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THE REPRODUCTION SYSTEM

The sperm are manufactured in the testicles which lie within the scrotum. Sperm production requires a slightly lower temperature than the rest of the body, this is possibly the reason why testes hang in the scrotum.

 

The testes are made up of long loops of fine tubes (called seminiferous tubules). Sperm production begins with immature sperm cells that grow and mature within the seminiferous tubules. The production of sperm begins at puberty and continues throughout a man's life into old age - the production of sperm is stimulated by the hormone FSH. It takes about 70 days for the sperm to mature. The testes also contain Leydig cells which produce the male hormone testosterone and Sertoli cells which mature the immature sperm.

Each seminiferous tubule ends in a structure called the epididymis, which lies above the testis. The epididymis is a single tube, highly coiled, and approximately 5 meters long. The sperm are stored in the epididymis for up to two weeks where they mature and acquire motility.

The vas deferens is a hollow tube that connects the epididymis to the urethra. During ejaculation, contractions of the vas propel the sperm into the urethra.

The urethra is a muscular pipe that connects the bladder to the outside via the penis, through which urine and sperm pass out of the body. The sperm enter the urethra at the time of ejaculation. Semen and urine never mix in a healthy man because the bladder sphincter contracts during orgasm thus, closing down the exit from the bladder to the urethra.

The prostate gland is a small gland that lies below the bladder and fits around the urethra.

The seminal vesicles (two reservoirs that connect to the vas deference) lie on either side of the prostate.

Fluids from the prostate gland (60% of the seminal fluid volume) and seminal vesicles (30% of the seminal fluid volume) form part of the seminal fluid, in which sperm are bathed. The seminal fluid contains chemicals and nutrients that are essential to the sperm health.

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The natural conception

Where does the conception take place under natural conditions?

The natural conception takes place when the sperm cells, after intercourse, swim up through the neck of the uterus and into the uterine tubes where they meet the egg and fertilise it. After the fertilisation of the egg in the uterine tube the egg cleaves, and after a couple of days the fertilised egg has moved down though the uterine tube and into the uterus. In the uterus the egg adheres to the endometrium and develops into a baby. This process is illustrated below:

In order for this process to take place it is important that the woman produces mature eggs, has an ovulation, has passage through the uterine tubes, and that the man sperm quality is all right.

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The interplay of the hormones and their influence on the fertility

In the ovaries the egg is matured and fully developed prior to the ovulation. The egg is caught by the uterine tube, which leads the egg to the uterus. If the egg is fertilised, it will adhere to the endometrium in the uterus and further develop. See below:

This process is steered by the hormones. If you know something about the influence of the hormones, it will be easier to understand the examinations and treatments you are going through.

Most of the superior sex hormones are formed in the pituitary gland, which is a gland in the brain. The hormone is transported via the blood to its destination (the organ), where a reaction takes place and a signal is sent back to the pituitary gland. Illustrated below:

Hormones UK Menstruation cycle UK

The individual hormones are briefly described:

GnRH-Gonadotropin releasing hormone. GnHR causes release of FSH and LH in the pituitary gland.

FSH (Follicle stimulating hormone) is formed in the pituitary gland and stimulates the growth of the follicles, which mature the eggs.

LH-Luteinizing hormone ?? is formed in the pituitary gland and causes ovulation.

Prolactine is formed in the pituitary gland and is in interplay with the breasts and the ovaries. Prolactine causes growth of the mammary gland during pregnancy and stimulate the milk production after childbirth. A too high level of prolactine can restrain the ovulation.

Estradiol is formed in the ovaries. During a menstrual cycle estradiol causes growth of the en endometrium in uterus. A high level of estradiol in the middle of the cycle results in an increase of LH, which leads to ovulation.

Progesterone. During the menstruationprogesterone together with estradiol prepare the endometrium in uterus to receive the fertilised eggs. During a pregnancy progesterone restrains the contractions of the uterus.

Androgens are formed in the ovaries and stimulate growth of the hair and the sexual instinct. An overproduction of androgens restrains ovulation.

HCG (human chorion gonadotropin) is formed in the placenta and prevents that the follicles are destroyed during pregnancy. We analyse this hormone to find out if you are pregnant.

During the menstruation the ovaries contain several small follicles, each of them containing an egg. The pituitary gland releases the hormone FSH, which leads to development of one single follicle to be approximately 20 mm in size. When the follicle is mature, the ovulating hormone LH is released.

The hormones are interdependent, and for some hormones the rule is that an increase in one hormone results in a decrease in another.

Here you can see how the follicles and the hormones develops throughout a normal menstruation cycle:


Transferring one IVF embryo, followed by a second, is almost as likely to result in a pregnancy as transferring two at a time.

