Regulation of the mitotic and meiotic cell cycles in the male germ line.

Mammalian gametogenesis provides a unique system in which to study cell-cycle regulation. Furthermore, understanding the genetic program controlling the mitotic and meiotic divisions of the germ line will provide insight into understanding infertility and new directions for contraception. Male and female germ cells have stages of cell-cycle regulation in common, including a mitotic proliferative stage, entry into meiosis, completion of a reductive division, and entry into a quiescent state awaiting signals at fertilization. However, the timing of these events - and, indeed, even the stage of development at which these events occurs - differs in the two sexes. The genes involved in controlling these specialized mitotic and meiotic cycles of mammalian germ cell differentiation are only now being identified. They include a complex array of kinases, phosphatases, regulatory proteins (e.g., cyclins), and an equally complex array of substrates, including components of the nuclear and cytoplasmic structures involved in cell division. This chapter provides an overview of our current understanding of cell-cycle regulation in mammalian mitotic cells and the importance of restriction points. A summary of observations regarding the expression of various cell-cycle regulatory genes in mouse gametes is provided, along with comments on interesting differences between mitotic and meiotic cells. Finally, the role of the novel A-type cyclin, cyclin A1, during male meiosis is discussed in depth.