Expression of X inactive specific transcript (XIST) and testicular morphogenesis in bovine fetuses

Inactivation of one of the 2 X chromosomes in the somatic cells of female m ammals is the process by which their X-linked gene products are equalized t o those of their male counterparts. In male mammals, however, a sex vesicle representing the condensed and transcriptionally silenced sex chromosomes is detected during early meiotic prophase. Since the exact stage of develop ment at which X inactivation is initiated in the bovine testis is not estab lished as yet, we undertook to study fetuses ranging in age from 30 to 180 days of gestation, to determine the transcriptional status of the Xist gene currently thought to be the prerequisite component of X inactivation. Our studies using reverse transcription polymerase chain reaction (RT-PCR) appr oach with primers designed to amplify a 463 bp product from a conserved reg ion of the first exon of bovine Xist gene, proved that Xist expression is e vident in bovine fetal testes as early as 50 days of gestation and that it continues at least to the end of the second trimester (180 days) of gestati on. Morphological studies on fetal testes during gestational stage spanning the period of Xist expression revealed the presence of large intra-tubular cells overtly resembling the prespermatogonia of postnatal bovine testes, at 50 days and preleptotene like cells as early as 90 days of gestation. We hypothesize that the expression of the Xist gene, or the recently discover ed Tsix gene antisense to Xist in orientation, may be related to the presen ce of these cells which participate in the morphogenesis of the fetal bovin e testis.