Sperm are manufactured in thread-like tubules that fill the two testes. These tubules, called seminiferous tubules, are lined with several layers of cells. Over a period of about three months, sperm cells in the layer closest to the tubule wall migrate to the tubule’s central passageway, called the lumen. These round. immature sperm cells gradually lengthen as they approach the lumen, developing the typical tadpole appearance of mature sperm. The head of the sperm contains the male genetic material, which will enter the egg and join the female genetic material during fertilization to form an embryo. The midpiece of the sperm provides the energy needed by the tail to propel the sperm forward. Results of treatment given to improve fertility may not be apparent for at least three months, the time from the beginning of sperm formation to the ejaculation of mature sperm.
THE EFFECT OF HORMONES ON SPERM PRODUCTION
Hormones initiate sperm production. The pituitary gland, located at the base of the brain, releases follicle stimulating hormone (FSH) and luteinizing hormone (LH). The primary action of FSH is to stimulate the cells in the seminiferous tubules to produce sperm. The primary action of LH is to stimulate the Leydig cells to produce testosterone, which is necessary for sperm production and development. These and other hormones work together to ensure that adequate amounts of healthy sperm are produced.
Outside the seminiferous tubules in the interstitium are the Leydig, cells. These cells produce the male hormone testosterone, which is responsible for the development of such masculine physical characteristics as body and facial hair. large muscles, and a deep voice. Testosterone also helps stimulate the sex drive and potency, or the ability to achieve and maintain an erection. Leydig cells almost always function well even if sperm production is poor.
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 disappeared.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).
Fertilization and Implantation:
Before ovulation occurs, the fimbria (finger-like ends) of the fallopian tube are positioned over the ovary. These fimbria move along the ovary until a follicle containing the ripened egg is found. The lining of a fallopian tube contains millions of hairlike projections, called cilia, that move back and forth creating suction. When ovulation occurs this suction sweeps up the egg, along with some of the fluid that has surrounded the egg in the follicle. The chemicals in the fluid signal the muscles of the fallopian tube to contract. These contractions propel the egg gently toward the uterus. After the egg is in the fallopian tube, it continues to ripen and prepare itself for the sperm. The egg is surrounded by a nourishing shell which the sperm must penetrate. Millions of sperm work hard to penetrate the shell, layer by layer. One sperm breaks through the shell and instantly a chemical reaction takes place, making it impossible for any other sperm to penetrate. The sperm tail, which has been so vital in propelling it to this point, is left outside and disintegrates. The new single cell that results is called a zygote and it forms a thick wall to prevent penetration by any other sperm. Pregnancy has begun.
The zygote now begins to divide into further cells, called blastomeres, which by the third day number about 12. This tiny cluster of new life then takes about 60 hours to make its way to the uterus, by which term it is made up of about 64 cells and is called a blastocyst. Already there are two distinct cell types: an outer layer of trophoblast cells which will develop into the placenta; and an inner cell mass which will eventually form the foetus. Two to three days later (about a week after fertilisaiton) the blastocyst embeds itself in the lining of the uterus. It has now subdivided into about 120 cells and starts to produce the hormone human chorionic gonadotrophin (HCG) , which sends a signal to the corpus luteum to carry on producing progesterone. If it didn’t , the lining of the uterus would break down and menstrual bleeding would start.
Stages of Implantation
In the second week after conception, the trophoblast cells continue to invade the uterine lining and the inner cell mass develops into an embryo. It is only a dot, but has already started to be differentiated into three different cell layers, the germ layers, which will each become a different part of the baby’s body.