What triggers meiosis in gonadal cells?
Ultimately, it seems that follicle-stimulating hormone (FSH), released from the pituitary gland, may be the direct signal for spermatocytes to enter meiosis [1]. It's worth noting, though, that these results come from in vitro studies, where spermatocytes were co-cultured with all cell types of seminiferous tubules in an artificial medium, which makes it a rather crude approximation of in vivo conditions: mimicking the complete spermatogenic cycle in vitro remains to be done [2].

It's a little bit more complicated with oocytes. They start meiosis during prenatal development, are arrested in diplotene stage of the first meiotic division, and then a surge of luteinizing hormone (LH), just before ovulation, makes them resume. But they are arrested again in the last phase, and only after conception do they finish meiosis. The pause occuring in prenatal development is thought to be induced by some kind of an "arrester" secreted by cells in the oocyte environment (specifically, mural granulosa cells) and sustained by high cAMP levels, that are produced in response to both intrinsic and extrinsic stimuli. [3]

Another matter, and quite interesting, is the issue of timing: meiosis starts in a distinct moment of a male mammal develompent, and then goes on continuously, but mammal oogenesis is a series of starts and stops. More on this can be found in [3] and [4].



[1] Tesarik, J., Guido, M., Mendoza, C. &amp; Greco, E. Human spermatogenesis in vitro: respective effects of follicle-stimulating hormone and testosterone on meiosis, spermiogenesis, and Sertoli cell apoptosis. J. Clin. Endocrinol. Metab. 83, 4467–4473 (1998). PMID: 9851795. Free access.

[2] Sousa, M., Cremades, N., Alves, C., Silva, J. &amp; Barros, A. Developmental potential of human spermatogenic cells co-cultured with Sertoli cells. Hum. Reprod. 17, 161–172 (2002). PMID: 11756382. Free access.

[3] Zhang, M. &amp; Xia, G. Hormonal control of mammalian oocyte meiosis at diplotene stage. Cell. Mol. Life Sci. Epub ahead of print. (2011). PMID: 22045555.

[4] Albertini, D.F. &amp; Carabatsos, M.J. Comparative aspects of meiotic cell cycle control in mammals. J. Mol. Med. 76, 795–799 (1998). PMID: 9846949.