This diagram shows the different maturational stages of ovarian follicles.

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Here’s a quick summary of when you see these different stages:

1. At birth, all you have are primordial follicles (about a million in each ovary).
2. At puberty, you start “recruiting” groups of about 50 primordial follicles at the beginning of each menstrual cycle.
   • These primordial follicles start down the pathway of maturation, moving through the stages listed above (unilaminar primary, multilaminar primary, secondary, and Graafian follicles).
   • Only one of the group of 50 follicles will make it to the Graafian follicle stage (the other 49 die off along the way in a process called “attrition” or “atresia”).
   • At the time of ovulation, the Graafian follicle ruptures, releasing the germ cell inside, which makes its way to the fallopian tube and down into the uterus.
3. In addition, over a woman’s lifetime, the primordial follicles in each ovary start dying off on their own (independent of the menstrual cycles). Their population dwindles over time until the age of menopause, when there are none left. Then menstrual periods cease (there are no primordial follicles left to recruit!).

By the way, here’s what is inside each follicle (spoiler: it’s always a primary oocyte):

• Primordial follicle: primary oocyte (arrested part way through meiosis I)
• Unilaminar primordial follicle: primary oocyte (arrested part way through meiosis I)
• Multilaminar primordial follicle: primary oocyte (arrested part way through meiosis I)
• Secondary follicle: primary oocyte (arrested part way through meiosis I)
• Graafian follicle: primary oocyte (arrested part way through meiosis I)

Wait, what?? What about the secondary oocyte and the ovum?

• At ovulation, the primary oocyte is “released” from its maturational arrest, and it turns into a secondary oocyte. YAY! This secondary oocyte starts undergoing meiosis II, but is arrested part way through.
• If no fertilization occurs, the secondary oocyte will never complete meiosis II, and will be shed during menstruation.
• If fertilization occurs, the secondary oocyte completes meiosis II, turning into an ovum. FINALLY!!!