Reproductive System | ICSE Class 10 Biology Notes

This note covers the human reproductive system: the main reproductive events, the male reproductive organs and accessory glands, the female reproductive organs, secondary sexual characters, the structure of the sperm and the egg, the menstrual cycle with menarche and menopause, fertilisation and implantation, the placenta and its functions, the foetal membranes and amniotic fluid, gestation, parturition and lactation, the roles of testosterone, oestrogen, progesterone and prolactin, and identical and fraternal twins.

What are the main events of human reproduction?

Humans reproduce sexually and are viviparous: the young one develops inside the mother's body and is born as a baby.

In sexual reproduction, germ-cells from two individuals join together. The male germ-cell or gamete is the sperm, and the female gamete is the ovum (egg).

The reproductive events in humans follow this order:

1. Gametogenesis: formation of sperms in the male and ova in the female.
2. Insemination: transfer of sperms into the female genital tract.
3. Fertilisation: fusion of a male and a female gamete, forming a zygote.
4. Formation of a blastocyst from the zygote, and its attachment to the uterine wall (implantation).
5. Gestation: embryonic development inside the uterus.
6. Parturition: delivery of the baby.

These events take place only after puberty, the period during adolescence when, as general body growth begins to slow down, the reproductive tissues mature.

There are remarkable differences between the sexes. Sperm formation continues even in old men, but the release of ova stops in women at around the age of fifty years.

Primary and accessory sex organs

The primary sex organs produce the gametes: the testes in the male and the ovaries in the female. Both also secrete sex hormones.

The ducts, glands and external genitalia are the accessory parts. They store and carry the gametes, add secretions, allow the gametes to meet and, in the female, support the growing baby.

What are the male reproductive organs and their functions?

The male reproductive system is located in the pelvis region. It includes a pair of testes, accessory ducts, accessory glands and the external genitalia.

Testes and scrotum

The testes lie outside the abdominal cavity in a pouch called the scrotum. The scrotum keeps the testes at a temperature 2 to 2.5 °C lower than the normal internal body temperature, which sperm formation needs.

In adults, each testis is oval, about 4 to 5 cm long and about 2 to 3 cm wide, and is covered by a dense covering.

Each testis has about 250 compartments called testicular lobules. Each lobule contains one to three highly coiled seminiferous tubules, in which sperms are produced.

Each seminiferous tubule is lined on its inside by two types of cells: male germ cells (spermatogonia), which divide to form sperms, and Sertoli cells, which provide nutrition to the germ cells.

The spaces outside the tubules, the interstitial spaces, contain small blood vessels and Leydig cells (interstitial cells). Leydig cells secrete the male hormones called androgens, mainly testosterone.

The testis therefore has two functions: it produces sperms, and it secretes testosterone.

Accessory ducts

The male accessory ducts store sperms and carry them from the testis to the outside. Sperms travel along this route:

1. The seminiferous tubules open into the rete testis.
2. From the rete testis, sperms pass into the vasa efferentia, which leave the testis.
3. The vasa efferentia open into the epididymis, which lies along the posterior surface of each testis.
4. The epididymis leads to the vas deferens, which ascends into the abdomen and loops over the urinary bladder.
5. The vas deferens receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct.
6. The urethra runs from the urinary bladder through the penis to its external opening, the urethral meatus.

The urethra is a common passage for both sperms and urine.

Penis

The penis is the male external genitalia. It is made of special tissue that helps in its erection, which makes insemination possible. Its enlarged end, the glans penis, is covered by a loose fold of skin called the foreskin.

A front view with part of one testis cut open. The labels are ureter, urinary bladder, seminal vesicle, prostate, bulbourethral gland, urethra, vas deferens, epididymis, vasa efferentia, rete testis, testicular lobules, testis, glans penis and foreskin. The vas deferens runs up from the epididymis and over the bladder.

See Fig. 2.1b in your NCERT textbook

Accessory glands and semen

The male accessory glands are a pair of seminal vesicles, a single prostate and a pair of bulbourethral glands.

Their secretions make up the seminal plasma, which is rich in fructose, calcium and certain enzymes. The bulbourethral secretion also lubricates the penis.

The fluid added by the prostate and seminal vesicles makes the transport of sperms easier and provides nutrition. The fructose is a source of energy for the sperms.

Secretions of the epididymis, vas deferens, seminal vesicle and prostate are essential for the maturation and motility of sperms.

Seminal plasma together with the sperms is called semen. A human male ejaculates about 200 to 300 million sperms during a coitus.

For normal fertility, at least 60 per cent of the sperms must have normal shape and size, and at least 40 per cent must show vigorous motility.

What are the female reproductive organs and their functions?

The female reproductive system lies in the pelvic region. It consists of a pair of ovaries, a pair of oviducts, the uterus, cervix, vagina and external genitalia.

