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Fatty tissue

Lung

Fatty tissue

Lung

The menopause

The menopause is the time when menstrual cycles cease. It usually occurs between the ages of 45 and 55. The ovaries stop responding to follicle-stimulating hormone and produce less of the female sex hormones oestrogen and progesterone. As a result ovulation and menstruation end, and once a woman has reached the menopause she is no longer fertile. In the years just before and after the menopause, hormone changes produce symptoms such as mood swings, hot flushes, vaginal dryness, and night sweats. The menopause may also result in long-term physical changes, such as osteoporosis.

Thin, brittle bone

Thin, brittle bone

Osteoporotic bone

The sex hormone oestrogen is needed to give bones strength. Low oestrogen levels after the menopause can result in osteoporosis, a condition in which the bones lose density and may become thin and brittle, as shown in this microscopic image.

Conception and pregnancy

All organisms reproduce. In human beings, reproduction involves two types of cell: sperm, produced by the testes in men, and eggs, produced by the ovaries in women. These cells each contain half a set of DNA (genetic material). They are brought together by sexual intercourse; if a sperm penetrates and fertilizes an egg, the man's and woman's DNA combine to form new cells. Conception occurs when these cells embed themselves in the uterus. During pregnancy, which lasts for about 40 weeks (9 months), the cells develop into a baby.

Fertilization

During sexual intercourse, sperm are expelled into the woman's vagina, then swim up through the uterus and into the fallopian tubes. If the sperm meet an egg, they try to pierce its coating. If a sperm succeeds, it sheds its tail and fuses with the nucleus of the egg, while chemical changes in the egg stop any more sperm from entering. In this way a new cell is formed, combining DNA from the man and the woman.

Tail of sperm

Head of sperm

Surface of egg

Tail of sperm

Head of sperm

Surface of egg

Sperm penetrating egg

The head of the sperm pushes through the egg's outer coating in order to reach the nucleus.

Sperm penetrating egg

The head of the sperm pushes through the egg's outer coating in order to reach the nucleus.

Beginning of pregnancy

The cell produced by the fusion of the egg and sperm is called a zygote. Within 2 days of fertilization, the zygote starts its journey along the fallopian tube towards the uterus, propelled by the muscular action of the tube's walls. At the same time, the zygote divides itself repeatedly to form a cluster of cells, which is called a morula. After 5-7 days, the cell cluster reaches the uterus. It embeds itself securely in the endometrium (the lining of the uterus) and continues to grow. From this moment onwards, the pregnancy is properly established. One part of the cell cluster grows into the endometrium and becomes the placenta, which will nourish the developing baby. The rest of the cells, from which the baby will grow, become an embryo.

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