So how do you tell if a man has normal fertility? What is normal semen? How is sperm produced? Sperm is the cell by which man shares in the process of fertilization. Millions of sperm dash forwards within every ejaculate to cross the path to the ovum inside the female’s tube (Fallopian tube). By the time they reach her, only one sperm out of these millions is still viable and strong enough to penetrate the ovum and fertilize her. The rest are rendered weak by the long path they have to cross and the chemical obstacles they face through out.
Sperm is made in the testis, a process called “spermatogenesis”, and exits the testis into the seminal track, that takes the sperm to the urethra (urine passage in the penis) from which it is ejaculated to the vagina. During its passage through the sperm track or seminal track, sperm meet the seminal fluid (semen is composed of sperm swimming in a fluid). This fluid is secreted from a gland attached to the seminal track and is called the “seminal vesicle”. Seminal fluid is not secreted from the testis. Sperm are.
For a sperm to be capable of fertilization it should have the following characteristics:
Normal Sperm, Normal Semen:
A normal sperm has a head, neck and tail. The head contains the genetic material and characteristics that mix with that of the ovum to form the baby (embryo). Above the head leys a bag containing a fluid. This fluid is a digesting enzyme that dissolves the external wall of the ovum to allow the sperm to penetrate it. The bag is called (acrosome) or (acrosomal sac). The fluid is called (acrosomal enzymes).
Sperm neck contains the energy factories (mitochondria) that produces ATP. ATP is the energy source upon which the tail depends for its side to side movement, that propels the sperm forwards.
As sperm make their way to the ovum inside the female’s body, they travel a very long distance and face a lot of lethal secretions that kill the majority and weaken the rest. It is therefore that a minimum of 20 million sperm per millimeter (ml) semen is present in every normal ejaculate. This allows a few sperm to eventually reach the ovum, and one of them may still retain enough power to penetrate her.
A minimum of 60% of sperm must be able to move. Movement is required for the sperm to reach the ovum. At least 25% of sperm should have the ability to move in a darting, high speed manner (forward progressive motility). This motility is necessary for the sperm to force itself into the ovum. Motility requires energy. Energy is extracted by the sperm neck from the sugary material (fructose) present in the seminal fluid and secreted by the seminal vesicles ( a gland that is present along the sperm track close to the prostate). It is the latter gland that secretes the seminal fluid, and not the testis. The testis produced sperm without its fluid.
Sperm has to have the bag of enzymes mentioned before (acrosomal cap) that is important to dissolve the outer wall of the ovum in preparation for sperm penetration into it.
Sperm should have a head containing normal genetic material in a normal for.
Sperm also needs a fluid to swim within (seminal fluid or seminal plasma) with adequate nutrients and other chemicals necessary for the sperm to perform its function (fructose, zinc…etc). This fluid should be free of harmful chemicals (reactive oxygen species), harmful organisms (bacteria) and harmful particles (Antisperm antibodies). Seminal fluid should have a normal viscosity. Otherwise, if it is too viscous, this will prevent sperm motility. Since this fluid is ejaculated in a coagulated form, it should contain liquefying enzymes that render it to a fluid state within 30 minutes at most following ejaculation. As mentioned before, this fluid is secreted mainly by the seminal vesicles (see later), and to a lesser extent by the prostate. Seminal fluid is white or grayish white, and should be differentiated from prosemen, which is a transparent fluid of low volume, emitted upon sexual excitation and not upon ejaculation.
Normally, semen is ejaculated at the very depth of the vagina adjacent to the cervix of the female, a point called (posterior fornix). Seminal fluid contains a substance called Prostaglandin (PG), that stimulates the uterus to contract, sucking the seminal fluid upwards. Semen and sperm move upwards across the cervix, uterus and into the Fallopian tubes.
The cervix is a major obstacle to sperm entry since it secretes the “cervical secretions” that are sometimes hostile to sperm. This is why IUI may be useful in bypassing the cervix in some cases.
In one of the two Fallopian tubes, one ovum is matured every month. Sperm reaching the tube meet the ovum. Due to the long pathway and the difficult obstacles they face, only few sperm reach the ovum despite millions have been ejaculated, and only very few among those are still strong enough to penetrate (fertilize) the ovum.
One of those sperm approaches the ovum and pours its acrosomal enzymes (see above) onto its wall, dissolving it. Once this happens, sperm starts moving in a vigorous manner (hyper activation) to drill its way into the ovum. Once one sperm is inside it, the ovum seals itself against entry of more sperm. The sperm that entered the ovum stops moving, and the genetic material in its head interacts with that of the ovum to form the nucleus of the embryo (upcoming baby).Fertilization is done.
