Sexual reproduction is the most common method of reproduction in animals (including human beings). The sexual reproduction in animals takes place in the following steps:
1. The male parent produces male gametes (male sex cells) called sperms. The sperm is a small cell with a long tail (flagellum) for movement.
2. The female parent produces female gametes (female sex cells) called ova (or eggs). The ovum (or egg) is a much bigger cell than the sperm, having a lot of cytoplasm.
The sperm enters into the ovum (or egg) and fuses with it to form a new cell called ‘zygote’. This process is called fertilisation. So, the zygote is a fertilised ovum (or fertilised egg)-
4. The zygote then divides again and again to form a large number of cells (all of which remain together). And ultimately zygote grows and develops to become a new baby.
From the above discussion we conclude that the whole process of sexual reproduction in animals involves the formation of sperms and eggs; joining together of sperm and egg to form a zygote and then the growth and development of zygote to form a baby animal.
In complex multicellular animals (like human beings) there are special reproductive organs to make sperms and eggs; to bring together sperms and eggs for fertilisation; and for the growth and development of zygote into a baby.
We will study all this in the human reproductive system. Before we describe the human reproductive system, we should know the meaning of the term ‘puberty’. This is discussed below.
When a child is small, sometimes it becomes difficult to tell from appearance whether it is a boy or a girl. This is because small boys and girls have the same body shape. A time of rapid growth and body changes starts in the early teens which make the boy and the girl appear different and also behave differently.
These changes start earlier in girls than in boys. We call the time between childhood and adulthood ‘adolescence.’ The production of male and female ‘sex hormones’ in the bodies of boys and girls increases dramatically at this stage and causes wide-ranging changes in their bodies.
The testes (in boys) and ovaries (in girls) make different hormones, so the boys and girls develop in different ways. Ultimately the boys and girls become sexually mature and their reproductive systems start functioning.
The age at which the sex hormones (or gametes) begin to be produced and the boy and girl become sexually mature (able to reproduce) is called puberty. Puberty tends to start earlier in females (girls) than in males (boys). Generally boys attain puberty at the age of 13 to 14 years while girls reach puberty at a comparatively lower age of 10 to 12 years.
On attaining puberty, the male gonads called testes start producing male gametes called sperms and the female gonads called ovaries start producing female gametes called ova (or eggs). In addition to producing sex cells (or gametes) male and female gonads (testes and ovaries) also produce and secrete sex hormones with the onset of puberty.
The testes produce the male sex hormone called testosterone, and the ovaries produce two female sex hormones, oestrogen and progesterone. The sex hormones play an important role in the process of reproduction because they make the reproductive organs to mature and start functioning. Puberty is the age at which the reproductive organs reach maturity and secondary sexual characteristics develop.
The various changes which occur in boys at puberty are: Hair grows under armpits and in pubic regions (genital area) between the thighs. Hair also grows on other parts of the body like chest and face (moustache, beard, etc.).
Body becomes more muscular due to the development of muscles. The voice deepens (or cracks). Chest and shoulders broaden. The penis and testes become larger. The testes start to make sperms. Feeling and sexual drives associated with adulthood begin to develop. All these changes in boys are brought about by the male sex hormone ‘testosterone’ made in tests.
The various changes which occur in girls at puberty are: Hair grow under armpits and pubic region (This change is the same as in boys). Mammary glands (or breasts) develop and enlarge. The hips broaden.
Extra fat is deposited in various parts of the body like hips and thighs. Fallopian tubes, uterus and vagina enlarge. Ovaries start to release eggs. Menstruation (monthly periods) start. Feelings and sexual drives associated with adulthood begin to develop.
All these changes in girls are brought about by the female sex hormones ‘oestrogen’ and ‘progesterone’ made in ovaries. Please note that the hormone ‘oestrogen’ is also written and spoken as ‘estrogen’.