Female Gonads: Ovaries
Produce:
- Gametes (eggs)
- Hormones (estrogens and progesterone)
Oogenesis
Overview
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The creation of eggs (genesis: creation)
- Eggs = ova (plural); ovum (singular)
- Occurs in the ovary.
- The first phase of egg development begins in the female fetus and is completed by the end of the third month of fetal development!
- The final phase of egg development is completed only if the egg is fertilized by a sperm.
Details
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Mitosis
- Oogonia become primary oocytes (both are diploid cells).
- Completed by the end of the third month of fetal development.
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Meiosis (pre-puberty)
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Primary oocytes begin the process of meiosis but are halted in prophase I.
- A newborn baby girl has about two million primary oocytes.
- No new primary oocytes will be produced during a female's life.
- Primary oocytes will die or be reabsorbed by the body so that about 400 000 remain at the beginning of puberty (normally between ages 11–14).
- Meiosis resumes, one oocyte at a time, beginning with the first menstrual cycle.
- Only 400–500 oocytes (at most) will be released during a woman's reproductive years.
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Meiosis (from puberty through menopause)
- Beginning at puberty, and continuing every month thereafter, one follicle will begin growing.
- Fourteen days after beginning its growth, and just before leaving the ovary, the primary oocyte will finish its first meiotic division and become a secondary oocyte.
- This process is stimulated by follicle stimulating hormone (FSH).
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Unequal cytokinesis occurs.
- Large cell will eventually become the egg (ovum).
- Small cell becomes a polar body.
- Both are haploid.
- Meiosis stops again!
- The secondary oocyte will be released from the ovary during ovulation without yet becoming an ovum!
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Completion of meiosis (post-fertilization)
- Penetration of sperm induces the secondary oocyte to go through its second meiotic division (again with unequal cytokinesis).
- The polar bodies eventually degenerate.
| Spermatogenesis |
Oogenesis |
| Original spermatogonium becomes four functional sperm. |
Original oogonium produces one functional egg and several non-functional polar bodies. |
| Spermatogonia go through mitosis daily for the entire life of a male, producing new primary spermatocytes, which then develop into sperm. |
Oogonia go through mitosis during the first three months of fetal development, producing all the primary oocytes that will be used during the life of a female. |
The Ovarian Cycle
- A 28-day cycle
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Follicular phase (days 1–14): a dozen or more follicles begin to grow in response to follicle stimulating hormone (FSH).
- A follicle is a primary oocyte together with special cells surrounding it that nourish the oocyte and secrete estrogens.
- Only one follicle will actually mature each month; the others that begin the process soon degenerate.
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Note:
- In the illustration below, it appears the follicle is moving around the ovary: this is not what actually happens.
- The follicle stays in one location, growing towards the surface of the ovary before ovulation.
- The different positions of the follicle facilitate learning the cyclic nature of the process.
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Ovulation (day 14): follicle ruptures.
- 8 to 10 hours prior to ovulation, the primary oocyte (diploid) undergoes the first meiotic division.
- This produces a secondary oocyte and an insignificant polar body (both haploid).
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Luteal phase (day 15–28): follicle cells transform.
- They become a glandular structure called the corpus luteum (word derivation: corpus, body; luteum, yellow).
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The corpus luteum secretes hormones:
- If fertilization does not occur, the corpus luteum disintegrates.
The Menstrual Cycle
- A 28-day cycle
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Menstrual flow phase (days 1–5): the lining of the uterus, the endometrium, is shed.
- Day one of the cycle is the first day of flow.
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Proliferative phase (days 5–14): the walls of the uterus build up (i.e., the endometrium grows).
- Stimulated by estrogens that are secreted from the growing follicle.
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Secretory phase (days 14–28): the endometrium continues to thicken.
- Stimulated by the estrogens and the progesterone that are secreted by the corpus luteum.
- Disintegration of the corpus luteum at the end of the ovarian cycle reduces the levels of estrogen and progesterone in the blood, so the endometrium is shed and the cycle begins again.