The ovum makes its way through the Fallopian tube into the uterus where it implants itself. Meanwhile, the first cell of the embryo divides and increases in number continuously.
The Normal Testis
Normally, man has two testes, positioned in a skin bag (scrotum). Adult testes should have a length of at least 2.5 cm. Length can be determined by examination or by ultrasonography. The two testes may differ in size, but this should be within a narrow limit. If the difference is great, this may be a sign of disease. They may also differ in position, one being lower than the other, and this is normal as long as it is within narrow limits and as long both are in the scrotum and not in the wall of the abdomen or inside the abdomen.
Normally, the testes may move up and down sue to contraction and relaxation of a muscle blanket that is present underneath the scrotal skin and around the testes. When the muscle contracts, the scrotum becomes tight and looks smaller than usual, and the testes move up. This is normal as long as they do not leave the scrotum into the abdomen. If they do, this is a condition that may require treatment and is called “retractile testes”.
Being ovoid in shape rather than spherical, the testis has a long axis and a short axis. The long axis should be almost vertical rather than horizontal. If this is not the case, the testis may be predisposed to a condition called (torsion of the testis).
Normally, the testis are smooth in contour. Any bumps or humps should be examined by ultrasonography to determine their nature.
Normally, the testis is slightly painful upon moderate pressure. Exaggeration or loss of this feeling is a sign of disease.
The testis hangs from the body by a stalk called “Spermatic Cord”. This is a collection of arteries, veins, nerves, lymphatic vessels and a duct. The duct is the “vas deferens”, a tube that takes sperm from the testis to the urethra in the penis. The arteries feed the testis with blood containing nutrients, oxygen and hormones. The veins take blood away from the testis, carrying waste products such as carbon dioxide, heat..etc. Lymphatic vessels carry away the excess fluid.
Among the possible diseases that affect the cord are:
Absence of the vas deferens
Varicocele (affecting the veins)
Hernia: is a condition where the abdominal viscera (such as intestine) or abdominal fluid make their way into the scrotum through the abdominal wall, next to contents of the spermatic cord.
The testis is normally surrounded by several layers between which a thin layer of fluid accumulates. If this layer increases in volume, it leads to enlargement of the scrotum (hydrocele)
The “epididymis” is a storage space for sperm right after they leave the testis and before they pass into the vas deferens. The epididymis is C-shaped and leys over the posterior surface of the testis. Its upper most part is called the “head”, middle part is the “body”, and lowest part is the “tail” that connects to the vas deferens.
The epididymis is not just a storage space, but it is also a site for maturation of sperm, and is where sperm acquire their motility.
After sperm is made in the testis, they assume the following pathway to the outside:
Sperm are stored in the epididymis for several days, where they acquire motility and maturation. It has an upper head, middle body, and lower tail.
Sperm ( not semen fluid) pass in a tube called the vas deferens. Is a 45cm long tube present on both sides. Though it is 1-2 mm thick, its internal lumen (opening) is much more narrow as its walls are thick. It conveys sperm from the testis and passes up the scrotum (scrotal vas), within the layers of the abdominal muscles (inguinal vas), into the depth of the pelvis (pelvic vas)
The vas ends in the prostate (ejaculatory duct) by joining a the urethra (urine track that passes into the penis). The right vas, left vas and urethra unite at this very point.
Before the vas enters the prostate, it is joined by the seminal vesicle, a gland that secretes the seminal fluid accounting for 80% of semen volume, regardless sperm content.
Hormonal Control of Sperm and Semen Production
FSH (Follicle Stimulating Hormone) is the hormone that directly stimulates the testis to produce sperm. It is secreted by the pituitary gland present in the brain. This hormone is present in both males and females. In the female, it stimulates the ovary to produce the follicle that contains the ovum.
The pituitary gland is sensitive to the activity of the testis. If activity is low, it raises the FSH level to stimulate the testis further to increase sperm in semen. Therefore, an FSH level higher than normal is an indication of low activity of the testis, which is why the pituitary raised the FSH level. High FSH level is the reason for infertility or low testicular activity, it is only a result and a sign of the low activity of the testis.
The pituitary also produces LH (Leutinizing Hormone), that stimulates the testis to produce the male sex hormone: Testosterone. Testosterone is responsible for sexual activity, masculinity, and to some extent for fertility and semen volume, among many other functions. Again, pituitary is sensitive to testosterone level and can regulate LH level accordingly.
Pituitary in turn is stimulated by a brain area called “hypothalamus”, that secretes the hormone “GnRH“ that controls the activity of the pituitary.