The human male and female reproductive systems are made from the same embryonic cells and are perhaps more similar in structure and function than is first apparent. There are two ovaries protected within the pelvic cavity. The ovary is the site of egg cell production. The egg cell is the female gamete and is haploid – it has only one chromosome from each homologous pair. The ovaries are also endocrine organs that produce the female sex hormones oestrogen and progesterone.
[Indeed differences between the gametes is the essential difference between male and female organisms. Females are always individuals who produce a small number of large, often immobile gametes. You can easily remember this: female – few, fixed, fat. Males are organisms that produce large numbers of small, motile games. Male – many, mini, motile.]
This diagram shows the human egg cell after it has been released from the ovary into the Fallopian tubes (or oviduct). The egg cell is coloured pink in the diagram above (if you are being picky it is not really an egg but a cell called a secondary oocyte but I won’t stress over this now…) The egg cell is surrounded by a thick jelly-like layer called the zona pellucida and then by a whole cluster of mother’s cells from her ovary – the corona radiata. The big idea to remember is that the egg cell is very large compared to sperm cells: it is one of the largest cells in humans with a diameter of about 500 micrometers.
The Fallopian tubes carry the egg down towards the uterus. The lining of the Fallopian tubes is covered in a ciliated epithelium. The cilia waft to generate a current that helps move the egg down towards the uterus. Sperm cells have to swim against this current to reach the egg in the tubes. The Fallopian tube is the usual site for fertilisation to occur.
Once fertilisation has occurred, the newly formed zygote divides over and over again by mitosis to form a ball of cells called an embryo. The embryo continues its journey down the Fallopian tube until it reaches the uterus. The uterus (womb) is a muscular organ with a thickened and blood-rich lining called the endometrium. Implantation occurs when the embryo attaches to the endometrium and over time, a placenta forms. The embryo develops into a foetus and remains in the uterus for 9 months.
The cervix is a narrow opening between the uterus and the vagina. It holds the developing foetus in the uterus during pregnancy but dilates (widens) at birth to form part of the birth canal. The vagina is the organ into which sperm are deposited from the man’s penis during sexual intercourse. The lining of the vagina is acidic to protect against bacterial pathogens and the sperm cells released into the vagina quickly start to swim away from the acidity in grooves in the lining. These grooves lead to the cervix and hence into the uterus.