Hormonal correlates of ovarian alterations in bovine freemartin fetuses

Abstract

Gonadal morphology and steroidogenic patterns of female bovine fetuses in twin pregnancies were compared with those of single male and female fetuses to determine the morphogenetic and functional correlates of ovarian alteration in freemartins. In male fetuses, seminiferous cords were detected by 40 days and Leydig cells by 45 days. The ovaries of 50-day fetuses showed clusters of steroidogenic cells forming medullary cords, which appeared to be the precursors of the follicular cells detected in the ovarian cortex of fetuses over 90 days old. Among the steroids produced by the testis of single male fetuses, testosterone was the most prominent, androstenedione was intermediate and estrone and estradiol were in trace amounts, whereas ovaries secreted low levels of testosterone and moderate amounts of androstenedione, estradiol and estrone. Ovarian estradiol secretion in the fetus peaked during 50 to 70 days of development while estrone was produced at elevated levels during 75 to 95 days. During the peak period for estradiol, steroidogenesis in the ovary appeared to follow the androstenedione-to-estrone-to-estradiol pathway. From the relative concentrations of steroids detected, it would appear that steroid synthesis declines in fetal ovaries, probably due to the inhibition of 17β-hydroxysteroid dehydrogenase, prior to that of aromatase. In contrast, in female fetuses of heterosexual twins (freemartins) an interruption of ovarian differentiation was evident as early as 55 days whereas testicular cords and cells resembling Leydig cells were apparent only in fetuses over 70 days old. The morphologic evidence of ovarian alteration was preceded by functional alterations in that the levels of estrogen were significantly reduced and androstenedione was elevated in freemartins as early as 40 days of fetal development. These initial changes were followed by elevated levels of testosterone and androstenedione after 75 days in virilized freemartin gonads coincident with the induction of Leydig cells in the ovary under the influence of the male co-twin. The higher levels of androstenedione secreted by freemartin gonads in early stages seem to further confirm that the androstenedione-to-testosterone pathway may be inherently inactive in normal bovine ovaries.