These parts, along with a pair of mammary glands, work together to support ovulation, fertilisation, pregnancy, birth and child care.

Ovaries

The ovaries are the primary female sex organs. They produce the female gamete (ovum) and several steroid hormones, the ovarian hormones oestrogen and progesterone.

The ovaries lie one on each side of the lower abdomen. Each is about 2 to 4 cm long and is connected to the pelvic wall and the uterus by ligaments.

Each ovary is covered by a thin epithelium that encloses the ovarian stroma, which has an outer cortex and an inner medulla. Ovarian follicles at different stages of development lie in the stroma.

When a girl is born, her ovaries already contain thousands of immature eggs. From puberty, some of them start maturing, and one egg is usually released every month by one of the ovaries.

Fallopian tubes (oviducts)

Each fallopian tube is about 10 to 12 cm long and runs from the edge of an ovary to the uterus. It has three parts:

• Infundibulum: the funnel-shaped part nearest the ovary. Its edges bear finger-like projections, the fimbriae, which help to collect the ovum after ovulation.
• Ampulla: the wider middle part, where fertilisation takes place.
• Isthmus: the last part, with a narrow lumen, which joins the uterus.

Uterus, cervix and vagina

The uterus (womb) is single and shaped like an inverted pear. It is supported by ligaments attached to the pelvic wall. The embryo implants and grows in it.

The uterus opens into the vagina through a narrow cervix. The cavity of the cervix, the cervical canal, together with the vagina forms the birth canal.

The wall of the uterus has three layers:

• Perimetrium: the thin outer membrane.
• Myometrium: the thick middle layer of smooth muscle, which contracts strongly during delivery of the baby.
• Endometrium: the inner glandular layer lining the uterine cavity, which undergoes cyclical changes during the menstrual cycle.

The vagina receives the semen during coitus, and the menstrual flow passes out through it.

External genitalia

The female external genitalia are the mons pubis (a cushion of fatty tissue covered by skin and pubic hair), the labia majora (fleshy folds around the vaginal opening), the labia minora (paired folds under the labia majora), the hymen and the clitoris.

A front sectional view. The uterus is shown with its uterine fundus, uterine cavity and three wall layers (endometrium, myometrium, perimetrium) labelled. A fallopian tube runs out on each side, bracketed into isthmus, ampulla and infundibulum, ending in fimbriae beside an ovary. Below are the cervix, cervical canal and vagina.

See Fig. 2.3b in your NCERT textbook

What are secondary sexual characters?

Definition: Secondary sexual characters are the features that appear at puberty and distinguish a mature male from a mature female, but are not themselves involved in producing gametes.

In the early teenage years, a whole new set of changes occurs that cannot be explained simply as the body getting larger. Body proportions change and new features appear.

Some changes are common to boys and girls. Thick hair grows in new places such as the armpits and the genital area, and may become darker. Thinner hair can appear on the legs, arms and face. The skin often becomes oily and pimples may develop.

Other changes differ between the sexes:

• In girls: breast size begins to increase, the skin of the nipples darkens, and menstruation begins.
• In boys: thick hair grows on the face, the voice begins to crack, and the penis occasionally becomes enlarged and erect, during daydreams or at night.

These changes happen slowly, over months and years. They do not all happen at the same time in one person, nor at an exact age. In some people they come early and quickly; in others, slowly.

Many of these changes, such as new hair-growth patterns, are signals that sexual maturation is taking place. The mammary glands are one of the female secondary sexual characteristics.

The changes are caused by the sex hormones: testosterone in boys and oestrogen in girls.

Androgens stimulate muscular growth, the growth of facial and axillary (armpit) hair, aggressiveness and a low pitch of voice. Oestrogen brings about female features such as a high pitch of voice and the development of the mammary glands.

What is the structure of a sperm and an egg?

How are the gametes formed?

Spermatogenesis takes place in the seminiferous tubules and begins at puberty. Diploid spermatogonia (46 chromosomes) multiply by mitosis; some become primary spermatocytes, and each undergoes meiosis to give four haploid spermatids (23 chromosomes), which change into sperms.

Oogenesis, the formation of the ovum, begins before birth. A couple of million gamete mother cells (oogonia) form in each fetal ovary, and no more are added after birth.

These cells enter the first meiotic division and stop at an early stage, as primary oocytes. Each primary oocyte becomes surrounded by granulosa cells, forming a primary follicle. Many follicles degenerate between birth and puberty, so only 60,000 to 80,000 primary follicles are left in each ovary at puberty.

In a growing follicle, the primary oocyte completes its first meiotic division. This division is unequal, giving a large haploid secondary oocyte and a tiny first polar body. The mature follicle is called the Graafian follicle.

