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Fertilisation can be defined as the fusion of the sperm nucleus with the egg nucleus to form a diploid cell known as zygote.
The fertilisation is internal in the human reproductive system. It is achieved by the insertion of the male organ, penis into the vagina of the female. The sperms are deposited in the vagina of the females during a process called as copulation or sexual intercourse. The release of sperms is called ejaculation and at one ejaculation there may be upto 600 million sperms.
Of these only a few thousands make the journey from the vaginal canal to the oviducts. This may take 4-8 hours. The life of the sperm is 1-3 days after the ejaculation.
Each sperm is a small cell of about 50um length and 2.5um diameter.
It consists of a head, middle piece and the tail regions. The head consists of the nucleus and a structure called the acrosome at the tip. The acrosome secretes lytic substances that break down the walls of the egg for fertilisation. The beating of the tail propels the sperm forward at the rate of 1-4 mm per minute.
Each egg or ovum is a spherical cell much bigger than the sperm. It mainly consists of a single nucleus and some reserve food made of lipid droplets.
In the fallopian tubes, many sperms surround an egg. However, only one enters the egg leaving behind the tail. The enzymes of the acrosome digest the several layers of tissue to reach the egg cytoplasm. Once the sperm is inside, the male and female nuclei become lighter and are called pro-nuclei. The two pro-nuclei fuse forming a zygote.
Once a sperm enters an egg, the thickening of the outer walls of the egg blocks the entry of other sperms.
Once the egg is fertilised, ovulation and menstruation do not take place. The fertilised structure, now called the zygote is diploid. It starts dividing and forms a blastocyst, the first stage of an embryo. The blastocyst gets implanted on the uterine walls. It then grows into a foetus.
The foetus remains attached to the mother through an umbilical cord which is embedded in a tissue called placenta at one end.
Foetus in Different Positions and Stages
The placenta in turn is embedded into the uterine wall and is richly supplied with blood vessels. The nutrients from the mother's blood pass into the umbilical cord and the waste from the foetus pass into the mother's blood through the placenta.
The foetus is fully grown and ready to emerge from the womb in 40 weeks. The uterine walls start muscular contractions called the labour that helps in pushing the foetus to the outside through the vaginal canal. The placenta detaches from the uterine wall once the baby is out. The umbilical cord has to be cut and tied near the baby's body.
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