Descemet’s membrane (posterior basement membrane) is the basal lamina of corneal endothelial cells. It is intensely positive to periodic acid–Schiff (PAS) and can be as thick as 10 m. Descemet’s membrane has a feltlike appearance and consists of an interwoven meshwork of fibers and pores. It separates the corneal endothelium from the adjacent corneal stroma.
Unlike Bowman’s membrane, Descemet’s membrane readily regenerates after injury. It is produced continuously but slowly thickens with age.
Descemet’s membrane extends peripherally beneath the sclera as a trabecular meshwork forming the pectinate ligament. Strands from the pectinate ligament penetrate the ciliary muscle and sclera and may help to maintain the normal curvature of the cornea by exerting tension on Descemet’s membrane.
The corneal endothelium provides for metabolic exchange between the cornea and aqueous humor.
The corneal endothelium is a single layer of squamous cells covering the surface of the cornea that faces the anterior chamber (see Fig. 24.4). The cells are joined by well-developed zonulae adherentes, relatively leaky zonulae occludentes, and desmosomes. Virtually all of the metabolic exchanges of the cornea occur across the endothelium. The endothelial cells contain many mitochondria and vesicles and an extensive rough-surfaced endoplasmic reticulum (rER) and Golgi apparatus. They demonstrate endocytotic activity and are engaged in active transport. Na/K-activated ATPase is located on the lateral plasma membrane.
Transparency of the cornea requires precise regulation of the water content of the stroma. Physical or metabolic damage to the endothelium leads to rapid corneal swelling and, if the damage is severe, corneal opacity. Restoration of endothelial integrity is usually followed by deturgescence, although corneas can swell beyond their ability for self-repair. Such swelling can result in permanent focal opacities caused by aggregation of collagen fbrils in the swollen cornea. Essential sulfated glycosaminoglycans that normally separate the corneal collagen fbers are extracted from the swollen cornea.
Human corneal endothelium has a limited proliferative capacity. Severely damaged endothelium can only be repaired by transplantation of a donor cornea. Recent studies indicate that the periphery of the cornea represents a regenerative zone of the corneal endothelial cells. However, soon after corneal transplantation, endothelial cells exhibit contact inhibition when exposed to the extracellular matrix of Descemet’s membrane. This fnding that inhibitory factors released by Descemet’s membrane prevent proliferation of endothelial cells has focused some current corneal research on reversal or prevention of this inhibition with exogenous growth factors.
The sclera is an opaque layer that consists predominantly of dense connective tissue.
The sclera is a thick fibrous layer containing flat collagen bundles that pass in various directions and in planes parallel to its surface. Both the collagen bundles and fibrils that form them are irregular in diameter and arrangement. Interspersed between the collagen bundles are fine networks of elastic fibers and a moderate amount of ground substance. Fibroblasts are scattered among these fibers (Plate 107, page 926).