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tud ith re fluid for stereology and the other frozen for testosterone measurement. Trunk blood was collected for testosterone RIA. The produced within the testis by the aromatisation of testosterone (Nitta et al. 1993) and has been shown to human post-meiotic germ cells (Pentikainen et al. 2000). (Zhou et al. 2002) and are thus the only cells in the mouse testis reported to express both ERs. 1993). How these changes influence Sertoli cell The effects of oestrogen are mediated by two distinctDOI: 10.1530/REP-07-0025receptors: oestrogen receptor a (ERa) and b (ERb), both q 2007 Society for Reproduction and Fertilitynumbers remains unknown. Male ERb knockout mice (bERKO), on the other hand, are reported to haveaffect the major cell types within the organ. Adminis-tration of exogenous oestrogen during development has been shown to reduce Sertoli cell numbers in the adul