Reproduction is the process of producing new individuals. In human reproduction the two sexes, male and female, each produce special types of reproductive cells, called gametes. The male gametes are the sperm (or spermatozoa) and the female gametes are the ova or eggs.

Ovum
Sperm

To produce a new individual, a sperm has to reach an ovum and fuse with it. The sperm nucleus then passes into the ovum and the two nuclei also fuse. This is fertilisation. The cell formed after the fertilisation of an ovum by a sperm is called a zygote. A zygote will grow by cell division to produce first an embryo and then a fully formed animal.

Fertilisation and development

In humans, the male produces millions of sperm, while the female produces a smaller number of eggs (usually one a month for about 40 years). Usually only one egg is fertilised at a time; two eggs being fertilised at the same time produces (non-identical) twins.

To bring the sperm close enough to the ova for fertilisation to take place, there is an act of mating or copulation. In mammals this act results in sperm from the male animal being injected into the female.

The sperm swim inside the female’s reproductive system and fertilise any eggs that are present. The zygote then grows into an embryo inside the body of the female.

The human reproductive system

Male

Table below summarises the functions of parts of the male reproductive system. Sperm are produced in the male reproductive organs, called the testes. The testes are held just outside the male body in the scrotum. Each testis produces both sperm cells and the hormone testosterone. Sperm cells are carried from the testes in the sperm ducts which transport them past the seminal vesicles and prostate glands. These glands together produce the seminal fluid in which the sperm cells travel. Seminal fluid containing sperm cells is known as semen. This fluid travels down the penis in the urethra and is transferred to the vagina during intercourse. The penis contains spongy tissue that can fill with blood during sexual arousal to cause and erection. Sperm production is a continuous process, which begins at puberty and continues throughout a man’s life.

The male reproductive organs side view
The male reproductive organs front view

The temperature in the scrotum is about 35C which is idle for sperm production

Role of testosterone

The hormone testosterone, which is produced by the testes, has important roles in the sexual development and reproductive behaviour in males.

  • During fetal development, testosterone cause the development of male genitalia.
  • At puberty, levels of testosterone rise and cause the development of male secondary sexual characteristics including growth of muscle, deepening of the voice, enlargement of the penis and growth of body hair.
  • Testosterone stimulates the continuous production of sperm and behaviour associated with the sex drive.

Functions of parts of the male reproductive system

PartFunction
epididymisa mass of tubes in which sperm are stored
peniscan become firm, to insert into the vagina of the female during sexual intercourse in order to transfer sperm
prostate glandadds fluid and nutrients to sperm to form semen
scrotuma sac that holds the testes outside the body, keeping them cooler than body temperature
seminal vesicleadds fluid and nutrients to sperm to form semen
sperm ductmuscular tube that links the testis to the urethra to allow the passage of semen containing sperm
testismale gonad that produces sperm
urethrapasses semen containing sperm through the penis, also carries urine from the bladder

Female

Table below summarises the function of parts of the female reproductive system. The female reproductive organs (gonads) produces ova and female sex hormones e.g. estrogen and progesterone. Female sex hormones are responsible for development and maintenance of secondary sexual characteristics. Mature eggs are released from the ovaries into the oviducts. The oviduct sometimes called the fallopian tube is a narrow muscular tube leading from the ovary to the uterus. The oviduct has a funnel like opening to make it easier for ova to enter the oviduct. Cilia on the inner lining help move the ovum to the uterus. The ovum is usually fertilised in the oviduct.

The female reproductive organs side view
The female reproductive organs front view

The uterus is a thick muscular organ that can stretch as the fetus increases in size during pregnancy. The smooth muscles in the uterine wall contract to expel the fetus during birth. The uterus is lined by a lining called the endometrium (uterine lining). The endometrium is richly supplied with blood vessels and is the site of implantation of the embryo post-fertilisation. It is broken down every month and flows out of the body in the process called menstruation. Cervix is a circular ring of muscle at the neck of the uterus. It opens into the vagina. It enlarges during birth to allow the passage of the fetus. Vagina is a thin-walled chamber where sperm is deposited during sexual intercourse. It forms the birth canal through which the baby is born.

