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Making the Gametes
Fertilisation is all about getting the gametes together.
It may not sound very romantic but that's all there is to it really. In both plants and animals, the male and female gametes meet and join. They form the zygote, the fertilised egg that becomes the new organism as it divides, grows and develops.
In plants it is all about the male gamete, the pollen, getting to the female gamete the ovum.
In animals, as you already know, the sperm and egg join to form the zygote which becomes the baby.
But how do the sperm and eggs get made?
The human male is rather a simple creature - well in terms of making sperm and his plumbing that is.
Sperm are made in a continual process in the testes. Each testis is a series of tubes in which the sperm develop.
When they are ready they are stored in the widened start of the spermduct. Then during sexual intercourse they are fired along the sperm duct towards the urethra - the tube that extends through the penis. This process of sperm emission is called ejaculation and is under reflex control using muscles.
As the sperm moves along into the urethra it gets mixed with the secretions from the prostate gland and the seminal vesicle. These give the sperm sugars and other chemicals to fuel them during their journey to the egg. This mixture of sperm and secretionis called semen.
Finally, when the sperm are ejaculated out of the penis there is about 3-5ml. of semen with the sperm only making up a small fraction of that. However there are usually about 200 - 500 million sperm in each ejaculation!
Eggs are formed in the ovary. Then when they are ready they are released one at a time each month. This is called ovulation.
The egg travels down the oviduct towards the uterus. If it is not fertilised it will pass down through the uterus, past the cervix and out of the vagina.
This all seems quite simple compared to men. But things are deceptive, it is the hormone control of ovulation that is the tricky bit!
If you study the wall of the uterus you see a roughly 28 day cycle. It begins with the start of the bleeding or menses, it is this that gives the cycle its name, the menstrual cycle.
The menstrual cycle has 4 stages to it.
- The lining of the uterus breaks down and the bleeding starts.
- Stretches from day 4 to day 14, this is when the lining is repaired.
- On day 14 the egg is released from the ovary.
- The maintenance stage when the uterus is maintained in case the egg is fertilised
The different stages of the cycle are controlled by a set of four hormones:
Oestrogen and progesterone are produced by the ovary.
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are produced in the pituitary gland in the brain.
What do they do?
Studying this diagram might help you.
FSH stimulates the ovary to get the egg ready for release. It also gets the ovary to secrete oestrogen.
Oestrogen causes the lining of the uterus to grow and get ready for the egg. It also helps to trigger the release of the egg.
LH triggers the release of the egg from the ovary once it is ready and enough oestrogen has been produced.
Progesterone maintains the uterus lining after the egg is released. When the level of progesterone falls the lining breaks down.
By knowing about what the hormones do, doctors have been able to help women to control their egg release. This allows fertility treatment and contraception.
The Pill is a widely used means of contraception which contains both progesterone and oestrogen. This keeps the oestrogen levels high which stops further egg release.
If FSH is given to women who have problems ovulating (producing and releasing eggs) it triggers oestrogen release in the ovary and stimulates egg release.
It works well although if you give too much you can get multiple births!