Macroscopic and histological variations in the cellular tapetum in dogs

Abstract

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.

Fig. 1.

The normal tapetum is almost triangular with rounded angles and smooth contour usually contacting with the optic nerve disc (arrow). Brightness of the tapetum reflects the thickness in tapetal tissue. N, nasal; T, temporal. Bar=5 mm.

Fig. 2.

The atypical tapetum showing variable shape and irregular contour. The tapetal area decreases from the nasal and ventral parts (left and ventral sides in each tapetum, respectively) of the tapetum (A–D). The Chihuahua dog (D) lacks the tapetum, and the dorsal part of its fundus is reddish-brown, which correspond approximately to the normal tapetal area both in location and size.

Fig. 3.

The thickest part of the typical tapetum (T). The retinal pigment epithelial layer (arrow) on the tapetum is unpigmented. Bar=100 µm.

Fig. 4.

Reconstructed topographic distribution of the tapetum thicknesses. The thickest part of the tapetum (central black area) is located dorso-temporally to the optic nerve disc (white circle). Dots show the periphery of the tapetum covered by the lightly pigmented retinal pigment epithelial layer. Oblique lines show the heavily pigmented retinal epithelial layer without the tapetal tissue. T, temporal.

Fig. 5.

Histological photographs of Fig. 2C. A, The thickest part of the tapetum (T) is covered by unpigmented retinal pigment cells (arrow). B, Unpigmented retinal pigment epithelial cells (arrow), which normally cover the tapetal tissue, are found in the broad area without tapetal tissue. Bar=100 µm.

Fig. 6.

Histological photographs of Fig. 2D. A, The unpigmented retinal pigment epithelial layers (arrow) are found in the reddish-brown area without the tapetum. B, Although the macroscopic tapetum is not observed, fragmented tapetal tissues (T) are rarely found in the reddish-brown area. Bar=100 µm.