eyes were obtained from 53 dogs including 5 beagles ( 23 years old ) , 1 chihuahua ( 10 years 
 old ) and 47 mixed breeds ( 1 to 13 years old ) of each gender that were euthanized by carbon 
 dioxide overdose ( 47 mixed breeds ) or deep anesthesia using pentobarbital sodium ( 100 mg / kg , 
 i.p . ) ( the other 6 dogs ) for an unrelated experiment to the eye by other group in our 
 faculty .
both eyes of all animals were removed within 2 hr of death . for a macroscopic study 
 ( all cases ) , the unilateral eyes were cut along the equatorial plane , and the tapetal fundus 
 was photographed by digital camera ( ex - z400 , casio , tokyo , japan ) . for a histological study 
 (
20 cases including 5 beagles , 1 chihuahua and 18 mixed breeds ) , following macroscopic 
 observations , the unilateral eyes were injected with 3% glutaraldehyde in 10% formalin into 
 the vitreous cavity .
after fixation , the posterior cup of each eye was dehydrated , embedded 
 in nitrocellulose ( collodion , wako , osaka , japan ) , cut at 3040 m and then 
 stained with hematoxylin - eosin or thionine . in 5 beagles ,
40-m sections of 
 tapetum with the retina and choroid were cut at 0.5 to 2 mm intervals and reconstructed 
 after measurement of the tapetum thickness and observations of the rpel .
these procedures 
 were carried out in accordance with the guidelines of the ethical committee of tottori 
 university , japan .
macroscopic observations : the normal tapetum was triangular with rounded 
 angles and smooth contour .
its base had usually contact with the optic nerve disc ( od ) , but 
 separated from the od in rare cases .
thus , the nasal angle of the tapetum was more acute 
 than the temporal angle .
the color tone of the tapetum was not even ; its center was 
 brightest ( fig . 1fig .
1.the normal tapetum is almost triangular with rounded angles and smooth contour 
 usually contacting with the optic nerve disc ( arrow ) .
n , nasal ; t , temporal . bar=5 mm . ) . based on histological observation , the color tone of the tapetum reflected the 
 thickness of the tapetal tissue .
the normal tapetum is almost triangular with rounded angles and smooth contour 
 usually contacting with the optic nerve disc ( arrow ) .
the atypical tapetum was smaller and more variable in shape , contour and color than the 
 normal one ( fig .
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 . ) .
the atypical tapetum disappeared from the periphery , especially from the nasal and 
 ventral parts . in mild cases ,
the atypical tapetum was irregular in contour and faded in the 
 nasal part ( fig .
in severe cases ( the 
 chihuahua ) , the fundus was devoid of the tapetum and had a reddish - brown area that was 
 similar in both location and extent to the normal tapetum ( fig .
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 .
bilateral differences in appearance of the tapetum were few through the normal and atypical 
 tapeta . regarding the relationship of the atypical tapetum to age , the occurrence ratio was 31% in 
 the atypical tapetum and 69% in the normal tapetum in dogs under age 5 ( 35 cases ) and was 
 72% in the atypical tapetum and 28% in the normal tapetum in dogs over age 6 ( 18 cases ) .
histological observations : in the normal tapetum , the rpel was almost 
 unpigmented on the tapetum ( fig . 3fig .
the retinal pigment epithelial layer 
 ( arrow ) on the tapetum is unpigmented . bar=100 m . ) .
the narrow peripheral zone of the tapetum was covered by the lightly pigmented 
 rpel .
thus , the distribution of tapetal tissue closely corresponded to the area of the 
 macroscopic tapetum .
the tapetum was thick in its central part , and its thickest part was 
 located dorso - temporally to the od ( fig .
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 .
t , temporal . ) . the thickest part of the typical tapetum varied in thickness , from 20 to 70 
 m ( average : 53 m , standard deviation : 17 
 m ) and consisted of maximum 20 layers of tapetal cell . the thickest part of the typical tapetum ( t ) .
the retinal pigment epithelial layer 
 ( arrow ) on the tapetum is unpigmented . bar=100
m . 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 .
t , temporal . in the atypical cases , in which the tapetum was markedly small and very irregular in 
 contour , the tapetum was very thin . in these cases ,
regardless of the shape of the atypical tapetum , the unpigmented 
 rpel showed a similar extent to that in the normal tapetum .
pigmentation of the choroid and 
 pigmented rpel was normal in all cases ( fig . 5fig .
