Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

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in the excretory portion has been suggested in studies of human tear flow. Evaluation of canalicular function in humans suggests that destruction of either canaliculus alone does not affect excretion of tears; in domestic animals, the lower canaliculus is considered to be the primary site for tear drainage.

      The clear cornea serves as a window for the eye with two critical optical properties, transparency and refractive power, both of which are essential for vision. The cornea, with the sclera, protects the inner components of the eye from injury through its exquisite structure, biomechanics, and sensitivity.

      Transparency

Schematic illustration of collagen fiber organization in the canine 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.

Image described by caption.

      Metabolism

      Steady‐state hydration in the cornea occurs when the endothelial leak and pump rates are equivalent; this process is termed the “pump–leak” mechanism. The leaky barrier function of the endothelium may at first seem counterintuitive, but most nutrients for the cornea, except oxygen, come from the AH. Thus, leakiness of the endothelium is essential to providing bulk fluid flow through a tissue that lacks blood and lymphatic vessels. Glucose transporters are found on both the apical and basolateral endothelial cell membranes that face the AH and stroma, respectively, to ensure transcellular glucose flux. The corneal epithelium converts glucose to glucose 6‐phosphate, where it is subsequently metabolized to pyruvate via glycolysis. Most of this pyruvate is then metabolized into lactate, but some is diverted into the tricarboxylic acid cycle to produce ATP. Glucose is also stored in the epithelium as glycogen, which can be used for energy under stressful conditions such as corneal injury. The corneal epithelium and keratocytes in the anterior stroma obtain oxygen for aerobic glycolysis from the PTF, while the endothelium and keratocytes in the posterior stroma receive their oxygen from the AH.

      Biomechanics

      The cornea is a thick‐walled, pressurized, partially intertwined, unidirectionally fibril‐reinforced laminate biocomposite, which imparts stiffness, strength, and extensibility to withstand both inner and outer forces that may alter its shape or integrity. A soft, fibrous connective tissue, like the cornea, usually is much stronger in the parallel versus perpendicular direction to the collagen fibrils. Consequently, the collagen fibrils are arranged into complex hierarchic structures, which give the cornea its anisotropic mechanical properties. The collagen lamellar architecture of the cornea varies dramatically between vertebrate species, with nonmammalian vertebrates exhibiting an orthogonal‐rotation arrangement with a marked increase in lamellar branching in species such that birds >> reptiles > amphibians > fish; in contrast, the mammalian species exhibit a random pattern.

      Sensitivity and Innervation

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