Spermatozoon Health Dictionary

A sexual or germ cell: for example, an OVUM or SPERMATOZOON.... gamete

Those embryonic cells with the potential to develop into ova (see OVUM) or spermatozoa (see SPERMATOZOON).... germ cell

See SPERMATOZOON.... sperm

The general term for any one of a group of HORMONES which govern the development of the sexual organs and the secondary sexual characteristics of the male. TESTOSTERONE, the androgenic hormone formed in the interstitial cells of the testis (see TESTICLE), controls the development and maintenance of the male sex organs and secondary sex characteristics. In small doses it increases the number of spermatozoa (see SPERMATOZOON) produced, but in large doses it inhibits the gonadotrophic activity of the anterior PITUITARY GLAND and suppresses the formation of the spermatozoa. It is both androgenic and anabolic in action. The anabolic e?ect includes the ability to stimulate protein synthesis and to diminish the catabolism of amino acids, and this is associated with retention of nitrogen, potassium, phosphorus and calcium. Doses in excess of 10 mg daily to the female may produce VIRILISM.

Unconjugated testosterone is rarely used clinically because its derivatives have a more powerful and prolonged e?ect, and because testosterone itself requires implantation into the subcutaneous fat using a trocar and cannula for maximum therapeutic bene?t. Testosterone propionate is prepared in an oily solution, as it is insoluble in water; it is e?ective for three days and is therefore administered intramuscularly twice weekly. Testosterone phenyl-propionate is a long-acting microcrystalline preparation which, when given by intramuscular or subcutaneous injection, is e?ective for four weeks. Testosterone enantate is another long-acting intramuscular preparation. Mesterolone is an e?ective oral androgen and is less hepatoxic: it does not inhibit pituitary gonadotrophic production and hence spermatogenesis is unimpaired. Testosterone undecanoate is also an e?ective oral form.... androgen

The condition characterised by lack of spermatozoa (see SPERMATOZOON) in the SEMEN.... azoospermia

Cloning – from the Greek klon, meaning a cutting such as is used to propagate plants – is essentially a form of asexual reproduction. The initial stages were ?rst successfully achieved in rabbits. In essence the technique consists of destroying the nucleus of the egg and replacing it with the nucleus from a body cell of the same species – either a male or a female. This provides the egg with a full complement of CHROMOSOMES and it starts to divide and grow just as it would if it had retained its nucleus and been fertilised with a spermatozoon. The vital di?erence is that the embryo resulting from this cloning process owes nothing genetically to the female egg. It is identical in every respect with the animal from which the introduced nucleus was obtained.

In 1997 the ?rst mammal to be cloned from the tissue of an adult animal was born. A technique that scientists have been trying to perfect for decades, the success of the Roslin Institute, near Edinburgh, in producing ‘Dolly’, a cloned sheep, has profound implications. Already some scientists are talking of cloning humans, although this has great medical, legal and ethical consequences. The key to the scientists’ success in producing Dolly was the ability to coordinate the fusion of a donor cell (from an adult) containing all its DNA with a recipient egg from which DNA had been removed. The di?culty of the technique is shown by the fact that, out of 277 fused pairs of cells where the donor cell was from adult tissue, Dolly was the only survivor and she has developed premature arthritis. Research suggests that cloning may be accompanied by a higher than normal incidence of congenital defects.

Since Dolly was born, other animal clones have been produced and American researchers have cloned the ?rst human embryo – which grew to six cells – with the aim of providing stem cells for therapeutic use. As a result the UK government passed emergency legislation to outlaw human cloning for reproductive purposes.... cloning

Conception signi?es the complex set of changes which occur in the OVUM and in the body of the mother at the beginning of pregnancy. The precise moment of conception is that at which the male element, or spermatozoon, and the female element, or ovum, fuse together. Only one-third of these conceptions survive to birth, whilst 15 per cent are cut short by spontaneous abortion or stillbirth. The remainder – over one-half – are lost very early during pregnancy without trace. (See also FETUS.)... conception

The study of the growth and development of an EMBRYO and subsequently the FETUS from the fertilisation of the OVUM by the SPERMATOZOON through the gestational period until birth. Embryology is valuable in the understanding of adult anatomy, how the body works and the occurrence of CONGENITAL deformities.... embryology

