Gametogenesis
Gametes are formed through a process called gametogenesis. This takes place in the gonads (testicles and ovaries). Gametes are formed by germ cells which, like all other human cells, have 46 chromosomes. During gametogenesis, the number of chromosomes is halved, through a special type of cell division called meiosis. Therefore, each gamete only has 23 chromosomes, instead of 46. When an egg and a sperm join together during fertilization to form a zygote, the total number of 46 chromosomes is restored. The new cell contains a set of
genetic material from the mother and a set from the father.
Sperm
Sperm are highly specialised cells that carry the genetic material of the male.
The formation and maturation of sperm is triggered by follicle stimulating hormone (FSH), released from the pituitary gland. The testes produce sperm continually at a rate of about 100 000 000 per day from puberty to old age.
There are around 120 million sperm in every milliliter of semen.
genetic material from the mother and a set from the father.
Sperm
Sperm are highly specialised cells that carry the genetic material of the male.
The formation and maturation of sperm is triggered by follicle stimulating hormone (FSH), released from the pituitary gland. The testes produce sperm continually at a rate of about 100 000 000 per day from puberty to old age.
There are around 120 million sperm in every milliliter of semen.
A sperm cell has three main parts: the head, the body and the tail. In the head of the sperm is the nucleus which contains chromosomes (the genetic information from the father). The body contains many mitochondria which provide energy to the tail, which makes the movement possible driving the sperm forward and allowing it to swim towards the egg.
The sperm determines the sex of the future child.
Oogenesis and the Menstrual Cycle
Oogenesis or production of mature eggs is a cyclic process, which includes the ovarian and uterine cycles. Both cycles are controlled and triggered by a series of hormones. Oogenesis begins very early in the life of a human female, when she’s still a developing fetus. In fact, a human female is born with all the eggs she’ll ever have. However, the eggs aren’t quite finished, because cell division pauses early in the first half of meiosis. These eggs lie dormant from birth until puberty, at which time the monthly cycle of a woman’s hormones restarts the development of eggs)
Women usually produced one ovum per month during her reproductive life, from puberty to middle age (menopause). The two ovaries take it in turns to produce an ovum, and one ovary releases a mature female gamete every 28 - 35 days. The cycle of producing and releasing mature ova is called the menstrual cycle. During the cycle a series of hormones will:
Women usually produced one ovum per month during her reproductive life, from puberty to middle age (menopause). The two ovaries take it in turns to produce an ovum, and one ovary releases a mature female gamete every 28 - 35 days. The cycle of producing and releasing mature ova is called the menstrual cycle. During the cycle a series of hormones will:
- prepare the uterus to receive any fertilised ova
- control the development of mature ova
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Ova
Eggs or female sex cells are large round cells that, like sperm, also have 23 chromosomes. It does not move on its own, and it contains food reserve in the form of yolk to feed the zygote for the first few days after fertilisation. The egg is covered by a complex protective layer, the zona pellucida and the corona radiata, which the sperm will have to cross in order to fertilise it. |
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The ovarian cycle – hormones control the development of ova
Follicular phase (lasts around 14 days): The ova develop from cells lining the ovary. This is triggered by follicle stimulating hormone (FSH) released from the pituitary gland (in the brain). FSH stimulates the development of several follicles in the ovary at a time, but usually only one egg reaches full maturity. As the follicle matures it will produce a sac around itself. The developing ovum within the sac is called a Graafian follicle.
Ovulation (which means the release of a mature egg from the ovary’s surface): Once the Graafian follicle is matured, and there is a high concentration of oestrogen, it moves to the surface of the ovary and bursts, releasing the ovum into the funnel of the oviduct. The release of the egg is stimulated by the luteinising hormone (LH) released by the pituitary gland. During this phase body temperature increases about 0.5 ºC
Follicular phase (lasts around 14 days): The ova develop from cells lining the ovary. This is triggered by follicle stimulating hormone (FSH) released from the pituitary gland (in the brain). FSH stimulates the development of several follicles in the ovary at a time, but usually only one egg reaches full maturity. As the follicle matures it will produce a sac around itself. The developing ovum within the sac is called a Graafian follicle.
Ovulation (which means the release of a mature egg from the ovary’s surface): Once the Graafian follicle is matured, and there is a high concentration of oestrogen, it moves to the surface of the ovary and bursts, releasing the ovum into the funnel of the oviduct. The release of the egg is stimulated by the luteinising hormone (LH) released by the pituitary gland. During this phase body temperature increases about 0.5 ºC
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Luteal phase: LH will also stimulate the development of corpus luteum from the remains of the follicle, which produces the hormone progesterone. Progesterone keeps the lining of the uterus ready for implantation and pregnancy. It also prevents FSH secretion which prevents the release of any more mature ova by feedback inhibition. (This ensures that only one fertilised ovum develops in the uterus at any one time). If there is no implantation, after 10-12 days the corpus luteum decays stopping production of progesterone and initiating a new cycle.
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