Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 74

The cornea serves as the most powerful refractive structure of the eye. Corneal clarity or transparency is a result of the lattice‐like organization of the stromal collagen fibrils as well as the transparency of the cells within the cornea (Figure 2.2). The state of relative dehydration, hypocellularity, unmyelinated nerve fibers, a nonkeratinized epithelium, and absence of blood vessels and pigment also contribute to corneal transparency. The corneal stroma comprises the bulk of the cornea and is responsible for 90% of its thickness. It is predominantly composed of water that is stabilized by an organized network of collagens, glycosaminoglycans (GAGs), and glycoproteins. The GAGs are important for maintaining the regular spacing between fibrils. The uniform thickness, small collagen fibrils arrange into parallel lamellae running at oblique angles to each other, and are separated by less than a wavelength of light (Figure 2.3). This formation results in a highly ordered, lattice‐like arrangement whereby short‐range order results in corneal transparency via destructive interference.

Figure 2.2 In the normal cornea (a), a cross section of the corneal fibrils demonstrates a nearly perfect lattice arrangement, with equidistant collagen fibrils permitting light transmission and concomitant transparency. In contrast, swelling of the cornea with edema (b) disrupts this highly ordered arrangement, resulting in light diffraction and an opaque, bluish cornea.

Quiescent keratocytes lie between collagenous lamellae to form a closed, exquisitely structured syncytium. These three‐dimensional, stellate‐shaped cells comprise a cell body with multiple, extensive dendritic processes that interact with other keratocytes. Abundant corneal crystallins (~25–30% of the intracellular soluble protein), such as aldehyde dehydrogenase and transketolase, minimize refractive differences in the keratocyte cytoplasm, thus ensuring transparency of these cells.

Figure 2.3 Schematic of collagen fiber organization in the canine cornea. The epithelium produces an anterior basement membrane with a complex surface topography consisting of a meshwork of fibers and holes. The anterior 10% of the cornea comprises unidirectional, interwoven collagen lamellae, while the posterior 90% consists of unidirectional, nonwoven collagen lamellae with a random orientation. Descemet's membrane, the specialized basement membrane of the endothelium, can be divided into the anterior banded and posterior nonbanded layers. The anterior banded layer is dominated by collagen VIII, which appears as a hexagonal network en face and parallel bands in transverse section. The surface topography posterior nonbanded layer has a rich network of intertwined fibers, but with a smaller pore size in comparison to the anterior basement membrane.

Upon corneal wounding, transformation of keratocytes to activated fibroblasts and myofibroblasts results in a dramatic increase in cell volume and subsequent dilution of corneal crystallins, with a concomitant increase in light scatter. Corneal scarring is thought to be due to alterations in the light scattering properties of keratocytes, in addition to changes to the extracellular matrix (ECM).

The epithelium and endothelium are responsible for maintaining the cornea in a relatively dehydrated state. Specifically, loss of the corneal epithelium or endothelium results in a 200% or 500% increase in corneal thickness, respectively, due to stromal edema. Anatomical integrity of the epithelium and endothelium provides two‐way physical barriers against the influx of tears and AH, respectively. However, the multiple‐layered epithelium provides a relatively impermeable barrier versus the leaky, single‐layered endothelium. The endothelium primarily maintains stromal deturgescence via active transport that is energetically maintained by sodium–potassium‐activated adenosine triphosphatases (Na⁺–K⁺‐ATPases).