Gonadotrophins, or gonadotrophic hormones, are hormones that control the activity of the gonads (i.e. the testes and ovaries). In the male they stimulate the secretion of TESTOSTERONE and the production of spermatozoa (see SPERMATOZOON); in the female they stimulate the production of ova (see OVUM) and the secretion of OESTROGENS and PROGESTERONE. There are two gonadotrophins produced by the PITUITARY GLAND. CHORIONIC GONADOTROPHIC HORMONE is produced in the PLACENTA and excreted in the urine.... gonadotrophins

A method of helping CONCEPTION to occur when a man is infertile (see INFERTILITY) because his sperm (see SPERMATOZOON) cannot penetrate either the cervical mucus at the entrance of the UTERUS or the barriers that surround the OVUM. The sperm, often treated chemically beforehand to increase motility, are injected directly into the uterus via the VAGINA.... intrauterine insemination

Twins who develop from a single OVUM fertilised by a single SPERMATOZOON. Also known as identical or uniovular twins (see MULTIPLE BIRTHS).... monozygotic twins

A less-than-normal number of sperm (see SPERMATOZOON) present in each unit volume of seminal ?uid (each ml of semen usually contains 20 million sperm). The condition may be permanent or temporary and is a major cause of INFERTILITY in men. It may be caused by ORCHITIS, an undescended testis, or VARICOCELE, and should be investigated.... oligospermia

A means of avoiding pregnancy despite sexual activity. There is no ideal contraceptive, and the choice of method depends on balancing considerations of safety, e?ectiveness and acceptability. The best choice for any couple will depend on their ages and personal circumstances and may well vary with time. Contraceptive techniques can be classi?ed in various ways, but one of the most useful is into ‘barrier’ and ‘non-barrier’ methods.

Barrier methods These involve a physical barrier which prevents sperm (see SPERMATOZOON) from reaching the cervix (see CERVIX UTERI). Barrier methods reduce the risk of spreading sexually transmitted diseases, and the sheath is the best protection against HIV infection (see AIDS/HIV) for sexually active people. The e?ciency of barrier methods is improved if they are used in conjunction with a spermicidal foam or jelly, but care is needed to ensure that the preparation chosen does not damage the rubber barrier or cause an allergic reaction in the users. CONDOM OR SHEATH This is the most commonly used barrier contraceptive. It consists of a rubber sheath which is placed over the erect penis before intromission and removed after ejaculation. The failure rate, if properly used, is about 4 per cent. DIAPHRAGM OR CAP A rubber dome that is inserted into the vagina before intercourse and ?ts snugly over the cervix. It should be used with an appropriate spermicide and is removed six hours after intercourse. A woman must be measured to ensure that she is supplied with the correct size of diaphragm, and the ?t should be checked annually or after more than about 7 lbs. change in weight. The failure rate, if properly used, is about 2 per cent.

Non-barrier methods These do not provide a physical barrier between sperm and cervix and so do not protect against sexually transmitted diseases, including HIV. COITUS INTERRUPTUS This involves the man’s withdrawing his penis from the vagina before ejaculation. Because some sperm may leak before full ejaculation, the method is not very reliable. SAFE PERIOD This involves avoiding intercourse around the time when the woman ovulates and is at risk of pregnancy. The safe times can be predicted using temperature charts to identify the rise in temperature before ovulation, or by careful assessment of the quality of the cervical mucus. This method works best if the woman has regular menstrual cycles. If used carefully it can be very e?ective but requires a highly disciplined couple to succeed. It is approved by the Catholic church.

SPERMICIDAL GELS, CREAMS, PESSARIES, ETC.

These are supposed to prevent pregnancy by killing sperm before they reach the cervix, but they are unreliable and should be used only in conjunction with a barrier method.

INTRAUTERINE CONTRACEPTIVE DEVICE (COIL) This is a small metal or plastic shape, placed inside the uterus, which prevents pregnancy by disrupting implantation. Some people regard it as a form of abortion, so it is not acceptable to all religious groups. There is a risk of pelvic infection and eventual infertility in women who have used coils, and in many countries their use has declined substantially. Coils must be inserted by a specially trained health worker, but once in place they permit intercourse at any time with no prior planning. Increased pain and bleeding may be caused during menstruation. If severe, such symptoms may indicate that the coil is incorrectly sited, and that its position should be checked. HORMONAL METHODS Steroid hormones have dominated contraceptive developments during the past 40 years, with more than 200 million women worldwide taking or having taken ‘the pill’. In the past 20 years, new developments have included modifying existing methods and devising more e?ective ways of delivering the drugs, such as implants and hormone-releasing devices in the uterus. Established hormonal contraception includes the combined oestrogen and progesterone and progesterone-only contraceptive pills, as well as longer-acting depot preparations. They modify the woman’s hormonal environment and prevent pregnancy by disrupting various stages of the menstrual cycle, especially ovulation. The combined oestrogen and progesterone pills are very e?ective and are the most popular form of contraception. Biphasic and triphasic pills contain di?erent quantities of oestrogen and progesterone taken in two or three phases of the menstrual cycle. A wide range of preparations is available and the British National Formulary contains details of the commonly used varieties.

