Figure 1.

Reproductive life cycle of a mammalian female. An oocyte and a spermatozoon will fuse to form a zygote and undergo multiple steps in embryogenesis. At about E6.5 in mouse, the PGC will be allocated and go through multiple steps to reach the genital ridge. In an XX mammal, the germ cell will form an oocyte that arrests at meiosis I (MI). During prenatal life in women, and in the perinatal period in mice, the oocyte will be encased in somatic cells to become primordial follicles. Upon recruitment into the growing pool, the oocyte increases in size during folliculogenesis. The LH surge will induce resumption of meiosis, release of the first body, arrest at MII, and subsequent ovulation of the oocyte into the fallopian tube. Fertilization with a spermatozoon will induce the completion of meiosis and release of the second polar body. The cycle continues in the next generation of females. During the reproductive cycle, there are multiple steps where significant oocyte loss is observed.