Functions of parts of the female reproductive system

PartFunction
cervixa ring of muscle, separating the vagina from the uterus
funnel of oviductdirects an ovum (egg) from the ovary into the oviduct
ovarycontains follicles in which ova (eggs) are produced
oviductcarries an ovum to the uterus, with propulsion provided by tiny cilia in the wall; also the site of fertilisation
urethracarries urine from the bladder
uteruswhere the fetus develops
vaginareceives the male penis during sexual intercourse; sperm are deposited here

Production of gametes

Spermatogenesis

Spermatogenesis is the production of mature sperm cells (spermatozoa) in the testis. More than 100 million sperm cells are produced each day in a process that takes place in the narrow seminiferous tubules making up each testis.

Sperm production and development takes place from the outer part of the seminiferous tubules towards the central lumen, where sperm cells are eventually released. Each tubule is enclosed in a basement membrane beneath which is an outer layer of germinal epithelium cells. These diploid cells divide regularly by mitosis to produce more diploid cells which enlarge and are known as primary spermatocytes.

Primary spermatocytes divide by meiosis and their first division produces tow haploid cells. The second division of these two cells results in four spermatids.

Section through sperm-producing tubules
Human sperm (x800). The head of the sperm has a slightly different appearance when seen in side view or in top view

Developing sperm are attached to Sertoli cells, which are also called nurse cells. These large cells assist the differentiation of immature spermatids into spermatozoa and provide nourishment for them.

Spermatozoa that have developed their tails detach from the Sertoli cells and are carried down the lumen of the tubule to the epididymis of the testis.

Structure of the testis

Hormones and sperm production

Sperm production is controlled by three hormones – follicle-stimulating hormone (FSH) and luteinising hormone (LH) from the pituitary gland, and testosterone produced by testes.

  • FSH stimulates meiosis in spermatocytes, to produce haploid cells
  • Testosterone stimulates the maturation of secondary spermatocytes into mature sperm cells.
  • LH stimulates the secretion of testosterone by the testis

Epididymis, seminal vesicles and semen production

Sperm cells are stored and mature in the epididymis where they also develop the ability to swim. Sperm cells are released at ejaculation in a nutrient-rich-fluid known as semen. Semen is produced by two seminal vesicles and the prostate gland. It is mixed with the sperm cells as they leave the epididymis and move along the sperm duct. Fluid from the seminal vesicles makes up about 70% of semen. It is rich in fructose, which provides energy for the sperm cells to swim, and it also contains protective mucus. The prostate gland produces an alkaline fluid that helps the sperm cells to survive in the acidic conditions of the vagina.

Oogenesis

Oogenesis produces female gametes, the ova. Unlike spermatogenesis, which takes place in an adult male, oogenesis begins in the ovaries of a female when she is still a fetus. Oogonia the germinal epithelial cells within the ovaries of the female fetus, divide by mitosis to produce more diploid cells. These enlarge to form primary oocytes, which are also diploid. Primary oocytes undergo the first stages of meiosis but this stops during prophase 1 leaving the primary oocyte surrounded by a layer of follicle cells in a structure known as the primary follicle.

Development now ceases but the ovaries of a baby girl contain around 300000 primary follicles at birth. The remaining stages of oogenesis are shown in Figure below

Stages in the development of one follicle in a human ovary. The arrows show the sequence of events

At puberty, development of the primary follicles continues. During each menstrual cycle, a few follicles proceed to complete the first division of meiosis. Two haploid cells are produced but the cytoplasm divides unequally so that one cell is much larger than the other. The larger cell is known as the secondary oocyte and the smaller cell is the polar body. The polar body degenerates and does not develop further.

The secondary oocyte, protected within its follicle, begins meiosis II but stops in the prophase II. At the same time, the follicle cells divide and produce a fluid that causes the follicle to swell. At the point of ovulation, the follicle bursts, releasing the secondary oocyte, which floats towards the oviduct (fallopian tube). Although ovulation is often described as the release of the ovum, the cell that is released is in fact still a secondary oocyte.