The main side-e?ect is an increased risk of cardiovascular disease. The lowest possible dose of oestrogen should be used, and many preparations are phasic, with the dose of oestrogen varying with the time of the cycle. The progesterone-only, or ‘mini’, pill does not contain any oestrogen and must be taken at the same time every day. It is not as e?ective as the combined pill, but failure rates of less than 1-per-100 woman years can be achieved. It has few serious side-effects, but may cause menstrual irregularities. It is suitable for use by mothers who are breast feeding.

Depot preparations include intramuscular injections, subcutaneous implants, and intravaginal rings. They are useful in cases where the woman cannot be relied on to take a pill regularly but needs e?ective contraception. Their main side-e?ect is their prolonged action, which means that users cannot suddenly decide that they would like to become pregnant. Skin patches containing a contraceptive that is absorbed through the skin have recently been launched.

HORMONAL CONTRACEPTION FOR MEN There is a growing demand by men worldwide for hormonal contraception. Development of a ‘male pill’, however, has been slow because of the potentially dangerous side-effects of using high doses of TESTOSTERONE (the male hormone) to suppress spermatogenesis. Progress in research to develop a suitable ANDROGEN-based combination product is promising, including the possibility of long-term STEROID implants. STERILISATION See also STERILISATION – Reproductive sterilisation. The operation is easier and safer to perform on men than on women. Although sterilisation can sometimes be reversed, this cannot be guaranteed and couples should be counselled in advance that the method is irreversible. There is a small but definite failure rate with sterilisation, and this should also be made clear before the operation is performed. POSTCOITAL CONTRACEPTION Also known as emergency contraception or the ‘morning after pill’, postcoital contraception can be e?ected by two di?erent hormonal methods. Levonorgesterol (a synthetic hormone similar to the natural female sex hormone PROGESTERONE) can be used alone, with one pill being taken within 72 hours of unprotected intercourse, but preferably as soon as possible, and a second one 12 hours after the ?rst. Alternatively, a combined preparation comprising ETHINYLESTRADIOL and levonorgesterol can be taken, also within 72 hours of unprotected intercourse. The single constituent pill has fewer side-effects than the combined version. Neither version should be taken by women with severe liver disease or acute PORPHYRIAS, but the ethinylestradiol/levonorgesterol combination is unsuitable for women with a history of THROMBOSIS.

In the UK the law allows women over the age of 16 to buy the morning-after pill ‘over the counter’ from a registered pharmacist.... contraception

The name given to the unborn child after the eighth week of development. Humans, like all animals, begin as a single cell, the OVUM, in the ovary (see OVARIES). After FERTILISATION with a SPERMATOZOON, the ovum becomes embedded in the mucous membrane of the UTERUS, its covering being known as the decidua. Increase in size is rapid, and development of complexity is still more marked. The original cell divides repeatedly to form new cells, and these become arranged in three layers known as the ectoderm, mesoderm and endoderm. The ?rst produces the skin, brain and spinal cord, and the nerves; the second the bones, muscles, blood vessels and connective tissues; while the third develops into the lining of the digestive system and the various glands attached to it.

The embryo develops upon one side of the ovum, its ?rst appearance consisting of a groove, the edges of which grow up and join to form a tube, which in turn develops into the brain and spinal cord. At the same time, a part of the ovum beneath this is becoming pinched o? to form the body, and within this the endoderm forms a second tube, which in time is changed in shape and lengthened to form the digestive canal. From the gut there grows out very early a process called the allantois, which attaches itself to the wall of the uterus, developing into the PLACENTA (afterbirth), a structure well supplied with blood vessels which draws nourishment from the mother’s circulation via the wall of the womb.

The remainder of the ovum – which within two weeks of conception has increased to about 2 mm (1/12 inch) in size – splits into an outer and inner shell, from the outer of which are developed two covering membranes, the chorion and amnion; while the inner constitutes the yolk sac, attached by a pedicle to the developing gut of the embryo. From two weeks after conception onwards, the various organs and limbs appear and grow. The human embryo at this stage is almost indistinguishable in appearance from the embryo of other animals. After around the middle of the second month, it begins to show a distinctly human form and then is called the fetus. The property of ‘life’ is present from the very beginning, although the movements of the fetus are not usually felt by the mother until the ?fth month.

