Medaka igf1 identifies somatic cells and meiotic germ cells of both sexes

Insulin-like growth factors (IGFs) play significant roles in regulating cell proliferation, differentiation and reproduction in diverse organisms. Here, we report the identification of medaka igf1 and the expression pattern of igf1 RNA in adult medaka gonads. Using RT-PCR, igf1 RNA was easily detected in several somatic organs and adult gonads of both sexes. Using chromogenic in situ hybridization, igf1 RNA in the ovary was found in stage I-IV oocytes and in the somatic cells throughout oogenesis. In the testis, igf1 RNA was present in meiotic products and the somatic cells surrounding male germ cells throughout spermatogenesis. We performed fluorescent in situ hybridization and immunostaining for the Vasa protein as a germ cell marker on gonadal cryosections. We showed that igf1 RNA in the ovary was most abundant in theca cells and meiotic oocytes at stage I-IV, which was detectable in granulosa cells and infrequently occurred in the mature oocytes at stage V but was absent in oogonia. The igf1 RNA in the testis was most prominent in Leydig cells and Sertoli cells and meiotic spermatids and sperm as well as detectable in spermaotgonia and spermatocytes. We conclude that differential igf1 RNA expression identifies medaka somatic cells and meiotic germ cells of both sexes.