After fertilisation the secondary oocyte completes meiosis II, becoming a mature ovum, and expels a second polar body, which degenerates. The empty follicle in the ovary develops to become the corpus luteum or yellow body, which produces the hormone progesterone.

Oogenesis and spermatogenesis compared

OogenesisSpermatogenesis
Similaritiesboth begin with production of cells by mitosis
in both cells grow before meiosis
in both two divisions of meiosis produce the haploid gamete
Differencesone secondary oocyte is produced per menstrual cyclemillions of sperm cells are produced continuously
only one large gamete is produced per meiosisfour small gametes are produced per meiosis
occurs in ovaries, which tend to alternate oocyte productionoccurs in testes, which both produce sperm cells
early stages occur during fetal developmentprocess begins at puberty
released at ovulation during the menstrual cyclereleased at ejaculation
ovulation ceases at menopausesperm production continues throughout an adult male’s life

Female sex hormones and the menstrual cycle

Ovaries produce two hormones, estrogen and progesterone. These hormones stimulate the development of female characteristics at puberty and also influence the changes in the uterus lining during the menstrual cycle and pregnancy. The pituitary gland in the brain produces two further hormones, luteinising hormone (LH) and follicle-stimulating hormone (FSH). FSH stimulates the development of immature follicles in the ovary, one of which will come to contain a mature egg cell. LH stimulates the follicle to release the egg and subsequently to form the corpus luteum.

The sequence of events begins at the start of menstruation which is often called a period. During the first four or five days of the cycle, the endometrium (lining) of the uterus is shed and leaves the body through the vagina. This indicates that fertilisation has not occurred during the previous month.

In this early part of the cycle, the pituitary gland secretes FSH, which stimulates the development of an immature follicle in the ovary. The follicle then secretes estrogen, which enhances the follicle’s response to FSH. As the level of estrogen rises, it also stimulates the repair of the uterus lining.

The menstrual cycle

As the follicle grows, estrogen levels rise to a peak around day 12 when they stimulate the release of LH from the pituitary gland. As LH levels reach their highest point, ovulation – the release of the egg cell from the follicle – takes place. Ovulation usually occurs at around the day 14 of the cycle. Immediately afterwards, LH stimulates the empty follicle to form the corpus luteum. Levels of estrogen begin to rise and as a result FSH and LH levels fall.

The corpus luteum secretes progesterone, which stimulates the thickening of the endometrium and prepares the uterus to receive an embryo. It also inhibits the production of FSH and LH.

If the egg cell is not fertilised, the LH level falls, the corpus luteum degenerates and progesterone and estrogen fall. The fall in progesterone stimulates the breakdown of the uterus lining. FSH is no longer inhibited, so a new follicle is stimulated and the cycle begins again.

Ovulation

The egg cells (ova) are present in the ovary from the time of birth. No more are formed during the female’s lifetime, but between the ages of 10 and 14 some of the egg cells start to mature and are released, one at a time about every 4 weeks from alternate ovaries. As each ovum matures, the cells around it divide rapidly and produce a fluid-filled sac. This sac is called a follicle and, when mature it projects from the surface of the ovary like a small blister. Finally, the follicle bursts and releases the ovum with its coating of cells into the funnel of the oviduct. This is called ovulation. From here, the ovum is wafted down the oviduct by the action of cilia in the lining of the tube. If the ovum meets sperm cells in the oviduct, it may be fertilised by one of them.

The released ovum is enclosed in a jelly-like coat called the zona pellucida and is still surrounded by a layer of follicle cells. Before fertilisation can occur, sperm have to get through this layer of cells and the successful sperm has to penetrate the zona pellucida with the aid of enzymes secreted by the head of the sperm.

Section through an ovary
Mature follicle as seen in section through part of an ovary (x30). The ovum is surrounded by follicle cells. These produce the fluid that occupies much of the space in the follicle

Mating and fertilisation

Mating

Sexual arousal in the male results in an erection. That is the penis becomes firm and erect as a result of blood flowing into the erectile tissue. Arousal in the female stimulates the lining of the vagina to produce mucus. This lubricates the vagina and makes it easy for the erect penis to enter.