During the ?rst few days after conception the eye begins to be formed, beginning as a cup-shaped outgrowth from the mid-brain, its lens being formed as a thickening in the skin. It is very soon followed by the beginnings of the nose and ear, both of which arise as pits on the surface, which increase in complexity and are joined by nerves that grow outward from the brain. These three organs of sense have practically their ?nal appearance as early as the beginning of the second month.

The body closes in from behind forwards, the sides growing forwards from the spinal region. In the neck, the growth takes the form of ?ve arches, similar to those which bear gills in ?shes. From the ?rst of these the lower jaw is formed; from the second the hyoid bone, all the arches uniting, and the gaps between them closing up by the end of the second month. At this time the head and neck have assumed quite a human appearance.

The digestive canal begins as a simple tube running from end to end of the embryo, but it grows in length and becomes twisted in various directions to form the stomach and bowels. The lungs and the liver arise from this tube as two little buds, which quickly increase in size and complexity. The kidneys also appear very early, but go through several changes before their ?nal form is reached.

The genital organs appear late. The swellings, which form the ovary in the female and the testicle (or testis) in the male, are produced in the region of the loins, and gradually descend to their ?nal positions. The external genitals are similar in the two sexes till the end of the third month, and the sex is not clearly distinguishable till late in the fourth month.

The blood vessels appear in the ovum even before the embryo. The heart, originally double, forms as a dilatation upon the arteries which later produce the aorta. These two hearts later fuse into one.

The limbs appear at about the end of the third week, as buds which increase quickly in length and split at their ends into ?ve parts, for ?ngers or toes. The bones at ?rst are formed of cartilage, in which true bone begins to appear during the third month. The average period of human gestation is 266 days – or 280 days from the ?rst day of the last menstrual period. The average birth weight of an infant born of a healthy mother (in the UK) is 3,200 g (see table).

The following table gives the average size and weight of the fetus at di?erent periods:

(See also PREGNANCY AND LABOUR.)... fetus

Intracytoplasmic sperm injection (ICSI) is the most signi?cant therapeutic advance in male INFERTILITY treatment in the past 30 years. The technique is used when in vitro fertilisation (IVF – see under ASSISTED CONCEPTION) is not possible because the man has very few, motile, normal sperm (see SPERMATOZOON), or when previous attempts at IVF have not produced a fertilised EMBRYO. ICSI requires a single sperm which is injected directly into the cytoplasm of an egg previously retrieved from the woman. Once fertilised, the embryo is transferred to her UTERUS. For men with no sperm in the semen, it may be possible to retrieve sperm by needle aspiration of the EPIDIDYMIS under local anaesthetic (see ANAESTHESIA). Other techniques involve microsurgical retrieval from the epididymis or TESTICLE under a general anaesthetic. Potential complications include scrotal pain, bruising, HAEMATOMA formation and infection. ICSI and surgical sperm-retrieval require extensive training and expertise and is currently available in only a few selected

infertility units. Safety concerns relate to a higher-than-expected rate of abnormalities in the SEX CHROMOSOMES after ICSI, and also the potential risk of transmitting paternal genetic defects in the Y chromosome to sons born after ICSI.... intracytoplasmic sperm injection

The fertilising chamber where the ovum is fertilised by the spermatozoon.... ootype

The single cell derived from the female, out of which a future individual arises, after its union with the SPERMATOZOON derived from the male. It is about 35 micrometres in diameter. (See FETUS; OVARIES.)... ovum

Non-sexual reproduction. In other words, development of the OVUM into an individual without fertilisation by a SPERMATOZOON. It is common in plants and has been produced in animals experimentally.... parthenogenesis

The pouch of skin and ?brous tissue, positioned outside the abdomen behind the root of the PENIS, within which the testicles (see TESTICLE) are suspended. It consists of a purse-like fold of skin, within which each testicle has a separate investment of muscle ?bres, several layers of ?brous tissue, and a serous membrane known as the tunica vaginalis. The extra-abdominal site means that the production and storage of sperm (see SPERMATOZOON) in the testicles is at a lower temperature than internal body heat. Temperature control is facilitated by contraction and relaxation of the scrotal muscles.... scrotum

Meiosis, or reduction division, is the form of cell division that only occurs in the gonads (see GONAD) – that is, the testis (see TESTICLE) and the ovary (see OVARIES) – giving rise to the germ cells (gametes) of the sperms (see SPERMATOZOON) and the ova (see OVUM).