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34 Egg development 78

When a girl is born she has two ovaries containing about two million immature eggs. But, by the time the girl reaches puberty and starts menstruating, many of the eggs that she started off with, will have

Each month an egg is usually produced by one of the ovaries. Before the egg is shed from the ovary (ovulation) it must be matured. The egg is matured inside a fluid filled sac called a follicle. The egg is surrounded by granulosa cells and fluid, these cells produce the female hormone estrogen.  The development of the follicles is controlled by chemical messages called hormones, which are produced by the pituitary gland. At the beginning of menstruation, the pituitary gland releases follicle stimulating hormone (FSH) into the bloodstream to stimulate a selected group of immature follicles to grow.  One of these follicles grows faster than the others and is called the dominant follicle, it is from this follicle that the egg will be released. The other follicles undergo follicular atresia i.e. shrink in size and disappear. As the dominant follicle grows, its granulosa cells produce an increasingly large quantity of the hormone estrogen that prepares the lining of the womb (known as the endometrium) to receive an embryo.

When the hormone estrogen level reaches its peak, the hypothalamus will send a chemical message (releasing hormones) to the pituitary gland to slow the production of FSH and trigger the release of a hormone called luteinizing hormone (LH). The ovary will detect this, and ovulation is set in motion. The follicle usually ruptures about 24 hours after the level of LH reaches its maximum. The follicle grows to a diameter of about an inch before it bursts and releases the egg. When the egg is released, it is swept into the Fallopian tube. 

     

Another effect of the Luteinizing hormone is to convert the granulosa cells to luteal cells. Once the follicle has released the egg, it collapses and becomes the corpus luteum. The corpus luteum secretes estrogen and progesterone in steadily increasing amounts; and about seven days after ovulation the progesterone level reaches its peak. The progesterone prepares the endometrium to receive the fertilized egg. The endometrium becomes thickened, (a maximum thickness of 8 to 12 millimeters) and more glandular (the endometrial cells secretes a fluid rich in nutrients). Ovulation usually precedes menstruation by about two weeks.

In most months, conception will not occur, and after about 10 days of ovulation, the corpus luteum gradually degenerates and the progesterone production will start to fall, this will result in cessation of growth and shedding of the endometrium (menstruation).

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Fertilization (Fertilisation)

The term fertilization describes the joining of the egg and sperm. It occurs in the outer third of the Fallopian tube near the ovary. It usually occurs within 36 hours of ovulation.

How do the sperm reach the Fallopian tube?

An average man ejaculates anything between 100 to 800 million sperm into the vagina. Some of these sperm trickle out of the vagina. Most just remain inside the vagina, where they rapidly die because of the vaginal acidity. A small proportion of sperm find their way into the cervix (the neck of the womb) which lies at the top of the vagina. Once sperm have entered the mucus, the majority will stay there; the cervical mucus acts as a reservoir. Over the next few days, a few sperm will swim up through the mucus and into the cavity of the womb. The other function of the cervical mucus is to increase the ability of the sperm to penetrate and fertilize the egg (this is known as sperm capacitation). The release of sperm from the mucus will continue until the mucus becomes thick. From the cavity of the womb the sperm will travel to the Fallopian tube. It is possible that the contraction of the womb acting like a pump may help in this transport mechanism. Sperm can be found in the Fallopian tube about 15 minutes after having been deposited into the vagina.

Of the millions of sperm in the ejaculate, only a few hundred will make the trip to the egg successively. As the sperm reaches the egg, the shell which covers the head of the sperm begins to dissolve; this will allow the sperm to penetrate the outer coat of the egg (known as the zona pellucida). As the sperm enters the egg its tail is left behind. As soon as one sperm enters the egg, the egg produces a chemical barrier that prevents any other sperm, of the thousands surrounding it, from penetrating the egg.

Although one sperm is required for fertilization, the enzymes from many sperm are required to breakdown the zona pellucida to allow the fertilizing sperm to penetrate the egg.

Once the egg is fertilized, it is known as a zygote.

Zygote - two pronucleate embryo

The zygote moves down the Fallopian tube towards the uterus by a combination of rhythmic contractions of the muscular wall, and the action of the cilia. The tube also supplies foodstuffs to the developing early embryo and removes waste products. The fertilized egg divides into two cells, each of these divide into two more and so on. The division occurs at intervals of about 15-18 hours. By the time it enters the uterus it has about 64 cells.

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Embryo implantation

Implantation of the embryo is the attachment of the embryo to the endometrium (lining of the womb). It occurs about a week after fertilization; and is a very delicate and complex process.

Once the embryo reaches the uterus, it may float around for 2-3 days, still growing. It does not stick to the lining of the uterus until about 7 days after fertilization when the process of implantation starts. By this time the embryo, now known as the blastocyst, will have two distinct types of cells: one type will form the afterbirth (known as the placenta) and the other will form the fetus. The embryo begins burrowing into the endometrium and the placenta starts to form - a pregnancy is achieved. At this stage, the placenta starts to produce the pregnancy hormone (human chorionic gonadotropin - hCG).

This hCG will continue to stimulate the corpus luteum so that progesterone production is maintained and the pregnancy can continue. The corpus luteum is functional for the first 8 weeks of pregnancy then the progesterone production is taken over by the placenta.

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The most common place for the blastocyst to implant is at the top and back of the womb.

Not all embryos implant and not all implanted embryos will  further develop to fetuses, and not all fetuses end in healthy babies. Sometimes adverse outcomes can result, such as miscarriages and ectopic pregnancy. However, the majority of couples will end in having healthy babies.

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