THE MECHANICS OF SEXUAL REPRODUCTION
Ejaculation
During intercourse, friction between the glans penis and the vaginal walls stimulates the nerve endings of the smooth muscle lining the male reproductive tract. When stimulation reaches a pre-ordained threshold, it triggers ejaculation. (This threshold differs from man to man, a result of inheritance, culture, taboos, training, etc.)
Ejaculation is a nervous reflex controlled by the spinal cord. It occurs in two parts:
1. Emission in which semen moves through the ejaculatory ducts (running through the prostate gland) and into the central tube of the penis the urethra
2. Ejaculation proper in which semen is propelled out of the urethra by contraction of pelvic muscles.
Contraction of muscles in the epididymis and vas deferens propel semen upwards towards the penis. Each vas deferens is the thickness of a pencil, yet the central channel running through it is only 0.25 to 0.33 mm in diameter the width of a coarse hair. The remaining thickness of the vas is composed of muscle, needed to milk the sperm up from the testes so quickly during ejaculation.
Sperm take a rather complicated route from the testes as a result of the testes descent through the abdomen during foetal development (see testicle descent). They pass up through the two vas deferens, over and behind the bladder, and into the ejaculatory ducts. From here, they pass into the urethra, where they are joined by secretions from the seminal vesicles and prostate gland.
The muscles surrounding the base of the penis, the bulbospongiosus (also called the bulbocavernosus) and the ischiocavernosus muscles constrict and help to propel semen through the penis. At the same time, the internal sphincter (valve) that closes off the neck of the bladder constricts, so sperm are directed out towards the tip of the penis, rather than upwards into the bladder. Retrograde ejaculation (where the sperm backtrack up into the bladder so nothing comes out of the end of the penis during ejaculation) is common after prostate operations, in which one of the bladder sphincters is often destroyed.
Figure 11: Orgasm
Orgasm
The first stage of orgasm, the excitement phase, occurs when stimuli from a number of sources (psychological, tactile, visual, olfactory, etc.) raise sexual interest and trigger an erection. (See erections.)
During sexual intercourse, increased amounts of the hormone adrenaline and its relative, noradrenaline (a neurotransmitter), are released from the adrenal glands. This increases the heart rate and the amount of blood pumped through the heart with each contraction. Palpitations may be felt and blood pressure rises. Breathing becomes rapid and flushing of the face and chest is common, along with perspiration. The nipples become erect and the scrotal skin thickens and contracts. The testes are reflexly drawn up towards the base of the penis and may increase in volume by as much as 50 per cent due to congestion with blood. The thickness of the penis at the coronal ridge of the glans increases to improve friction, and drops of lubricating fluid from Cowper's glands ooze out of the tip of the penis. These changes all occur during the plateau phase of orgasm, which can last from a few seconds to several minutes, even as long as an hour if sexual intercourse is deliberately prolonged. If the level of stimulation is inadequate, orgasm does not occur and sexual arousal will subside. If stimulation is adequate, the physical effects noted during the plateau phase become more intense and culminate in orgasm.
The orgasmic phase consists of an intensely pleasurable sensation variously described as emanating in the brain, the penis, the testicles or everywhere. It is accompanied in the male by a varying number of major muscle contractions (typically 38) followed by several smaller ones. Nerve impulses spread via the pudendal nerves and cause rhythmic, wave-like contractions of the pelvic floor muscles and sometimes of the thigh muscles as well. Contraction of muscles lining the reproductive tract propel sperm up from the testes and out through the penis. Male orgasms usually last from 310 seconds, and rarely longer than 15 seconds.
During orgasm, a number of brain chemicals are released: prolactin hormone, phenylethylamine (also found in chocolate) and endorphin. The latter two are addictive, and abstinence may result in cravings and mild depression.
Heart rate and blood pressure peak during orgasm, and hyperventilation is common. The rectal sphincter may contract and involuntary vocalizations occur as pleasurable sensations wash through the body.
After orgasm, a period of resolution follows when heart rate, blood pressure and genital blood flow gradually return to normal. Nerves that trigger relaxation of the muscles lining the reproductive tract come into play, and the arteries supplying blood to the penis close down. Muscle fibres lining the cavernous spaces in the corpus spongiosum and corpora cavernosa also contract, reducing the volume of blood that the spongy tissue can hold. This relieves the pressure on the outlet veins and maximizes venous drainage so that flaccidity soon occurs. The resolution phase occurs rapidly over the space of a few minutes, providing orgasm has occurred. If the plateau phase does not end in orgasm, resolution may take several hours. This results in pelvic congestion and heavy, dragging sensations in the loins, and testes which can be uncomfortable.
After a successful male orgasm, an absolute refractory period is seen, in which further orgasm is impossible. This is prob-ably related to the high levels of adrenaline flowing round the body. Inhibitory centres in the brain may be switched on as well. In young males, the refractory period is short, often only a few minutes, but in most males past middle age it lasts at least 20 minutes, and often longer. Interestingly, a new sexual partner may arouse interest enough to shorten the usual refractory period.
