Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 41

The corneal endothelium is a single layer of flattened cells lining the inner (posterior) cornea (see Figure 1.24). The regenerative ability of the endothelium varies with species and age. In most species, active mitosis of the endothelium occurs primarily in only immature animals. Specular microscopy and SEM of adult eyes reveal that the cells are usually hexagonally shaped. Closer inspection by SEM reveals that the surface is spotted with small microvillae and pores, and that the lateral edges of one cell interdigitate with another. In young canines (i.e., one to four weeks of age), many of the cells do not have the typical hexagonal shape. Pronounced pleomorphism has also been observed in kittens and rabbits.

Transmission electron microscopy (TEM) reveals the extensive, lateral, convoluted interdigitations between adjacent cells in the dog. The cell junctions, including zonulae occludentes and maculae adherentes, are located at the lateral cell margins. The abundance of mitochondria, smooth and rough endoplasmic reticulum, and a variety of vesicles indicates that these cells are metabolically active. There is gradual loss of the hexagonal shape in older animals due to a gradual decrease in the overall cell density of the endothelium. In young dogs, endothelial density is greater than 3000 cells/mm² with approximately 3600 cells/mm² in dogs less than one year old. As animals age, endothelial density can gradually decrease to 50% or less of that number.

With a smaller population of cells, the endothelial cells spread out and produce more pump sites to compensate for increasing leakage. An age‐related decrease in the density of corneal endothelial cells results in little change in overall corneal thickness, but if the cell density continues to decrease, however, the cells become too attenuated, resulting in the pumps being unable to withstand the increasing leakage with concomitant corneal thickening and loss of optical clarity. This point is known as corneal decompensation, and it usually occurs when the endothelial cell density decreases to between 500 and 800 cells/mm².