Human Reproductive Systems: Making and Delivering Sex Cells

1Two parents, two kinds of sex cell

Humans reproduce sexually, which means that a new person starts from two cells, one from each parent: a sperm and an egg. These are the gametes, or sex cells. They are made by meiosis (see the previous lesson), which is why each carries 23 chromosomes, half the number found in a body cell.

The organs that make gametes are called gonads. The male gonads are the testes and the female gonads are the ovaries. Both also make hormones that control reproductive development and function. This lesson follows the structures that produce, store and transport gametes, and the hormones that coordinate them. The next lesson follows what happens when a sperm meets an egg.

Biology describes the male and female systems as they typically occur. People vary, and the aim here is only to describe the structures that produce sperm and eggs and the steps that allow a pregnancy to begin.

2The male reproductive system

Testes and the scrotum

The two testes sit in a muscular sac, the scrotum, outside the main body cavity. Sperm production works best a little cooler than the core of the body. The scrotum keeps the testes about 2 to 4 °C below core body temperature, and muscles in its wall can raise or lower the testes to adjust the temperature. Each adult testis is about 4 to 5 cm long.

Making sperm

Most of each testis consists of tightly coiled tubes called seminiferous tubules, where sperm are made. Stem cells divide by mitosis, and some of the resulting cells then go through meiosis, yielding four haploid cells that mature into sperm. Sperm production begins at puberty and continues throughout adult life. A complete cycle of sperm formation takes about 64 days, and the testes make roughly 100 to 300 million sperm each day.

The path of sperm

1. Seminiferous tubules. Sperm form here.
2. Epididymis. A long, tightly coiled tube on the surface of each testis. Sperm mature here and gain the ability to swim. The trip takes about 12 days on average, and mature sperm are stored in the tail of the epididymis.
3. Vas deferens. At ejaculation, sperm move into this muscular tube, which runs up from the scrotum.
4. Urethra. The vas deferens joins the urethra, which runs through the prostate and the penis and carries semen out of the body.

Accessory glands and semen

Sperm travel in a fluid called semen. Sperm account for only about 5% of it. The rest comes from three kinds of gland.

• Seminal vesicles provide about 60% of the volume, including fructose, a sugar that fuels the sperm's movement.
• The prostate gland (about the size of a walnut) adds an alkaline fluid that helps semen to thicken after ejaculation and then liquefy again so the sperm can move on.
• The bulbourethral glands release a lubricating fluid before ejaculation that also clears traces of urine from the urethra.

Hormones in the male

The brain controls the testes through hormones. The hypothalamus releases GnRH, which prompts the pituitary gland to release LH and FSH. LH stimulates cells in the testes to make testosterone, which supports sperm production and the development of male features such as muscle and bone growth. FSH acts on supporting cells in the tubules to help sperm develop. Testosterone and a hormone called inhibin travel back to the brain and tell it to reduce its signals. This is a negative feedback loop, which keeps the hormone levels balanced.

The sperm cell

A sperm is built for one job: delivering a set of chromosomes to an egg. It is one of the few human cells that moves under its own power. Its structure reflects this.

3The female reproductive system

Ovaries and follicles

The two ovaries are almond-sized organs. Each contains follicles, each of which is an immature egg cell (an oocyte) surrounded by supporting cells. Most follicles never reach ovulation: roughly 99 percent degenerate at some stage.

Uterine tubes

Each ovary lies next to a uterine tube (also called a fallopian tube) with a fringed end that captures the egg after it is released. Smooth muscle contractions and tiny moving hairs (cilia) carry the egg along the tube. Fertilization usually occurs in the wider middle section of the tube.

Uterus, cervix and vagina

The uterus is a muscular organ with a thick wall. Its inner lining, the endometrium, thickens under hormonal control each cycle to prepare for a possible embryo, and part of it is shed if no pregnancy occurs. The lower end of the uterus is the cervix, which opens into the vagina, a muscular canal about 10 cm long. The vagina receives sperm, serves as the birth canal, and is the exit for menstrual flow.

The egg cell and oogenesis

The formation of eggs, oogenesis, is quite different from the formation of sperm.