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 
 m . ) . while the chihuahua dog macroscopically lacked the tapetum , unpigmented rpel showed 
 histologically a similar distribution to that in the normal tapetum ( fig .
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 .
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 
 m . histological photographs of fig .
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 .
a wide variation in extent and color of the tapetum has been reported in normal dogs [ 9 , 11 , 16 ] .
the tapetum is poorly developed in the toy breeds 
 including the chihuahua and in dogs with a merle coat color [ 11 , 16 ] .
pigmentation of the choroid 
 correlates closely with the development of the tapetum [ 10 , 11 ] .
interestingly , the albino ferret 
 and the siamese cat lack choroidal pigmentation , but have the tapetum [ 22 , 23 ] . according to thibos 
 et al .
, the choroid in the 
 siamese cat contains melanocytes , but is defective in pigment production .
thus , the siamese 
 is completely free from pigment in the choroid but has the tapetum , because tapetal cells 
 derive from neural crest cells as well as choroidal melanocytes .
the blue - eyed white cat has 
 no tapetum and no choroidal pigmentation , because neural crest cells do not migrate into the 
 choroid .
all cases observed in this study were normal in pigmentation of the choroid . in 
 this study ,
unpigmented rpel was equally distributed in the dog retina regardless of the 
 extent and shape of the tapetal area . in the chihuahua
, the tapetum was not found , but the 
 unpigmented rpel was similar in distribution to that of the normal tapetum .
the developmental relation of the unpigmented rpel 
 to the tapetum has not been studied in dog . according to a study of the bovine eye ,
the 
 adult pattern of pigmented rpel in nontapetal zones and unpigmented rpel in the presumptive 
 tapetal zone is well established in the early stage of pregnancy . in other word ,
the extent of the unpigmented rpel is decided before 
 the development of the tapetum .
thus , compartmentalization of unpigmented and pigmented rpel 
 would not be directly related to the development or regression of the tapetum . in this study ,
histologically , tapetal layers decreased in number throughout the atypical tapetum . 
 therefore , thin tapetal parts , namely the periphery of the tapetum , disappeared first , and 
 the contour of the tapetum became irregular with shrinkage of the tapetum . according to 
 wyman and donovan , the degree of beadiness of 
 the tapetum increases with aging .
beadiness refers to the mosaic pattern observed in 
 ophthalmoscopic examination at the periphery of the tapetum .
thus , it is suggested that the tapetum in 
 the dog tends to decrease in size macroscopically with aging as the number of tapetal cell 
 layers decreases due to tapetal cell degeneration .
it is speculated 
 that the atypical tapetum results from tapetal cell degeneration in addition to congenital 
 hypoplasia of the tapetum .
for example , smaller - sized breeds have significantly smaller 
 tapetal area . in this study ,
the normal tapetum 
 had a wide variety of thickness ( 53  17 m ) .
several studies have shown selective tapetal cell degeneration following 
 administration of drugs , including a beta - adrenergic blocking agent sch 19927 , a macroride antibiotic ( rosaramicin ) , an azaride antibiotics ( cp 62,993 ) , an aromatase inhibitor ( cgs 14796c )   and an antipsychotic agent ( 1192u90 ) .
tapetal cells in the dog are packed with long slender rods , which are rich in zinc [ 12 , 24 , 25 ] .
zinc is essential for preservation of the structure of normal 
 tapetal rods , because the congenitally abnormal tapetum which lacks zinc in its tapetal rods 
 is not visible in ophthalmoscopy and shows an absence of the regular arrays of tapetal rods 
 .
drugs that chelate zinc cause a reversible 
 tapetal decoloration in dogs [ 8 , 19 ] . even in human in developed countries ,
zinc deficiency is common in 
 the elderly , and zinc supplementation delays the progression of age - related macular 
 degeneration and reduces the risk of loss of vision .
if zinc deficiency is not uncommon in elderly dogs , 
 tapetal cells may gradually degenerate for zinc deficiency with aging .
cysteine , to which zinc readily binds , is necessary for the biosynthesis of glutathione , 
 which is quantitatively the most important antioxidant and radical scavenger .
the 
 age - related decrease in the body s cysteine and /or glutathione level plays a causative role 
 in various ageing related degenerative processes . 

as the age - related decrease in zinc and cysteine may synergistically increase tapetal cell 
 degeneration , there may be a trend toward an increase of tapetal cell degeneration with 
 aging .
 abstractwe 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 . 
MATERIALS AND METHODS
RESULTS
DISCUSSION