In the act of copulation, the male inserts the penis into the female’s vagina. The sensory stimulus that this produces causes a reflex in the male, which results in the ejaculation of semen into the top of the vagina.

Fertilisation

The sperm swim through the cervix and into the uterus by wriggling movements of their tails. They pass through the uterus and enter the oviduct, but the method by which they do this is not known for certain. If there is an ovum in the oviduct, one of the sperm may bump into it and stick to its surface. The acrosome at the head of the sperm secretes enzymes which digest part of the egg membrane. The sperm then enters the cytoplasm of the ovum and the male nucleus of the sperm fuses with the female nucleus. This is the moment of fertilisation and is shown in more detail in Figure below.

a) Sperms swim towards ovum
b) Follicle scales are scattered, possibly by enzymes produced by sperms
c) One sperm gets through the zona pellucida
d) Sperm passes through cell membranes and enters cytoplasm
e) The sperm nucleus and egg nucleus fuse

Fertilisation of an ovum

Although a single ejaculation may contain over three hundred million sperm, only a few hundred will reach the oviduct and only one will fertilise the ovum. The released ovum is thought to survive for about 24 hours; the sperm might be able to fertilise an ovum for about two or three days. So there is only a short period of about 4 days each month when fertilisation might occur. If this fertile period can be estimated accurately, it can be used either to achieve or to avoid fertilisation (conception).

The fertilised egg has 23 chromosomes from the mother and 23 from the father, bringing its chromosome number to 46. It is called a zygote.

The stages of fertilisation

Pregnancy and development

The fertilised ovum (zygote) first divides into two cells. Each of these divides again, so producing four cells. The cells continue to divide in this way to produce a solid ball of cells, an early stage in the development of the embryo. This early embryo travels down the oviduct to the uterus. Here it sinks into the lining of the uterus, a process called implantation. The embryo continues to grow and produce new cells that form tissues and organs. After 8 weeks, when all the organs are formed, the embryo is called a fetus. One of the first organs to form is the heart, which pumps blood around the body of the embryo.

a) after 2 weeks
b) about 5 weeks
c) 8 weeks

Human embryo the first 8 weeks

As the embryo grows , the uterus enlarges to contain it. Inside the uterus enlarges to contain it. Inside the uterus the embryo becomes enclosed in a fluid-filled sac called the amnion or water sac, which protects it from damage and prevents unequal pressures from acting on it. The fluid is called amniotic fluid. The oxygen and food needed to keep the embryo alive and growing are obtained from the mother’s blood by means of a structure called the placenta.

HCG is excreted in the urine of a pregnant woman and it is this hormone that is detected in a pregnancy test with the use of monoclonal antibodies

Functions of placenta and umbilical cord

The blood vessels in the placenta are very close to the blood vessels in the uterus so that oxygen, glucose, amino acids and salts can pass from the mothers blood to the embryos blood see Figure below

The exchange of substances between the blood of the embryo and the mother

So the blood flowing in the umbilical vein from the placenta carries food and oxygen to be used by the living, growing tissues of the embryo. In a similar way, the carbon dioxide and urea in the embryo’s blood escape from the vessels in the placenta and are carried away by the mother’s blood in the uterus. In this way the embryo gets rid of its excretory products.

There is no direct communication between the mother’s blood system and that of the embryo. The exchange of substances takes place across the thin walls of the blood vessels. In this way, the mother’s blood pressure cannot damage the delicate vessels of the embryo and it is possible for the placenta to select the substances allowed to pass into the embryo’s blood. The placenta can prevent some harmful substances in the mother’s blood from reaching the embryo. It cannot prevent all of them, however: alcohol and nicotine can pass to the developing fetus. If the mother is a heroin addict, the baby can be born addicted to the drug.

Some pathogens such as the rubella virus and HIV can pass across the placenta. Rubella (German measles), although a mild infection for the mother, can infect the fetus and results in major health problems, including deafness, congenital heart disease, diabetes and mental retardation. HIV is potentially fatal.

The placenta produces hormones, including oestrogens and progesterone. It is assumed that these hormones play an important part in maintaining the pregnancy and preparing for birth, but their precise function is not known. They may influence the development and activity of the muscle layers in the wall of the uterus and prepare the mammary glands in the breasts for milk production.

