The present study was undertaken to investigate whether rat spermatogonial stem cells can differentiate into developmentally competent round spermatids during co-culture with Sertoli cells. Type-A spermatogonia and Sertoli cells were prepared from 7-d-old Wistar-strain male rats, and seeded at 4 x 10(6) cells/ 4 mL/35-mm dish (Day 0). They were co-cultured at 37 degrees C for 3 d and at 34 degrees C for the subsequent 7d in 5% CO(2)/air. Round spermatid-like cells (approximately 15 microm in diameter) were first observed on Day 5. A flow cytometric analysis showed that a single peak of haploid cells was detected in the cell populations harvested on Day 10. The participation of the spermatid-like cells to full-term development was examined by microinjection into activated oocytes. The oviductal transfer of 143 microinseminated oocytes resulted in only 8 implantation sites (6%), but no viable offspring. The expression of the round spermatid-specific marker gene, PRM-2, was confirmed in the Day 10 cell population by RT-PCR; however, no mRNA of two other haploid makers, TP1 or TP2, was detected. These results suggested that rat type-A spermatogonial cells underwent meiosis during the primary co-culture with the Sertoli cells, based on morphology, flow cytometry and PRM-2 expression, but the normality of the spermatid-like cells was not supported by microinsemination and TP1/2 expression.