Although mammals produce either sperm or eggs depending on their sex, we found oocytes in the testes of newborn MRL/ MpJ male mice. In the present study, we report the morphological characteristics of testicular oocytes, the postnatal change of oocyte number per testis, and the expression of a few oocytespecific genes in the testes of MRL/MpJ mice. The testicular oocytes had a diameter of 50-70 lm and were surrounded by zonae pellucidae, which were observed between oocytes and follicular

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... ial cells. Ultrastructurally, the testicular oocytes contained numerous microvilli and cortical granules, receiving cytoplasmic projections from follicular epithelial cells. The testicular oocytes appeared as early as at birth, and the largest number was found on Day 14. The testicular oocytes were detected in only MRL strains and B6MRLF1, but not in C57BL/6, C3H/He, BALB/c, DBA/2, A/J, and MRLB6F1. The expression of the oocyte-specific genes Zp1, Zp2, Zp3, and Omt2a was detected in testes from MRL/MpJ mice. These results suggest that newborn male MRL/MpJ mice with XY chromosomes can produce oocytes in their testes and that one of the genes causing this exists on the Y chromosome.