Progression of germ cells through meiosis is regulated by phosphorylation events. We previously showed the key role of cyclin dependent kinases in meiotic divisions of rat spermatocytes co-cultured with Sertoli cells (SC). In the present study, we used the same culture system to address the role of mitogen-activated protein kinases (MAPKs) in meiotic progression. Phosphorylated ERK1/2 were detected in vivo and in freshly isolated SC and in pachytene spermatocytes (PS) as early as 3 h after seeding on SC. The yield of the two meiotic divisions and the percentage of highly MPM-2-labeled pachytene and secondary spermatocytes (SII) were decreased in co-cultures treated with U0126, an inhibitor of the ERK-activating kinases, MEK1/2. Pre-incubation of PS with U0126 resulted in a reduced number of in vitro formed round spermatids without modifying the number of SII or the MPM-2 labeling of PS or SII. Conversely, pre-treatment of SC with U0126 led to a decrease in the percentage of highly MPM-2-labeled PS associated with a decreased number of SII and round spermatids, These results show hat meiotic progression of spermatocytes is dependent on SC-activated MAPKs. In addition, high MPM-2 labeling was not acquired by PS cultured alone in Sertoli cell conditioned media, indicating a specific need for cell-cell contact between germ cells and SC. (C) 2007 Elsevier Inc. All rights reserved.