Fig. 2

Schematic of the key stages of oocyte maturation and folliculogenesis. Upper panel: Oocyte growth, entry into meiosis and meiotic arrest. Meiotic maturation begins with GVBD, followed by the first meiotic division, divalent separation, extrusion of the first polar body, and arrest in the middle of the meiosis II. Oocytes ovulate at MII stage. Sister chromatids will separate after fertilization. Middle panel: Hormone levels in the corresponding states of oocyte maturation and follicular development. Growth of the oocyte and follicle can be classified into distinct stages. From primordial to secondary follicle development gonadotropin may be not needed. At this stage the follicular development may be termed gonadotropin-independent. Transition of the follicle from the preantral to early antral stage is primarily controlled by intraovarian regulators, the gonadotropin at this stage may not be required, termed as gonadotropin-responsive. The following growth past antral stage to the preovulatory stage may be gonadotropin-dependent. Lower panel: Follicle developmental stages and the sizes of follicles are indicated, with the oocyte eventually reentering meiosis and being released from the follicle