Two types of sperm cells are produced: one contains 22 autosomes and a Y sex chromosome (see SEX CHROMOSOMES); the other, 22 autosomes and an X sex chromosome. All the ova, however, produced by normal meiosis have 22 autosomes and an X sex chromosome.

Two divisions of the NUCLEUS occur (see also CELLS) and only one division of the chromosomes, so that the number of chromosomes in the ova and sperms is half that of the somatic cells. Each chromosome pair divides so that the gametes receive only one member of each pair. The number of chromosomes is restored to full complement at fertilisation so that the zygote has a complete set, each chromosome from the nucleus of the sperm pairing up with its corresponding partner from the ovum.

The ?rst stage of meiosis involves the pairing of homologous chromosomes which join together and synapse lengthwise. The chromosomes then become doubled by splitting along their length and the chromatids so formed are held together by centromeres. As the homologous chromosomes – one of which has come from the mother, and the other from the father – are lying together, genetic interchange can take place between the chromatids and in this way new combinations of GENES arise. All four chromatids are closely interwoven and recombination may take place between any maternal or any paternal chromatids. This process is known as crossing over or recombination. After this period of interchange, homologous chromosomes move apart, one to each pole of the nucleus. The cell then divides and the nucleus of each new cell now contains 23 and not 46 chromosomes. The second meiotic division then occurs, the centromeres divide and the chromatids move apart to opposite poles of the nucleus so there are still 23 chromosomes in each of the daughter nuclei so formed. The cell divides again so that there are four gametes, each containing a half number (haploid) set of chromosomes. However, owing to the recombination or crossing over, the genetic material is not identical with either parent or with other spermatozoa.... meiosis

Twins occur about once in 80 pregnancies, triplets once in 6,000, and quadruplets about once in 500,000. Quintuplets are exceedingly rare. Such is the natural state of a?airs.

In recent years, however, the position has been altered by the introduction of the so-called fertility drugs, such as CLOMIPHENE, and human menopausal gonadotrophin which, through the medium of the PITUITARY GLAND, stimulate the production of ova (see OVUM). Their wide use in the treatment of INFERTILITY has resulted in an increase in the number of multiple births, a recognised hazard of giving too large a dose.

Twins may be binovular or uniovular. Binovular, or fraternal, twins are the result of the mother’s releasing two ova within a few days of each other and both being fertilised by separate spermatozoa (see SPERMATOZOON). They both develop separately in the mother’s womb and are no more alike than is usual with members of the same family. They are three times as common as uniovular, or identical, twins, who are developed from a single ovum fertilised by a single spermatozoon, but which has split early in development. This is why they are usually so remarkably alike in looks and mental characteristics. Unlike binovular twins, who may be of the same or di?erent sex, they are always of the same sex.

So far as fraternal, or binovular, twins are concerned, multiple pregnancy may be an inherited tendency; it certainly occurs more often in certain families, but this may be partly due to chance. A woman who has already given birth to twins is ten times more likely to have another multiple pregnancy than one who has not previously had twins. The statistical chance of a third pair of twins is 1:512,000. Identical twins do not run in families.

The relative proportion of twins of each type varies in di?erent races. Identical twins have much the same frequency all over the world: around 3 per 1,000 maternities. Fraternal twins are rare in Mongolian races: less than 3 per 1,000 maternities. In Caucasians they occur two or three times as often as identical twins: between 7 (Spain and Portugal) and 10 (Czech and Slovak Republics and Greece) per 1,000 maternities. They are more common in Afro-Caribbeans, reaching 30 per 1,000 maternities in certain West African populations.

Rarely, uniovular twins may not develop as separate individuals, being physically joined in some way. They are called conjoined or (traditionally) Siamese twins. Depending on the extent of common structures shared by the infants – this ranges from a common umbilical cord to twins with conjoined heads or a common liver – the infants may be successfully separated by surgery. (See CONJOINED TWINS.)

