electrolyte gradients produced by the endothelial transport channels. Corneal hydration varies from anterior to posterior and increases closer to the endothelium. Though controversial, a novel layer in the posterior part of the corneal stroma (pre-Descemet layer or Dua’s layer) has been reported. This layer may be important during deep anterior lamellar keratoplasty.
Schlötzer-Schrehardt U, Bachmann BO, Tourtas T, et al. Ultrastructure of the posterior corneal stroma. Ophthalmology. 2015;122(4):693–699.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.