Reproduction in humans

The birds and the bees, doing the deed, getting hot and heavy.. whatever you wanna call it, here’s a page explaining the science behind sexual reproduction in humans.

Sexual vs asexual reproduction

Sexual reproduction involves the fusion of two gametes (sex cells) from two parents. Gametes are created by meiosis which produce cells with half the genetic information (haploid). When two haploid gametes join during fertilisation, an organism is produced which is genetically distinct from either of its parents.

Asexual reproduction occurs when cells from one parent divide by mitosis to produce a whole new organism. Since there is no mixing of genetic information, the offspring is genetically identical (a clone) of its parent. Organisms which can reproduce asexually include yeast, bacteria and fungi. Even some plants can produce clones of themselves, in a process called vegetative propagation. The disadvantage to asexual reproduction is that there is no variation in the offspring, making them equally susceptible to harmful environmental pressures such as disease or drought. However, it is a much quicker method than sexual reproduction as the organism doesn’t need to waste time finding a mate.

Male reproductive system

Testes (singular: testi) are the site of sperm and testosterone production. Sperm travels along a tube called the sperm duct, where it mixes with seminal fluid which provides the sperm with nutrients. The combination of sperm with the seminal fluid creates semen. During ejaculation, the semen travels along the urethra and out of the body through the penis. The penis is also used to pass urine out of the body, via the urethra.

Female reproductive system

Female gametes are called ova (singular: ovum) which are found in the ovaries. After an egg matures in the ovaries, it is released into the oviduct (fallopian tube) and moves through the tube towards the uterus. The oviduct is lined with ciliated epithelial cells, which have finger-like projections called cilia to move and sweep the ovum along the tube. It is in the oviduct where fertilisation takes place, if the ovum successfully encounters a sperm cell. Once in the uterus, the fertilised egg (zygote) implants itself into the uterine lining where it will develop into an embryo. If no fertilisation occurs, the uterine lining will shed and pass out of the body during menstruation.

A circular ring of muscle called the cervix is found at the end of the uterus and it keeps the baby in position during pregnancy. The vagina leads from the cervix to the outside of the body and it holds the penis during sexual intercourse. The urethra is a tube separate from the vagina, for allowing urine stored in the bladder to pass out of the body.

Hormonal control of the menstrual cycle

Follicle stimulating hormone (FSH) is produced by the pituitary gland in the brain and causes the maturation of an egg in the ovary. As the egg matures, it releases estrogen which stops FSH being produced so that only one egg matures per cycle. Estrogen also causes thickening of the uterus lining and stimulates the pituitary gland to release luteinising hormone (LH), which causes release of the egg from the ovary. If the egg is fertilised, it will implant itself into the uterus lining, which is maintained by the hormone progesterone. If the egg is unfertilised, the uterus lining breaks down, resulting in menstruation.

Secondary sexual characteristics

Estrogen and testosterone are responsible for the formation of secondary sexual characteristics in women and men respectively, which is just a fancy way of saying the changes that occur during puberty.

• Estrogen: causes the hips to widen, development of breasts, growth of pubic hair and menstruation.

• Testosterone: initiates sperm production, lowering of vocal cords and the growth of pubic hair.

Pregnancy

If the timings right and the stars are aligned, there is a chance that a sperm cell might just be lucky enough to meet and fuse with the ovum (egg cell) to produce a fertilised egg called a zygote. The zygote divides by mitosis into a ball of cells called an embryo which implants itself into the wall of the uterus. The embryo is surrounded by a liquid called amniotic fluid which cushions the baby and acts as a shock-absorber, protecting the embryo. A placenta also forms and is connected to the embryo through the umbilical cord. The placenta anchors the baby in the uterus and provides the developing embryo with substances (such as oxygen and glucose) as well as removing any waste products (like carbon dioxide).

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