SEX-DIFFERENTIATION IN THE EUROPEAN EEL - HISTOLOGICAL ANALYSIS OF THE EFFECTS OF SEX STEROIDS ON THE GONAD

The effects of sex steroids on sex differentiation in the European eel were studied. The steroids, 17 alpha-methyltestosterone (MT) and 17 a lpha-ethynylestradiol (EE), were given in the diet to 6-8 cm elvers an d to 15-18 cm and 22-25 cm yellow eels. In our rearing conditions a ve ry large percentage of the untreated eels developed as males. No mascu linizing effect of MT could be demonstrated. The EE, administered at a dose of 10 mg kg(-1) of diet to 6-8 cm elvers and 15-18 cm eels, indu ced ovarian differentiation in about 90 and 65% of eels respectively, while in the control <5% of females was recorded. In 22-25 cm yellow e els a moderated feminizing effect was observed. Histological analysis of the gonads of treated eels showed that sex steroids affect the gona dal structure. The androgen stimulates hypertrophy of compact connecti ve tissue, early differentiation of Leydig cells, Sertoli cells and ea rly formation of the spermatic duct. Oestrogen inhibits the differenti ation of these structural components and stimulates the differentiatio n of follicular cells and an ovarian structure. The involvement of gon adal structural components is discussed in relation to the effect of s teroid treatment and to the peculiarities of sex differentiation in th e European eel. (C) 1995 The Fisheries Society of the British Isles