Antenatal care

Antenatal or prenatal refers to the period before birth. Antenatal care is the way a woman should look after herself during pregnancy, so that the birth will be safe and her baby healthy.

The mother-to-be should make sure that she eats properly, and perhaps take more iron and folic acid than she usually does to prevent anaemia. If her job is a light one, she may go on working for the first 6 months of pregnancy. She should not do heavy work, however or repeated lifting or stooping.

Pregnant women who drink or smoke are more likely to have babies with low birth weights. These babies are more likely to be ill than babies of normal weight. Smoking may also make a miscarriage more likely. So a woman who smokes should give up smoking during her pregnancy. Alcohol can cross the placenta and damage the fetus. Pregnant women who take as little as one alcoholic drink a day are at risk of having babies with lower than average birth weights. These underweight babies are more likely to become ill.

Heavy drinking during pregnancy, sometimes called binge drinking can lead to deformed babies. This risk is particularly great in the early stages of pregnancy when the brain of the fetus is developing, and can result in a condition called fetal alcohol syndrome (FAS). At that stage the mother may not yet be aware of her pregnancy and continue to drink heavily. A child suffering from FAS can have a range of medical problems, many associated with permanent brain damage. All levels of drinking are thought to increase the risk of miscarriage.

During pregnancy, a woman should not take any drugs unless they are strictly necessary and prescribed by a doctor. In the 1950s, a drug called thalidomide was used to treat the bouts of early morning sickness that often occur in the first 3 months of pregnancy. Although tests had appeared to show the drug to be safe, it had not been tested on pregnant animals. About 20% of pregnant women who took thalidomide had babies with deformed or missing limbs.

If a woman catches rubella (German measles) during the first 4 months of pregnancy, there is danger that the virus may affect the fetus and cause abortion or still-birth. Even if the baby is born alive, the virus may have caused defects of the eyes (cataracts), ears (deafness) or nervous system. All girls should be vaccinated against rubella to make sure that their bodies contain antibodies to the disease.

Twins

Sometimes a woman releases two ova when she ovulates. If both ova are fertilised, they may form twin embryos, each with its own placenta and amnion. Because the twins come from two separate, ova, each fertilised by a different sperm, it is possible to have a boy and a girl. Twins formed in this way are called fraternal twins. Although they are both born within a few minutes of each other, they are no more alike than other brothers or sisters.

Another cause of twinning is when a single fertilised egg, during an early stage of cell division, forms two separate embryos. Sometimes these may share a placenta and amnion. Twins formed from a single ovum and sperm must be the same sex, because only one sperm (X or Y) fertilised the ovum. These ‘one-egg’ twins are sometimes called identical twins because unlike fraternal twins they will closely resemble each other in every respect.

Birth

The period from fertilisation to birth takes about 38 weeks in humans. This is called gestation period. A few weeks before the birth, the fetus has come to lie head downwards in the uterus, with its head just above the cervix.

Growth and development in the uterus

When birth starts, the uterus begins to contract rhythmically. This is the beginning of what is called labour. These regular rhythmic contractions become stronger and more frequent. The opening of the cervix gradually widens (dilates) enough to let the baby’s head pass through and the contractions of the muscles in the uterus wall are assisted by muscular contractions of the abdomen. The amniotic sac breaks at some stage in labour and the fluid escapes through the vagina. Finally, the muscular contractions of the uterus wall and abdomen push the baby head-first through the widened cervix and vagina. The umbical cord, which still connects the child to the placenta, is tied and cut. Later, the placenta breaks away from the uterus and is pushed out separately.

The sudden fall in temperature felt by the newly born baby stimulates it to take its first breath and it usually cries. In a few days, the remains of the umbilical cord attached to the baby’s abdomen shrivel and fall away, leaving a scar in the abdominal wall, called the navel.

Process of birth

Induced birth

Sometimes, when a pregnancy has lasted for more than 38 weeks or when examination shows that the placenta is not coping with the demands of the fetus, birth may be induced. This means that it is started artificially.

