All mammalian embryos grow up female unless told not to, with their 'supporting' cells turning into granulosa cells, which occur in the ovaries. Only when a Y-chromosome gene called Sry triggers the supporters to specialize as testicular cells that nurture growing sperm — Sertoli cells — does maleness follow.
A team led by Yoshiakira Kanai of the University of Tokyo has created a line of transgenic mice in which the turning on and off of Sry can be very tightly controlled. Using these mice, they have shown that Sry activation must occur during a six-hour window eleven days after mating if the adult is to have Sertoli rather than granulosa cells.
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Developmental biology: Swing time. Nature 456, 678 (2008). https://doi.org/10.1038/456678c
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DOI: https://doi.org/10.1038/456678c