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Sources of oestrogen in the testis and reproductive tract of the male.

The cytochrome P450 aromatase (P450arom) is the terminal enzyme responsible for the irreversible transformation of androgens into oestrogens and is present in the endoplasmic reticulum of various tissues throughout at least the phylum of vertebrates. The CYP 19 gene is unique and its expression is regulated in a tissue and more precisely in a cell-specific fashion via the alternative use of several promoters located in the first exons. The P450arom has been immunolocalized in germ cells of the mouse, brown bear and rooster. According to age, aromatase activity has been measured in immature and mature rat Leydig cells as well as in Sertoli cells, whereas in the pig, ram and human aromatase is mainly present in Leydig cells. In the adult rat testis, four complementary approaches (RTPCR, in situ hybridization, immunocytochemistry and the tritiated water assay) demonstrate that not only somatic cells but also mature germ cells represent a source of oestrogen synthesis. Taking into account the widespread distribution of oestrogen receptors (ER alpha & ER beta) in testicular cells and the genital tract of the male on the one hand, and the cross-talk between sex steroids and growth factors, and between membrane receptors and nuclear receptors for steroids on the other hand, it is anticipated that understanding of the pathophysiological roles of these 'female' hormones in the male will advance understanding of the hormonal regulation of male reproductive function. One of the future goals is to define oestrogen-targeted genes in the male gonad and indeed, a lot of work is now focused on this specific area in order to clarify the role of oestrogens in the reproductive tract of the male as well as to elucidate the regulation of aromatase gene expression.