1. Before birth. Cells in the ovary begin meiosis and form primary oocytes. The oocytes pause partway through meiosis I and stay paused until puberty.
2. Each cycle. Shortly before ovulation, a surge of the hormone LH restarts meiosis I in one follicle. The division is unequal: it produces one large secondary oocyte that keeps nearly all of the cytoplasm, and a tiny polar body that breaks down.
3. Ovulation. The follicle wall breaks down and the secondary oocyte is released from the ovary.
4. Meiosis II. The oocyte pauses again. Meiosis II is completed only if a sperm penetrates it. The result is one haploid egg (ovum) and another small polar body.

The unequal division keeps the large store of cytoplasm in a single cell. The egg supplies not only half of the chromosomes but also nearly all of the cytoplasm and mitochondria of the zygote, which is why the mitochondria (and their DNA) are inherited from the mother.

4The menstrual cycle

Unlike sperm production, which is continuous, the release of eggs is cyclical. The menstrual cycle coordinates three things: the maturing of a follicle, ovulation, and the preparation of the uterine lining. The average cycle lasts 28 days, but individual cycles typically range from 21 to 32 days.

The hormone pathway has the same basic structure as in the male. GnRH from the hypothalamus prompts the pituitary to release FSH and LH. FSH drives follicle growth. LH stimulates follicles to make estrogen. Estrogen at first acts by negative feedback. When one dominant follicle remains and produces very high estrogen, however, the feedback reverses (positive feedback) and triggers the LH surge that causes ovulation. After ovulation, progesterone from the corpus luteum maintains the lining and suppresses new follicle development. If no pregnancy occurs, the corpus luteum degrades, hormone levels fall, and menstruation begins.

5Comparing the two gametes

Timing: the window of opportunity

A pregnancy can start only if a sperm and an egg are in the same place at the right time. Two survival times set the limits. An egg remains able to be fertilized for only about 24 hours after ovulation, and it must be fertilized in the outer part of the uterine tube. Sperm, by contrast, can survive for 3 to 5 days in the uterine tubes.

Numbers that seem puzzling

A male body makes sperm in the hundreds of millions per day, while a female system releases a single egg in each cycle. This enormous difference comes with another surprising fact: of the hundreds of millions of sperm released in one ejaculation, only a few thousand reach the uterine tubes. The rest are lost along the way. The acidic fluid of the vagina, the mucus of the cervix and immune cells in the uterus all reduce their numbers.

Several features explain the strategy. Sperm are small and cheap to make, so they can be produced in huge numbers. The egg is large, contains the cytoplasm for early development, and is limited in number. The very large number of sperm improves the odds that some will reach the egg, and because many sperm release enzymes that clear a path through the layers around the egg, the large number is in fact part of how fertilization succeeds.

On the female side, the numbers also look wasteful: follicles are lost throughout life, and only about 1 in 100 reaches ovulation. In both sexes, reproduction involves producing far more gametes than will ever become a new individual.

6Keeping the system working

A few details show how the structures are adapted to protect the gametes and the reproductive tract.

• Temperature control. The scrotal muscles raise the testes toward the body when it is cold and lower them when it is warm, keeping sperm production at its best temperature.
• Protection against infection. Helpful Lactobacillus bacteria in the vagina keep its fluid acidic, which helps protect against infection. Practices that disturb this balance, such as douching, can interfere with this protection.
• Feedback control. The hormone loops in both sexes keep levels in balance. In females, the loop changes during the cycle: negative feedback at first, then a brief positive feedback that triggers ovulation.
• Cilia and muscles. The egg has no tail. It is moved along the uterine tube by wave-like muscle contractions and cilia, both of which respond strongly to estrogen around the time of ovulation.

7Summary

• Gametes are produced in the gonads by meiosis: sperm in the testes and eggs in the ovaries.
• Sperm form in the seminiferous tubules, mature in the epididymis, and travel through the vas deferens and urethra, with fluids added by glands to form semen.
• Eggs develop in follicles, are released at ovulation, and travel along a uterine tube, where fertilization usually occurs; the uterus provides the place for development.
• Oogenesis gives one large egg and small polar bodies; spermatogenesis gives four sperm.
• Hormones such as GnRH, FSH, LH, testosterone, estrogen and progesterone coordinate the system by feedback loops.

?Quick check

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