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Male Genital Tract
The male genital tract includes the testes (or testicles), the epididymis, the vas, the prostate and the urethra. The testes lie within the scrotum. The testes are mainly made up of long loops of fine tubes called seminiferous tubules in which the sperm are produced. The end of each seminiferous tubule is attached to the epididymis which lies next to the testis. The newly formed sperm pass out of the testis into the epididymis where they are stored and complete their development into mature sperm. The vas is a hollow tube which carries the sperm from the epididymis to the urethra. The prostate is a small gland that lies just below the bladder and fits around the urethra rather like a collar. The seminal vesicles lie on either side of the prostate. Fluids produced by the prostate and seminal vesicles form part of the ejaculated semen. The urethra is a muscular tube through which sperm and urine pass out of the body. The sperm enter the urethra at the time of ejaculation.
Sperm are produced within the testes (testicles) in long loops of fine tubes called seminiferous tubules. The development of the sperm is controlled by chemical 'messengers' called hormones. These hormones are produced by the pituitary gland in the brain. The pituitary releases two hormones - luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH stimulates the production and release of testosterone from the testis. FSH in conjunction with testosterone stimulates sperm production. Once the sperm are formed, they are stored in the epididymis where they mature.
Female Genital Tract
The female genital tract includes the ovaries, Fallopian tubes, uterus (womb), cervix, vagina and vulva. The ovaries lie at each end of the Fallopian tubes. Each month an egg is produced by one of the ovaries. The egg lies within a fluid-filled sac called a follicle, which bursts releasing the egg into the Fallopian tube. The Fallopian tubes lie on either side of the uterus and connect the uterus to the ovaries. The egg is then fertilized by the sperm in the part of the Fallopian tube closest to the ovary. The fertilized egg then passes down the tube to the uterus. The uterus (womb) is the organ in which the baby will develop. The inner lining of the uterus (endometrium) becomes thicker in preparation to receive the fertilized egg. It is this lining that is shed during the menstrual period. The cervix (the neck of the womb) lies at the top of the vagina. The vagina is the canal which lies between the vulva and the cervix. The vulva are the fleshy folds that surround the entrance to the vagina.
Development of the egg
Eggs are produced within the ovary. The egg is attached to the side wall of a fluid-filled sac called a follicle. The development of the follicles is controlled by chemical 'messengers' called hormones. These hormones are produced by the pituitary gland in the brain. At the beginning of the menstrual cycle, the pituitary releases follicle-stimulating hormone (FSH), which stimulates the ovary to produce follicles. One of these follicles grows faster than the others and is known as the 'dominant follicle'. It is from this follicle that the egg will be released. The ovaries also produce hormones, one of which is called estrogen. As the follicle(s) develops, the level of estrogen produced by the ovaries rises. This stimulates the pituitary to release luteinizing hormone (LH), which in turn stimulates further growth of the dominant follicle. Within 24 hours of the level of LH reaching its maximum, the follicle bursts releasing the egg. When the egg is released, it is swept into the Fallopian tube. The egg then begins to move slowly down the tube to be fertilized in the outer third of the Fallopian tube, closest to the ovary. Once the follicle has released the egg, it becomes the corpus luteum. The corpus luteum secretes the hormone progesterone, which prepares the lining of the uterus (the endometrium) for implantation of the egg.
Fertilization and Development
Fertilization is the joining of the egg and the sperm in the outer third of the Fallopian tube closest to the ovary. It can occur only when an egg, released by the ovary, meets sperm which have swum up through the cervix and uterus into the Fallopian tube. Fertilization normally occurs within 36 hours of the release of the egg. As the sperm approaches the egg, the cap covering the head of the sperm begins to dissolve. This allows the head of the sperm to penetrate the outer coat of the egg. As the sperm penetrates the egg, the tail is left behind (the tail is needed only to allow the sperm to move). Only one sperm will successfully fertilize the egg.
Once fertilized, the egg, which is now called a zygote, moves down the Fallopian tube towards the uterus. As it does so, the zygote begins to grow rapidly and, by the time it reaches the uterus 2-3 days later, it is called an embryo. The embryo then imbeds itself into the wall of the uterus and a pregnancy is achieved. This occurs about 7 days after fertilization.