This is often done by carefully breaking the membrane of the amniotic sac. Another method is to inject a hormone, oxytocin, into the mother’s veins. Either of these methods bring on the start of labour. Sometimes both are used together.

Feeding and parental care

Within the first 24 hours after birth, the baby starts to suck at the breast. During pregnancy the mammary glands enlarge as a result of an increase in the number of milk-secreting cells. No milk is secreted during pregnancy, but the hormones that start the birth process also act on the milk-secreting cells of the breasts. The breasts are stimulated to release milk by the baby sucking the nipple. The continued production of milk is under the control of hormones, but the amount of milk produced is related to the quantity taken by the child during suckling.

Milk contains the proteins, fats, sugar, vitamins and salts that babies need for their energy requirements and tissue-building, but there is too little iron present for the manufacture of haemoglobin. All the iron needed for the first weeks or months is stored in the liver of the fetus during gestation.

The liquid produced in the first few days is called colostrum. It is sticky and yellow, and contains more protein than the milk produced later. It also contains some of the mother’s antibodies. This provides passive immunity to infection.

The mother’s milk supply increases with the demands of the baby, upto 1 litre per day. It is gradually supplemented and eventually replaced entirely by solid food, a process known as weaning.

Cow’s milk is not wholly suitable for human babies. It has more protein, sodium and phosphorus, and less sugar, vitamin A and vitamin C, than human milk. It is less easily digested than human milk. Manufacturers modify the components of dried cow’s milk to resemble human milk more closely and this makes it more acceptable if the mother cannot breastfeed her baby.

Cow’s milk and proprietary dried milk both lack human antibodies, whereas the mother’s milk contains antibodies to any diseases from which she has recovered. It also carries white cells that produce antibodies or ingest bacteria. These antibodies are important in defending the baby against infection at a time when its own immune responses are not fully developed. Breastfeeding provides milk free from bacteria, whereas bottle-feeding carries the risk of introducing bacteria that cause intestinal diseases. Breastfeeding also offers emotional and psychological benefits to both mother and baby.

Other advantages of breastfeeding over bottle-feeding include the following:

  • There is no risk of an allergic reaction to breast milk
  • Breast milk is produced at the correct temperature
  • There are no additives or preservatives in breast milk
  • Breast milk does not require sterilisation since there are not bacteria present that could cause intestinal disease.
  • There is no cost involved in using breast milk
  • Breast milk does not need to be prepared
  • Breastfeeding triggers a reduction in the size of the mother’s uterus

In vitro fertilisation

In vitro fertilisation (IVF) is a technique used to help couples who have been unable to conceive naturally. There are many reasons for infertility. Males may have a low sperm count, blocked or damaged sperm ducts or be unable to achieve an erection. Females may fail to ovulate or have blocked or damaged oviducts, or produce antibodies in cervical mucus that destroy sperm.

The first step in IVF treatment is an assessment of whether the couple are suitable for treatment. If so, the woman is injected with FSH for about 10 days. This hormone causes a number of egg cells to mature at the same time in her ovaries. Just before the egg cells are released from the follicles, they are collected using a laparoscope. The egg cells are matured in culture medium for up to 24 hours before sperm cells are added to fertilise them. Fertilised egg cells are incubated for about three days until they have divided to form a ball of cells. These embryos are checked to make sure they are healthy and developing normally. Either two or three will be selected and placed into the woman’s uterus for implantation. The pregnancy is then allowed to continue in the normal way. Any remaining embryos can be frozen and stored for use later.

Arguments for and against IVF treatment

Arguments in favour of IVFArguments against IVF
Enables infertile couples to have a familyUnused embryos produced by IVF are frozen for a limited period and then destroyed
Couples willing to undergo IVF treatment must have determination to become parentsEmbryos are selected for implantation so humans are choosing which should have the chance of life
Embryos used in IVF treatment can be screened to ensure they are healthy and do not have certain genetic conditions that would be inheritedMultiple births often result from IVF and this increases the risks of mother and babies
IVF techniques have led to further understanding of human reproductive biologyInfertility is a natural phenomenon whereas IVF is not and some religions object to it on this basis
Some causes of infertility are due to genetic conditions, which may be passed on to children born as a result of IVF