In females, there seems to be no refractory period, so multiple orgasms are possible. Female orgasms lasting up to a minute have also been claimed.
If ejaculation does not occur over a prolonged period, sperm start to build up inside the vas deferens. Some get broken down and reabsorbed while others dribble through the end of the vas deferens into the urethra and wash away unnoticed in the urine. Eventually, nature will take control and sperm past their sell-by date will be discharged via a nocturnal emission (wet dream).
Semen
Semen is made up of a solution of spermatozoa in seminal fluid. This is usually ejaculated in a set order. The first few drops of ejaculate tend to come from Cowper's lubricating glands. The next portion consists of prostatic secretions which are free of sperm and contain substances which give semen its characteristic smell. Then follows the sperm-rich secretions from the two epididymes which make up the middle part of the ejaculate. Finally, the last fraction of the ejaculate consists of the thick, viscous secretions from the seminal vesicle.
This order of ejaculation is not invariable, however, and reflex spasm in different parts of the male tract can result in semen being ejaculated 'out of order'. This doesn't seem to cause a problem, as fractions rapidly mix once inside the female tract.
As a general rule, the volume of ejaculate averages around 2.75 3.4 ml after three days' abstinence. The volume varies considerably both within and between individuals. After a prolonged ejaculatory abstinence, semen volume may increase dramatically to as much as 13 ml. Studies show that between 13 and 33 per cent of semen volume is derived from prostatic secretions, 4680 per cent comes from the seminal vesicles and around 10 per cent from the two epididymes. The ratio of prostatic to seminal secretions remains constant within each individual, regardless of the frequency of sexual activity.
Fresh semen is a thick, milky, turbid, white-yellow fluid with a slight opalescence. It is permeated with sticky, glass-like fibres and contains granules resembling sago or tapioca. Yellow pigments (flavines) derived from the seminal vesicles are often seen as coloured streaks.
Initially, semen is thick and clotted. It almost immediately coagulates due to a reaction between an enzyme from the prostatic secretions (proteinase, or clotting enzyme) acting on a sticky protein within the seminal vesicle secretions. The semen then forms a thick, gelatinous clot. This is thought to be an evolutionary remnant. Semen in many promiscuous lower animals clots to form a cervical plug. This effectively blocks the female cervix and prevents semen from another male impregnating the female.
In humans, other prostatic enzymes immediately start to break down proteins in the seminal clot to their amino acid constituents. Within 520 minutes after ejaculation, the semen liquefies again.
More than 32 different chemicals have been isolated from semen, including 24 amino acids, glucose, fructose, citric acid, vitamin C, vitamin B12, sulphur, zinc, potassium, magnesium, calcium, copper and several hormones. After ejaculation, male hormones are broken down by enzymes so the female is not exposed to excessive amounts.
Semen is rich in a number of hormone-like chemicals known as prostaglandins. The name is derived from the prostate gland, where they were first identified, but they are now known to be produced by most body tissues. Those present in semen are mainly secreted by the seminal vesicles.
Prostaglandins have a number of actions and are important in controlling inflammation within the body. Those present in the semen are thought to make the female cervix open and 'pout' so sperm can swim through more easily. It is also pos- sible that they make the female orgasm more intense, thereby triggering strong muscle contractions which help to suck sperm up through the female reproductive tract in eddy currents.
Sperm and the Female Reproductive Tract
Sperm cannot survive for long within the hostile, acid vagina usually for less than six hours. They need to swim into the protective alkaline cervical mucus to survive. Only 1 per cent manage this, with 99 per cent of semen being flushed from the vagina by leakage.
The cervical mucus is exceptionally sperm-friendly in the middle of the female's menstrual cycle, during her fertile phase. The molecules are then aligned in parallel so sperm can swim through with ease and form a reservoir with the thin, glistening and semi-liquid mucus.
During the first few days after intercourse, a constant stream of sperm swim up from the cervical mucus towards the Fallopian tubes and a possible descending egg. During in-vitro fertilization studies, sperm have been found within the Fallopian tubes within 3060 minutes after ejaculation. Some sperm are found within minutes, but they are often dead presumably exhausted.
During the second half of the menstrual cycle, and if the woman is using a hormonal method of contraception, cervical mucus is not sperm-friendly. The molecules within are entangled and the mucus is thick, sticky and scant. Sperm become trapped and cannot easily swim through or form a reservoir.
Sperm Capacitation
By the time sperm are ejaculated, most are fully mobile. Mature sperm recovered during ejaculation seem unable to fertilize an egg, however, and if they do so take several hours before this is even attempted. In contrast, sperm aspirated from the uterus or Fallopian tubes seem keen to attempt fertilization immediately they sense an egg.
