Descemet s membrane

The cornea is the first structure of the eye to be in contact with incident light. It is composed of five distinct layers lying parallel to its surface the outer epithelium, which is continuous with the epithelial layers of the conjunctiva the epithelial basal lamina the keratocyte-containing stroma, which is a collagen structure arranged so that it is transparent Descemet s membrane and, finally, the endothelium adjacent to the aqueous humour. 

Cornea Epithelium Bowman s membrane Stroma (substantia propria) Descemet s membrane Endothelium... 

Microscopically, the cornea shows a rather simple and multilayered structure that can be divided into six layers the epithelium, basement membrane, Bowman s layer, stroma, Descemet s membrane, and endothelium. The corneal tissue consists of three different cell types epithelial cells, keratocytes (corneal fibroblasts), and endothelial cells. The outermost corneal surface is covered with the preocular tear film, which is functionally associated with the cornea. The epithelial surface must be kept moist and smooth, a role played by the tear film in conjunction with a spreading function of the eyelids during blinking motions. Furthermore, the tear film provides a protection against infectious agents that may gain access into the eye. 

It constitutes the basement membrane of the comeal endothelium, separating it from the stroma. It is a strong, amorphous, and elastic membrane. It is about 10 pm in adulthood. It is synthesized by the endothelial cells from the fourth month of embryonic life. The Descemet s membrane has got two distinct sides ... 

A specificity of the Descemet s membrane is the presence of types IV, V, VI, and VIII collagen, all arranged in an ordered structure. 

On this level, the collagen fibers change their orientation and lose the uniformity of size and interspace that are specific to the stroma. The cellularity increases (fibroblast, macrophages, lymphocytes, plasmocytes, melanocytes, Langerhans cells). The Descemet s membrane disappears. 

The organization of type VIII collagen that has been studied most closely is that in Descemet s membrane, where it forms a hexagonal lattice... 

FIGURE 24.2 Cornea. EPITH, epithelium ENDOT, endothelium BM, basement membrane BWN, Bowman s layer ST, stroma DSM, Descemet s membrane ZO, zonulae occludentes. 

Descemet s membrane covers the posterior surface of the stroma. It is a single-cell layer, 5-10 pm thick, and also composed of collagen. The corneal endothelium, a single-cell layer lining the posterior surface of the stroma, is rich in phospholipids, permeable to lipid-soluble materials, and almost impermeable to ions. 

Mouth Posterior epithelium Stratified squamous Descemet s membrane Controls fluid transport... 

Figure 3.4. Structure of cornea. The structural components of cornea include epithelium, corneal stroma, collagen lamellae and Bowman s and Descemet s membranes.

The outer surface of the cornea is covered with a smooth layer of stratified corneal epithelium (Figure 3.4). The lower layer of cells is columnar in shape and rests on a basement membrane that sits on top of a thick limiting structure termed Bowman s membrane derived from the corneal stroma below. The corneal stroma is composed of parallel bundles of collagen fibrils termed lamellae and rows or layers of branching corneal fibroblasts termed keratocytes. The posterior of the cornea is covered with a low cuboidal epithelium with a wide basement membrane (Descemet s membrane) and rests on the posterior portion of the corneal stroma. The corneal epithelium is normally under tension due to the pressure that is present in the anterior chamber just behind the cornea. The intraocular pressure is between 10 and 20 mm of mercury and is enough to cause the cornea to contract about 5% when it is excised from the eye. Therefore this pressure must be transferred between epithelium via cell-cell junctions. 

The cornea is a five-layered structure, which comprises the epithelium (superficial layer), Bowman s membrane, stroma, Descemet s membrane and the endothelium, as shown in Figure 12.1. 

The cornea is composed of five layers (see Figure 1) epithelium, Bowman s membrane stroma, Descemet s membrane, and endothelium. 

Descemet s membrane is the basement membrane of the endothelial cells. It is comprised of collagen fibers arranged in a hexagonal pattern and embedded in a matrix. 

Histologic cross-section of the cornea reveals five identifiable layers epithelinm. Bowman s layer, stroma, Descemet s membrane, and endothelium. Fluid surrounds the cornea in the forms of the tear film in front and the aqneons behind. The various corneal layers combine to form a structure that is approximately 633 mcm thick at the inferior periphery, 673 mcm at the superior periphery, and 515 mcm thick centrally. The adult corneal diameter measmes 11 to 12 mm horizontally and 9 to 11 mm vertically, creating a horizontally oriented ellipse. The radius of ciuvature of the central 3-mm optical zone ranges between 7.5 and 8.0 mm. 

Keratoconus tends to progress over 7 to 8 years and then stabilizes.The severity of keratoconus varies among patients and often is asymmetric when comparing the two eyes. In some keratoconic patients the progressive corneal thinning proceeds to such an extent that Descemet s membrane ruptures. In this event a sudden influx of aqueous into the cornea occurs, known as acute hydrops. 

Descemet s membrane, reestablishing stromal deturges-cence. Conservative therapeutic measures may be instituted during this resolution period, including the use of topical 5% sodium chloride drops during the day and 5% sodium chloride ointment instilled into the conjunctival sac at bedtime. Broad-spectrum topical ophthalmic antibiotics may be instituted to protect the compromised cornea from secondary bacterial infection. 

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