Prince Masahito

Life-span Studies on Spontaneous Tumor Development in the Medaka (Oryzias latipes)

A total of 961 medaka, separated chronologically from the first to the fifth year of life, were examined for spontaneous tumor development. While no liver tumors were found in either male or female medaka under the age of 1 year and the incidence in 2‐year‐old fish was relatively low (males 1.9% and females 1.7%), they became more common with advancing age. The incidence was higher in females than in males from 3 to 5 years of age, reaching 7.1% in 5‐year‐old female stock. These liver tumors included a total of 12 adenomas and 9 hepatocellular carcinomas. The hepatocellular carcinomas were histologically well differentiated and were all observed in female medaka. Spontaneous tumors occurring in organs other than the liver were rare and sporadic. Four squamous cell carcinomas, 5 melanomas and 4 lymphosarcomas were observed with no sexual or pronounced age bias being evident. The squamous cell carcinomas developed in the surface epithelium with local invasion into the dermis. Melanomas occurred in the abdominal cavity and demonstrated systemic invasion into various parts of the body. Three out of the 4 lymphosarcomas arose from the inner part of the operculum suggesting that these tumors were of thymic origin. They also showed extensive invasion. The data indicate a particular susceptibility of older female medaka to liver but not other tumor development.

Polycystic kidney and renal cell carcinoma in Japanese and Chinese toad hybrids

Frequent development of renal cell carcinomas in hybrids between Japanese toads (Bufo japonicus) and imported Chinese toads (Bufo raddei) was first reported by 2 of our authors in 1987. Such renal tumors of toads had never been observed previously in the laboratory. To confirm the observation and to establish a new animal model system, hybrids between female Japanese and male Chinese toads were newly generated from 3 pairs of parents and pathological changes in their kidneys were examined sequentially over 6 years. In hybrids from 2 of the 3 pairs, bilateral polycystic kidney developed at a high frequency from 3 months after fertilization, this being associated with the emergence of atypical, premalignant‐appearing cells in proximal tubules. Papillary lesions developed after 12 months and renal cell carcinomas after 48 months. Such pathological changes were never seen in non‐hybrid Chinese or Japanese toads. Electron microscopy showed no evidence of any viral participation. This unique toad model may prove useful for investigation of the underlying mechanisms of genetically determined renal cell carcinogenesis.

Age dependence of O6-methylguanine-DNA methyltransferase activity and its depletion after carcinogen treatment in the teleost medaka (Oryzias latipes)

O6-Methylguanine-DNA methyltransferase (O6-MT) is considered to play an important role in the repair of DNA lesions induced by alkylating carcinogens in a wide range of animals. The activity of O6-MT was compared in liver extracts from the teleost medaka (Oryzias latipes) at various ages (3-5 years old) reared under natural conditions. O6-MT activity decreased significantly with advancing age. When medaka were exposed continuously to the alkylating agent methylazoxymethanol (MAM) acetate at levels of 0.1, 0.15 and 0.3 ppm in water, O6-MT activity was markedly reduced from days 1 to 7, with a slight increase thereafter. Furthermore, when fish were exposed to MAM acetate at levels of 1-2 ppm for 1 h and then maintained in normal tap water, O6-MT activity remained suppressed for 2 weeks, followed by a partial recovery.

Highly Sensitive Method for Immunohistochemical Detection of Poly(ADP-Ribose) in Various Animal Species

Poly(ADP-ribose) has been expected to play some regulatory roles in nuclear functions. However, its amount and distribution in vivo have not yet been fully elucidated. An immunohistochemical method should be useful for the study of the natural occurrence of poly(ADP-ribose) at the cellular level. We have developed a highly sensitive immunohistochemical method for detecting poly(ADP-ribose) in conventional histological sections (1, 2).