The longer sperm remain in the female tract, the stickier they become, so they are better able to stick to the outside of the egg. This maturation process is known as capacitation and is probably triggered by female secretions. Sperm can also be capacitated by incubation with tissue fluids a technique which increases the chance of successful artificial insemination. During capacitation, proteins and zinc
(see zinc and sperm) coating the sperm are stripped off, which increases their fertilizing power.
Fertilization
Once a sperm senses an egg, it becomes activated. Three events occur during activation. First, the sac of enzymes at the sperm head (acrosome) swells and opens out to expose the enzymes within. This is known as the acrosome reaction. These enzymes at the sperm head will digest the egg coat and allow the sperm to drill a hole through.
Secondly, sperm tail movements change from a regular, undulating, wave-like motion to a vigorous whiplash motion that propels sperm forwards in jerky lurches. This seems to help penetration of the outer egg shell.
Thirdly, changes occur within the membrane surrounding the sperm head. These changes allow it to stick to the egg membrane and fuse with it, once it has dissolved its way through the outer shell. Fusion of the sperm and egg membranes is essential for the sperm nucleus to pass from the sperm head into the egg during fertilization.
The process of sperm activation needs to occur near the egg, as activation significantly shortens a sperm's lifespan. It is now thought that the egg releases chemicals which attract sperm towards it. Other egg chemicals seem to trigger sperm activation. The sperm- meets-egg scenario is therefore not as reliant on chance as previously thought.
Immediately after one sperm head successfully penetrates the egg cell, a minute electrical charge flashes through the egg membrane, triggering a chain reaction. This immediately hardens the egg membrane, so that no further sperm can stick to it or dissolve their way through.
Altogether, the time taken for a sperm to stick to the outer egg shell, penetrate and trigger the hardening reaction of the inner egg membrane is around 1020 minutes. Once a sperm successfully fertilizes an egg, it loses its tail, which remains outside the egg shell. The sperm nucleus then oozes from the sperm head into the egg cell and eventually fuses with the egg nucleus. A new individual is now on the long road to implantation and development. It is not until the fertilized egg has successfully implanted in the womb lining and started to create a placenta, however, that pregnancy can be said to have started.
Sperm and Gender of Offspring
When the number of chromosomes within each primary spermatocyte is split up during meiosis, so that the spermatids only receive half the usual number of chromosomes (23 instead of 46), one of the chromosome pairs that gets split up is unequal. This pair, known as the sex pair, consists of a fat X (female) chromosome and a smaller Y (male) chromosome. As a result of being split up, half a man's sperm will possess a Y-sex chromosome (but no X) and the other half will contain an X chromosome (but no Y).
In females, the sex pair is equal and consists of two X chromosomes. During meiosis in the female, all eggs therefore end up with one X chromosome each.
The Y chromosome provides all the genetic information needed for the development of male sexual characteristics. Without any input from a Y chromosome, a foetus will develop into a female. Therefore, if a sperm containing an X chromosome fertilizes an egg, the resultant offspring will possess an XX-sex pair and will develop into a female. If the egg is fertilized by a sperm containing a Y chromosome, the offspring will possess an XY-sex pair and will develop into a male.
It is always the sperm that dictate the sex of the offspring, not the egg.
Sperm containing the fatter X chromosome are slightly heavier and swim more slowly than sperm containing a Y chromosome. As the Y chromosome is comparatively light, Y sperm can swim faster. This small but significant difference between the X and Y sperm accounts for the fact that roughly 105 boys are born for every 100 girls.
Scientific methods designed to separate sperm into X and Y fractions use this difference in weight and speed. Sperm can be filtered through a test-tube containing a viscous solution of human albumin protein. The lighter and smaller male sperm tend to sink to the bottom of the tube more quickly than the larger, heavier female sperm. Fractions rich in X or Y sperm can then be separated out and used during artificial insemination to reduce the risk of sex-linked disorders in offspring, for example. The ethics of using these fractions to determine the sex of a child for aesthetic, family balancing purposes is still in question.
Newer methods of sperm separation involve labelling them with a fluorescent dye. This process is known as FISH Fluorescence In Situ Hybridization. Semen is then agitated to break it up into droplets, each one of which contains a single sperm. Drops containing the larger female sperm glow more brightly than drops containing a male sperm. Each drop is then electrically charged, with male sperm made positive and female sperm negative, and separated out using electrically charged plates. This method of sperm enrichment can produce samples containing 85 per cent female X sperm and 75 per cent male Y sperm, compared with the more usually 50:50 per cent mix.
At present this technique is only licensed for medical use for example to reduce the risk of having a child with an heredi- tary, sex-linked disorder such as Muscular Dystrophy which affects only boys and not for cosmetic (social) reasons.
Interestingly, it seems that divers are more likely to father daughters than sons. It was recently found that hyperbaric chambers significantly lower blood testosterone levels, and this seems to favour the female X sperm. Studies among Australian abalone divers and Swedish navy divers do show a preponderance of female offspring.
