1 Anatomy of the Cornea and Pre-Descemet Layer Cornea, the anteriormost refractive surface is the major refractive component of the eye. The normal vertical and horizontal diameter is about 11.5mm and 12mm, respectively. The radius of curvature is 7.8mm anteriorly and 6.5mm posteriorly. It measures about 0.52 to 0.54mm thick in the center. Human corneal ultrastructure can be divided into five different layers, namely the epithelium, the Bowman layer, the stroma, the Descemet membrane layer, and the endothelium (► Fig. 1.1).1,2 Recently a new layer has been noted that varies from normal stroma by a few histological characteristics, and it has been popularly termed as pre-Descemet layer (PDL) or Dua layer.3 The most superficial layer, present in contact with the tear film layer is made up of stratified squamous nonkeratinized cells. There are five to six layers and three types. The lowermost cells are columnar basal cells that are attached to the underlying basement membrane with hemidesmosomes. The wing cells occupy the layer above the basal cells and are usually two to three layers thick. The outermost layer consists of squamous cells that possess microvilli or plicae for adhesion with the overlying tear film. Bowman layer is an acellular, basement membrane layer, formed by randomly arranged collagen fibrils. Constituents of this layer are believed to be synthesized and secreted by both epithelial cells and stromal keratocytes. Collagen fibrils in this layer are smaller than those of the stroma. Collagen types I, V, and VII are some of the types of collagen present in this layer, along with proteoglycans. The stroma forms the major portion of the cornea. It is made up of regularly arranged collagen fibrils spaced by proteoglycans, namely dermatan sulphate and keratin sulphate. It also contains the modified fibroblasts named keratocytes. Lumican and keratan are the core proteins of keratan sulfate proteoglycan. Collagen types I, V, VI, and XII are distributed widely in the lamellae. This layer has been recently identified by Dua et al as it showed few differences from normal corneal stroma.3 It measured about 10.15 ± 3.6 µm (► Fig. 1.2) thick (range 6.3–15.83 µm) in Dua et al’s study.3 It was observed to consist of five to eight thin lamellae of tightly packed collagen bundles that were oriented in longitudinal, transverse, and oblique directions. In immunohistochemistry staining, CD34 was negative in the PDL, which showed the lack of keratocytes in this layer, unlike in the stroma. Collagen types IV and VI seemed to be more positive for PDL compared with the corneal stroma. Another report shows the separation of PDL to be randomly arranged cleavages in collagen layers measuring 4.5 to 27.5 µm with a variable presence of keratocytes.4 This is a defined sheet of collagen (► Fig. 1.3) with an anterior banded and a posterior nonbanded zone. This is a specialized basement membrane of the endothelium that lies in front of the endothelial cells. It is approximately 10 ± 2 µm thick. Fig. 1.1 Anatomical layers of the cornea. BL, Bowman layer; DM, Descemet membrane; PDL, pre-Descemet layer. Fig. 1.2 Pre-Descemet layer (PDL) seen in histopathological section. Arrows demonstrate the PDL layer with the Descemet layer and the endothelial complex. The endothelium is a monolayer of hexagonal cells arranged in a mosaic pattern (► Fig. 1.4). The average density of corneal endothelial cells at birth is approximately 4000 cells/mm2. There are active pumps present in the endothelial cells that function continuously for nurturing the corneal layers and maintaining transparency. The PDL layer can be harvested by pneumatic dissection, which involves injecting air from the limbus into the midperipheral stroma of the donor cornea with a 30-gauge needle. The type 1 large bubble, which starts from the center and is well circumscribed and thicker, is the complex containing endothelium, Descemet membrane, and PDL (► Fig. 1.5). The bubble with PDL extends to the periphery approximately 7 to 8.5mm and does not usually go beyond 9mm. The overlying Descemet membrane layer can be peeled, and the bubble of PDL stays intact due to its fibrous nature. The type 1 bubble was known to withstand pressure up to 1.4 bars as compared to an isolated Descemet membrane bubble, which popped at 0.6 bars (► Fig. 1.6). The bubble can also be separated using viscoelastic or storage medium. The findings from an additional study that evaluated the cleavage plane of Descemet membrane after dissection provided the evidence for the existence of a physiological cleavage plane between the interfacial matrix, the anteriormost adhesive zone of the Descemet membrane, and the corneal stroma, suggesting a relatively weak attachment that can be disconnected by mechanical forces.5 The epithelium’s unique functions include light refraction, protection, transmittance, and survival over an avascular bed. The function of light refraction is obtained by its smooth, wet apical surface and its extraordinarily regular thickness. The layer traverses the high sensory nerve supplies that terminate in the suprabasal and squamous cells. The epithelium has a high regenerative or proliferative property and grows centripetally from the existing limbal stem cells. The layer protects the cornea from external injury, pathogens, and fluid loss. The mitogenic activity takes place in the basal cells and moves apically. The apical surface has microplicae and a glycocalyx that help in tear film adhesion for forming the smooth ocular surface (► Fig. 1.7). Persistent epithelial defect can happen in chronic ocular surface problems or upper eyelid or tear film abnormalities.
1.1 Introduction
1.2 Corneal Ultrastructure
1.2.1 Epithelium
1.2.2 Bowman Layer
1.2.3 Stroma
1.2.4 Pre-Descemet Layer, or Dua Layer
Descemet Membrane
Endothelium
1.3 Isolation of Pre-Descemet Layer
1.4 Applied Anatomy of Corneal Layers
1.4.1 Epithelium
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