As in all other organisms reproduction is the formation of new individuals of the same species. Sexual
reproduction is the only method of reproduction in our species. Sexual reproduction involves the fusion of specialised haploid sex cells. The fusion of sperm and egg cell is called fertilisation. Fertilisation results in the formation of a diploid zygote from which a new individual develops.
Both the male and female reproductive structures have 3 levels of organisation:
1. Production of sex cells.
2. Transport tubes.
3. Glands to secrete hormones.
The Male Reproductive System
The gonad is the name for the organ that produces sex cells in organs. The male gonads are called the testes.
The testes are contained in the scrotum.
The testes produce the sperm cells by meiosis. The temperature must be lower than body temperature for this to occur. There are tubules that are lines with sperm producing cells. Testosterone, the male sex hormone, is also produced in the testes. Once the sperm are produced they mature in the epidymis. This structure is located outside of the testis. If they are not released within about 6 weeks they are broken down and released to the bloodstream by a process called resorption. The sperm are carried to the urethra by the sperm duct. The urethra carries both sperm and urine.
The sperm cells are carried within a liquid called semen. The semen is produced by the seminal vescicles, the
prostate gland, and Cowpers glands. The semen also contains nourishment for the sperm cells.
Sperm cells are released by ejaculation. About 50-300 million sperm cells are released at one time.
Sperm cells, also called spermatozoa, are haploid containing 23 chromosomes. Their production begins at puberty.
The penis is adapted to place sperm cells into the female. The tip is called the glans. Erection occurs when blood rushes into the penis.
THE STRUCTURE OF THE SPERM CELL
Watch an animation about sperm production.
Take some online quizzes on the male reproductive parts.
Quiz # 1
Quiz # 2
Quiz # 3
Quiz # 4
Male hormones are produced by the pituitary gland during puberty. They are:
During the period of pregnancy testosterone causes the development of primary male sex characteristics. These include the development of the penis and the other male reproductive parts.
Later in life, at puberty, testosterone causes the enlargement of the reproductive parts as well as the development of secondary sexual characteristics. These are characteristics that distinguish males from females.
Male secondary sexual characteristics included:
2. enlarged larynx producing a deeper voice
3. wider shoulders
4. greater skeletal muscular development
5. growth in height and weight
The most common cause of male infertility is the low production of sperm. There are many causes of low sperm production. Stress, alcohol and drug abuse, high temperature of the testes, and low testosterone production are all causes.
THE FEMALE REPRODUCTIVE SYSTEM
Ovaries produce eggs and female hormones. At puberty there are about 40,000 diploid eggs. Each egg is enclosed in a group of cells called a follicle. About 20 haploid eggs are produced each month. Usually all but one die. The haploid egg cell is called the ovum and is surrounded by the Graafian follicle which produces the female hormone called oestrogen. Ovulation is the release of the egg from the follicle. This occurs when the follicle bursts.
The fallopian tubes are about 12 cm long and have ends that are funnel shaped. These ends collect the egg after ovulation. Cilia and peristalsis move the egg along the tube. The egg will die in the tube if it is not fertilised.
The uterus, also known as the womb, is made of involuntary muscle. It is lines with the endometrium. This lining thickens with cells and blood every month. This happens in order to nourish the embryo (if present). The opening of the uterus is called the cervix.
The vagina is a muscular tube which allows the sperm to enter the female as well as the baby to exit. It is lined with mucous secreting cells. The uretrhra opens near the vagina. The vagina is protected by folds of skin called the vulva. The hymen partially blocks the entrance of the vagina. It is broken by sexual intercourse or with the use of tampons.
Take some short online quizzes on the female reproductive parts:
Quiz # 1
Quiz # 2
Quiz # 3
Quiz # 4
Quiz # 5
THE MENSTRUAL CYCLE
The menstrual cycle occurs every 28 days from puberty to menopause (the end of the females reproductive life). It occurs only if fertilisation of the egg has not taken place.
The typical events of the menstrual cycle are:
Day 1 to day 5-
a. The endometrium breaks down and is shed from the body. This is called menstruation.
b. Meiosis occus in the ovary to produce a new egg surrounded by the Graafian follicle.
Day 6 to day 13-
a. Oestrogen is produced by the Graafian follicle. Oestrogen also stimulates the endometrium to thicken again. One Graafian follicle with one egg develops.
b. Oestrogen stimulates the production of LH (leuteinising hormone)
a. The surge of LH stimulates ovulation.
b. The egg enters the funnel of the Fallopian tube. It can be fertilised for the next 48 hours.
Day 15 to day 26-
a. The corpus luteum (yellow body) develops from the remains of the Graafian follicle. This produced progesterone and some oestrogen. The progesterone causes the endometrium to continue to thicken. It also prevents new eggs from forming.
b. The egg that was released at day 14 will die if it is not fertilised.
c. If fertilisation did not take place the corpus luteum begins to degenerate.
Day 26 to day 28-
a. Oestrogen and progesterone levels decline.
HUMAN REPRODUCTION - leavingbio.net
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