Sequential histological changes in the liver of medaka exposed to methylazoxymethaol acetate

Abstract

We performed a medaka bioassay for the carcinogenicity of methylazoxymethaol acetate (MAM-Ac) to examine the sequential histological changes in the liver from 3 days after exposure until tumor development. The medaka were exposed to MAM-Ac at a concentration of 2 ppm for 24 hours, and were necropsied at 3, 7, 10, 14, 21, 28, 35, 42, 49, 60, and 91 days after exposure. MAM-Ac induced four cases of hepatocellular adenoma and one case of hepatocellular carcinoma in 8 fish after 60 or 91 days of exposure. Histological changes in the liver until tumor development were divided into three phases. In the cytotoxic phase (1–10 days), MAM-Ac-exposed hepatocytes showed vacuolar degeneration and underwent necrosis and apoptosis, resulting in multiple foci of hepatocyte loss. In the repopulation phase (14–35 days), the areas of hepatocyte loss were filled with hepatic cysts and the remaining hepatocytes were surrounded by hepatic stellate-like cells (or spindle cells) and gradually disappeared. In the proliferation phase (42–91 days), the original hepatic parenchyma was regenerated and progressively replaced by regenerative hyperplastic nodules and/or liver neoplasms. The medaka retained a strong hepatocyte regenerative ability in response to liver injury. It is considered that this ability promotes the proliferation of initiated hepatocytes in multistep carcinogenesis and influences the development of liver tumor over a short period in medaka.

Spontaneous seminoma in medaka (<i>Oryzias latipes</i>)

“…In zebrafish, spontaneous seminomas have been relatively known, and chemically-induced seminomas have been reported following N-ethyl-N-nitrosourea, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), or 7,12-dimethylbenz[a]anthracene (DMBA) treatments 6,7 . Conversely, in medaka (Oryzias latipes), spontaneous tumor development in organs other than the liver, including testis, is rare 8 , and the frequency of germ cell tumors in previous reports was 0% (0 out of 961 males and females in life-span studies on spontaneous tumor development) 9 and 0.26% (14 males and 12 females, including chemically exposed medakas, out of approximately 10,000 males and females in carcinogenesis studies) 2 . We encountered spontaneous testicular tumors in two adult medakas (Cases I and II).…”

Failure to gulp surface air induces swim bladder adenomas in Japanese medaka (<i>Oryzias latipes</i>)

“…Although the utility of these assays is limited for screening unknown chemicals for potential carcinogenicity, they serve as a shorter-term assay 39 and provide a more sensitive evaluation for hepatocarcinogens than assays with rodents 40 . In particular, the alkylating agents, diethylnitrosamine 41 and methylazoxymethaol 42 , induce liver tumors within just 2–3 months in medaka. The limited ability to repair DNA adducts in the medaka liver might be an important factor contributing to the high sensitivity for alkylating agent-induced hepatocarcinogenicity 42 , 43 .…”

“…In particular, the alkylating agents, diethylnitrosamine 41 and methylazoxymethaol 42 , induce liver tumors within just 2–3 months in medaka. The limited ability to repair DNA adducts in the medaka liver might be an important factor contributing to the high sensitivity for alkylating agent-induced hepatocarcinogenicity 42 , 43 . In the present study, as the swim bladder tumors were observed from 13 dph and onwards, it was revealed that tumors in medaka developed in an extremely short-term period, not only in the liver but also in the swim bladder.…”