A Convenient Alternative Method for Quantifying Endocrine Disrupting Effect of Chemicals Using Secondary Sexual Characteristics of Medaka (Oryzias latipes): The Node Count Method

Abstract

An efficient and convenient method that effectively quantify the endocrine disrupting effect of chemicals by using secondary sexual characteristics (SSCs) of the S-rR strain medaka (Oryzias latipes) was developed and demonstrated in this study. Gonad somatic index (GSI) and gonadal morphology are the main gross morphological characters in fish used to detect the effects of endocrine disrupters; however, their low sensitivity has been identified as their weakness. The method we proposed in the present study involved counting the number of nodes in the second-last ray of the anal fin, could be an efficient and convenient alternative method to OECD guidelines that require counting all number of papillary processes and joint plates with the papillary processes on every ray of the anal fin. The S-rR strain medaka was exposed to 17β-estradiol at three different concentrations (16, 32, or 64 ng･l-1) for three consecutive generations. Exposure was initiated on fertilized eggs and performed approximately for 70 days for each generation. At the termination of exposure, three characters: number of nodes in the second-last ray, maximum number of papillary processes on a ray, and number of rays with papillary processes, were observed. The results showed that the number of nodes represents a viable quantitative indicator for assessing endocrine disrupting chemicals (EDCs). Furthermore, it was confirmed that the node count method was an efficient and convenient alternative method to OECD guidelines that allows better understanding of the biological significance of the SSCs of medaka.