Human reproduction
Introduction
Previous chapters looked at the reproduction process on molecular and cellular levels. Different types of reproduction in plants and animals were also discussed. This chapter looks at human reproduction and reproductive systems.
Human reproductive systems
Human reproductive systems are made of different organs and tissues that are involved in the production and maturation of sex cells and sex-related hormones.
Female reproductive system
See image 1.
The female sex cell is called an ova. It is also referred to as egg. Eggs are produced in sex cell producing glands called ovaries. Females have two ovaries. Ovaries also produce hormones called progesterone and oestrogen. Progesterone is very important for successful implantation of a fertilised egg. Oestrogen is a hormone that is responsible for female sexual development and female secondary sex characteristics. In other words it makes females look like females.
All females' eggs are formed in the ovaries before birth and remain inactive until puberty. Once an organism reaches puberty, eggs are 'distributed' (about 20 at a time) for possible fertilisation in a process called the menstrual cycle. A new-born girl will have about half a million eggs in each ovary but only a few hundred of these eggs will reach a fully-formed stage. It takes about 14 days for the egg to reach this stage. The mature egg moves into the fallopian tube in the process called ovulation. It stays there for about seven days. After that it moves into the uterus. If the egg has been fertilised it implants into the wall of the uterus and after nine months develops into a baby. An unfertilised egg is discarded with the uterine lining in a process called menstruation, or period, through the female sex organ, called the vagina. When females 'run out' of eggs their periods stop. This process is called menopause.
Male reproductive system
See image 2.
Male sex cells called sperm are produced in the testes. The optimal temperature for sperm is lower than normal body temperature. That is why in the process of evolution male humans came to have the glands which produce their sex cells - the testes - outside their bodies. Testes are located in a skin 'sack' called a scrotum. The male hormone testosterone is also produced by the testes. It stimulates blood flow, the growth of certain tissues and is responsible for male secondary sexual characteristics. Because of testosterone males look like males.
Testes are made of tightly coiled tubes called seminiferous tubules. Sperm cells are formed there on a daily basis. Later, sperm move to another type of tubules network called epididymis. That is where sperm cells reach the stage where they can move using their 'tails'- flagella. Sperm is released via a sperm duct that runs through the male sex organ called the penis. As sperm cells pass through the sperm duct, glands called prostate and seminal vesicle add fluid to them. The fluid addition speeds up the sperm's movement and helps in the process of fertilisation. Sperm combined with these fluids is called semen.
Fertilisation
During fertilisation, the egg and sperm fuse together. See image 1. Sex cells are brought in contact through the process of sexual intercourse. During this process the penis is inserted into the vagina and semen is ejaculated. Men produce about 280-400 million sperm in each ejaculation. Once inside the vagina, sperm cells move through the cervix and uterus towards the fallopian tubes, 'searching' for eggs. This process can take about 30-60 minutes. An egg attracts sperm by releasing special chemicals. Many sperm cells 'get lost' on their way by getting stuck in the vagina or moving toward the wrong fallopian tube. Only the strongest cells reach the egg. When sperm cells find an egg they surround it, sticking to its wall. Sperm enzymes dissolve the egg's protective coating and make a hole in it. Only the strongest and most persistent sperm cell will get inside. After that, the egg becomes impenetrable so other sperm cannot enter it.
From zygote to baby
A fertilised egg is called a zygote. When a sperm cell joins an egg, a zygote is formed in the fallopian tube. The zygote moves towards the uterus where it will be implanted and mature into a baby. The process of this relocation is really slow. It takes about five days. While the zygote moves, it grows. By the time it reaches the uterus it is a cluster of about 80 cells. Once in the uterus, this cluster of cells becomes a blastocyst - a hollow ball of cells, filled with fluid. After eight weeks the heart starts beating and all the major organ systems will begin to form. This cell formation becomes an embryo. Later, the embryo becomes a foetus. In the female's body, the foetus is surrounded by a protective sack of amniotic fluid and it is fed through the umbilical cord. The umbilical cord is attached to the placenta. The placenta is a membranous organ that develops during pregnancy. It lines the uterine wall and is attached to the umbilical cord. After about nine months the baby is pushed out by strong contractions of the uterus called labour.
See animation 1.
Reproductive problems
The inability to produce children is called infertility. Both men and women can be infertile. Some people are born infertile and others acquire this condition. Radiation, infections, sexually transmitted diseases, drugs and excessive alcohol consumption can cause infertility. Some infertility cases can be treated using reproductive techniques such as IVF (in-vitro fertilisation), ovulation induction to enhance the production of eggs, surgery to repair reproductive organs and intra-uterine insemination to increase the chances of an egg's fertilisation by sperm.
