Spatial relationships among the cellular tapetum, visual streak and rod density in dogs.

The dog visual system is well suited to dim light conditions due to rod-dominated retina and the reflective tapetum. The topographical distributions of rods and thickness of the tapetum of the dog were quantified in retinal whole mounts stained with thionine, and spatial relationships among the tapetum, rod density and visual streak of high ganglion cell density were elucidated. The relationship between the retina and tapetum was analyzed in parasagittal sections stained with thionine or hematoxylin-eosin. The tapetum was thick in its center, and the thickest part consisted of 9 to 12 tapetal cell layers. Rod density ranged from 200,000 to 540,000/mm(2). Maximum rod density was found in the area dorsal to the visual streak, and the density in that area was significantly higher than the rod density in the visual streak and accorded spatially with the thickest part of the tapetum. The horizontal visual streak was found over the horizontal line through the optic disc in the temporal half and extended slightly into the nasal half. The central area of the highest density of ganglion cells was approximately located midway between the nasal and temporal ends of the visual streak. The visual streak was located within the tapetal area, but ventrally to the thick part of the tapetum.

Macroscopic and histological variations in the cellular tapetum in dogs.

We aimed to document macroscopic variations in the cellular tapetum in the dog, to provide a histologic description of the macroscopic results and to evaluate the correlation between the macroscopic appearance and aging. Fifty three dogs including 5 beagles, 1 Chihuahua and 47 mixed breeds of each gender were used. For a macroscopic study, the fresh tapetal fundi were photographed using digital camera. For a histological study, the glutaraldehyde-formalin fixed eyes were embedded in nitrocellulose and stained with hematoxylin-eosin or thionine. The normal tapetum was triangular with the rounded angles and the smooth contour. The atypical tapetum was smaller and more variable in shape, contour and color than the normal one. In severe cases, the fundus was devoid of the tapetum. The atypical tapetum tended to increase in frequency with aging. Retinal pigment epithelial cells on the normal tapetum were unpigmented. In the eye with the atypical tapetum, regardless of tapetal size and shape, unpigmented retinal pigment epithelial cells showed a similar distribution to that on the normal tapetum, even in a dog without a tapetum. Although there is a congenitally hypoplastic tapetum, the atypical tapetum tends to increase in incidence and severity with aging.