抄録
In order to assess the effects of genetic factors on spontaneous tumor development in various organs/tissues, especially the reproductive organs of Donryu and F344 rats, F1 hybrids between these two strains were investigated. Litters were obtained by mating virgin Donryu or F344 females with F344 or Donryu males, respectively. They were weaned at 4 weeks of age and divided into two hybrid groups: Group A, (Donryu female × F344 male) F1 (male n=49, female n=48) and group B, (F344 female × Donryu male) F1 (male n=53, female n=56). These two groups of F1 hybrids were observed with their parents (Donryu male n=19, female n=30; F344 male n=21, female n=29) until 120 weeks of age. The incidences of persistent estrus in both F1 groups from 8 to 12 months of age were in between those for their Donryu and F344 mothers, being slightly higher in group A than in group B. They increased thereafter and reached about 90% and 80% at 18 months of age in groups A and B, respectively. In females of group A, the incidences of endometrial hyperplasias and adenocarcinomas were comparable with those in the Donryu strain, and higher than those in group B, whose values were in turn elevated as compared to their F344 mothers. In contrast, endometrial stromal polyps and Leydig cell tumors of the testes were observed with similar incidences in both hybrids as well as F344 rats, but at markedly lower levels in the Donryu strain. The frequency of mononuclear cell leukemias was very low in both F1 groups of each sex. Various tumors were also observed in many other organs/tissues such as the pituitary, thyroid, pancreas, adrenals, mammary and prostate glands and their incidences were relatively high in both groups A and B, but their variation between the F1 groups was small. These results indicate that development of uterine endometrial hyperplasias/carcinomas in F1 hybrids might be related to hormonal conditions depending on the genetic background of the mothers, but other tumors are considered to be influenced by other factors. The effects of genetic background on the occurrence of spontaneous tumors in hybrids are very complex and differ not only with the site but also the tumor types.
