The female reproductive system, the ovary and oogenesis
The female reproductive system has two ovaries, two Fallopian tubes, the uterus and the vagina. It makes eggs and female hormones, and it gives a safe place for the embryo. The ovaries lie on each side of the pelvic cavity, each about the size of a peeled almond. Each ovary holds thousands of ova at different stages. In about thirty years of reproductive life roughly 400 of them mature, and each cycle one follicle matures and releases an egg. Each Fallopian tube has a funnel-shaped opening near the ovary with finger-like processes and a lining of cilia that moves the egg onward. The uterus is a muscular sac with a thick wall and a glandular lining. It ends in the cervix, which opens into the vagina. The embryo grows in the uterus for nine months. The vagina is a muscular tube of about 7 cm with folds that allow it to widen at birth. From about 12 to 15 years, monthly changes of the ovary and uterus begin, and at about 45 to 50 years the ovaries become less active: this is menopause. A section of an ovary shows follicles in different stages and the Graafian follicle, which becomes the corpus luteum after the egg is released. Oogenesis has three phases. Multiplication (oogonia, 2N) and growth (primary oocytes, 2N) take place in the embryo. In maturation, meiosis I gives a secondary oocyte and a first polar body, both haploid. Meiosis II of the secondary oocyte happens at the moment a sperm enters, and gives an ovum and a second polar body. The first polar body may also divide, so in all there are three polar bodies. The ovum has a thin coat held by hyaluronic acid, which the enzymes of many sperm together dissolve at one point. For any question about a person’s own cycle or body, the person to ask is a doctor or a trusted adult.
| Organ | Job |
|---|---|
| Ovary | holds the eggs in follicles; makes female hormones |
| Fallopian tube | receives the egg; cilia move it towards the uterus |
| Uterus | muscular, with a glandular lining; the embryo grows here for nine months |
| Cervix | the end of the uterus, opening into the vagina |
| Vagina | muscular tube, about 7 cm, with folds that allow widening at birth |
| From one primary cell (2N = 46) | Final cells | |
|---|---|---|
| Spermatogenesis | meiosis I and II | 4 sperm, each with 23 |
| Oogenesis | meiosis I and II | 1 ovum (23) and 3 polar bodies (23 each) |
Compare the two, with a grown-up. Draw a chart with two columns, one for sperm and one for egg. Write for each: where it is made, how many final cells result from one primary cell, the chromosome number at the end, and one feature of its shape. Then answer in one line: why is it correct that a woman’s eggs begin to form before she is born? (Use the phases of the lesson.) For any question about your own body, a doctor or a trusted adult is the one to ask.