Spontaneous testicular lesions were assessed in 50 control purpose-bred mal
e beagle dogs. They were selected from 13 toxicology studies conducted over
the period 1988-1999. Age of the dogs at study termination varied from 8-2
0 months with an average age of 13 months. Regardless of age, the most sign
ificant finding was bilateral segmental hypospermatogenesis in 15/50 (30%)
of the dogs. Cross sections of tubules with hypospermatogenesis were distri
buted randomly throughout the testes and were characterized by reduced prop
ortions of germ cells, tubular shrinkage, and Sertoli cell prominence. Thes
e changes were occasionally associated with giant cells, with cellular debr
is, and in 6/15 (40%) with atrophic tubules devoid of germ cells, indicatin
g a degenerative process. Focal subcapsular tubular atrophy or hypoplasia (
tubules Lined by Sertoli cells only) was also found in 9/35 (26%) of dogs w
ithout hypospermatogenesis. Inhibited spermiation with retention of mature
sperm in tubules was seen in 6/50 dogs, 3 of which also showed hypospermato
genesis. Other findings of high incidence but low prevalence included tubul
es with multinucleated giant cells, swollen spermatocytes, or apoptotic ger
m cells. These latter changes are probably a constituent of normal spermato
genesis. In conclusion, about 30% of control beagle dogs show segmental hyp
ospermatogenesis, which may be associated with degenerative changes, and an
additional 18% of the dogs exhibit focal tubular atrophy/hypoplasia in oth
erwise normal testes. These changes have to be distinguished from compound-
related toxic effects.