Parents of twins, triplets, etc. can obtain advice and help from the Twins and Multiple Births Association (TAMBA).... multiple births

The production of mature sperm (see SPERMATOZOON) in the testis (see TESTICLE). The sperm cells originate from the outermost layer of the seminiferous tubules in the testis: these multiply throughout reproductive life and are transformed into mature spermatozoa, a process that takes up to 80 days.... spermatogenesis

See SPERMATOZOON.... spermatozoa

A narrow tube that leads from each testis (see TESTICLE) through the PROSTATE GLAND to join a tube from the seminal vesicles to form the ejaculatory duct. Sperm (see SPERMATOZOON) and seminal ?uid pass through this duct during ejaculation.... vas deferens

This is the cell produced when an OVUM is fertilised by a SPERMATOZOON. A zygote contains all the hereditary material for a new individual: half comes from the sperm and half from the ovum. After passing down the Fallopian tube, when the zygote starts dividing, it becomes implanted in the UTERUS and develops into an EMBRYO.... zygote

n. the caplike structure on the front end of a spermatozoon. It breaks down just before fertilization (the acrosome reaction), releasing a number of enzymes that assist penetration between the follicle cells that still surround the ovum. Failure of the acrosome reaction is a cause of male infertility (see also andrology).... acrosome

Stem CELLS develop a few days after an egg (ovum) is fertilised by a spermatozoon and starts developing to form an EMBRYO. These master cells are crucial to the development of a normal embryo. They contain a specialised ENZYME that gives them the facility to divide inde?nitely, developing into the many di?erent specialised cells that comprise the various tissues in the body – for example, skin, blood, muscle, glands or nerves.

In a highly signi?cant advance in research, a scienti?c team in the United States obtained stem cells from newly formed human embryos

– donated by women who had become pregnant after successful in vitro fertilisation – and successfully cultivated these cells in the laboratory. This achievement opened the way to replicating in the laboratory, the various specialised cells that develop naturally in the body. UK government legislation constrains the use of human embryos in research (see ETHICS) and the ethical aspects of taking this stem-cell culture technique forwards will have to be resolved. Nevertheless, this discovery points the biological way to the use of genetic engineering in selecting di?erentiated specialised cells from which replacement tissues could be grown for use as transplants to rectify absent or damaged tissues in the human body.

Research into potential use of stem cells has raised expectations that in the long term they may prove to be an e?ective regenerative treatment for a wide range of disorders including PARKINSONISM, ALZHEIMER’S DISEASE, type-2 diabetes (see under DIABETES MELLITUS), myocardial infarction (see HEART, DISEASES OF), severe burns, osteoporosis (see under BONE, DISORDERS OF) and the regeneration of blood to replace the need for BONE MARROW TRANSPLANT. Recent research has shown that adult stem cells may also be stimulated to produce new cell lines. If successful, this would eliminate the need to use embryos and thus resolve existing ethical dilemmas over the use of stem cells.... stem cell

Every man has two testicles or testes which are the sexual glands. In the fetus, they develop in the abdomen, but before birth they descend into a fold or pouch of skin known as the SCROTUM. Each testicle consists of up to 1,000 minute tubes lined by cells from which the spermatozoa (see SPERMATOZOON) are formed. Around 4·5 million spermatozoa are produced per gram of testicle per day. These tubes communicate with one another near the centre of the testicle, and are connected by a much coiled tube, the EPIDIDYMIS, with the ductus, or VAS DEFERENS, which enters the abdomen and passes on to the base of the bladder. This duct, after joining a reservoir known as the seminal vesicle, opens, close to the duct from the other side of the body, into the URETHRA where it passes through the PROSTATE GLAND. Owing to the convolutions of these ducts leading from the testicles to the urethra, and their indirect route, the passage from testicle to urethra is over 6 metres (20 feet) in length. In addition to producing spermotozoa, the testicle also forms the hormone TESTOSTERONE which is responsible for the development of male characteristics.... testicle

n. the fusion of a spermatozoon and an ovum. Rapid changes in the membrane of the ovum prevent other spermatozoa from penetrating. Penetration stimulates the completion of meiosis and the formation of the second polar body. Once the male and female pronuclei have fused the zygote starts to divide by cleavage.... fertilization

(polyspermia) n. fertilization of a single ovum by more than one spermatozoon: the development is abnormal and the embryo dies.... polyspermy

n. (pl. pronuclei) the nucleus of either the ovum or spermatozoon after fertilization but before the fusion of nuclear material. The pronuclei are larger than the normal nucleus and have a diffuse appearance.... pronucleus

the thick membrane that develops around the mammalian oocyte within the ovarian follicle. It is penetrated by at least one spermatozoon at fertilization and persists around the *blastocyst until it reaches the uterus. See ovum.... zona pellucida