Structure of a sperm

A sperm is a microscopic structure made of a head, a neck, a middle piece and a tail. A plasma membrane covers the whole body of the sperm.

• Head: contains an elongated haploid nucleus. Its front part is covered by a cap-like acrosome, filled with enzymes that help the sperm to fertilise the ovum.
• Neck: a short region joining the head to the middle piece.
• Middle piece: has numerous mitochondria, which produce energy for the movement of the tail.
• Tail: a long tail whose movement makes the sperm motile, so that it can swim towards the ovum.

Half the sperms carry an X chromosome and half carry a Y chromosome.

A long sperm with its head bracketed at the top. The labels are plasma membrane, acrosome (the cap at the tip), nucleus containing chromosomal material, neck, middle piece, mitochondria (energy source for swimming) in the middle piece, and a long curving tail.

See Fig. 2.6 in your NCERT textbook

Structure of an egg

The egg released from the ovary at ovulation is the secondary oocyte. It is large and keeps the bulk of the nutrient-rich cytoplasm of the primary oocyte. It has a haploid nucleus, which always carries an X chromosome.

The cytoplasm is enclosed by a plasma membrane. Inside the Graafian follicle, the secondary oocyte forms a new membrane around it, the zona pellucida.

A narrow perivitelline space separates the ovum from the zona pellucida. Outside the zona pellucida lie the cells of the corona radiata, which come from the follicle cells.

The egg has no tail and does not swim; it is carried along the fallopian tube. If it is not fertilised, it lives for about one day.

A round ovum in the centre, ringed by the perivitelline space and then a thick zona pellucida. Outside this is a loose layer of irregular cells labelled cells of the corona radiata. Three sperms are drawn approaching the ovum from outside.

See Fig. 2.10 in your NCERT textbook

What happens during the menstrual cycle?

Definition: The menstrual cycle is the reproductive cycle of female primates, such as monkeys, apes and human beings. It is the cycle of events from one menstruation to the next.

In human females, menstruation is repeated at an average interval of about 28 or 29 days. One ovum is released (ovulation) in the middle of each cycle. The changes in the ovary and the uterus are controlled by pituitary and ovarian hormones.

The cycle has four phases:

1. Menstrual phase: the cycle starts with menstrual flow, which lasts for 3 to 5 days. The endometrial lining of the uterus and its blood vessels break down, and the resulting liquid of blood and mucus comes out through the vagina.
2. Follicular (proliferative) phase: primary follicles in the ovary grow into a fully mature Graafian follicle, and the endometrium regenerates by proliferation. The pituitary hormones LH and FSH rise gradually and stimulate the follicles, which secrete oestrogen.
3. Ovulatory phase: LH and FSH reach a peak in the middle of the cycle, at about day 14. This rapid rise of LH, the LH surge, makes the Graafian follicle rupture and release the ovum (ovulation).
4. Luteal (secretory) phase: the remaining parts of the Graafian follicle become the corpus luteum. It secretes large amounts of progesterone, which keeps the endometrium ready for implantation of a fertilised ovum.

If the ovum is not fertilised, the corpus luteum degenerates. The endometrium then breaks down, leading to menstruation and the start of a new cycle.

Four stacked panels over days 1 to 29. Pituitary hormones: LH shows a tall sharp peak around day 14, and FSH a smaller peak at the same time. Ovarian events: developing follicles grow into a mature follicle, ovulation, then a developing and a regressing corpus luteum. Ovarian hormones: oestrogen peaks around ovulation and again in the luteal phase; progesterone rises after ovulation and peaks in the luteal phase. Uterine events: menses at the start, then the endometrium thickens through the follicular (proliferative) and luteal (secretory) phases until the next cycle begins.

See Fig. 2.9 in your NCERT textbook

Pregnancy and missed periods

During pregnancy, all events of the menstrual cycle stop and there is no menstruation. Lack of menstruation may indicate pregnancy, but it can also be caused by other factors such as stress or poor health.

Menarche and menopause

The first menstruation begins at puberty and is called menarche. In human beings, menstrual cycles stop at around 50 years of age; this is called menopause.

Regular, cyclic menstruation is a sign of the normal reproductive phase of a woman's life, which lasts from menarche to menopause.

Menstrual hygiene

Cleanliness during menstruation is very important. Bathe regularly and use sanitary napkins or clean homemade pads, changing them every 4 to 5 hours as needed.

Wrap used napkins in paper before disposal; do not throw them into toilet drainpipes or the open. Wash hands with soap after handling them.

How do fertilisation and implantation take place?

Definition: Fertilisation is the fusion of a sperm with an ovum to form a diploid zygote. In humans it is internal and takes place in the ampulla of the fallopian tube.

Steps of fertilisation

1. Insemination: during coitus, semen is released by the penis into the vagina.
2. The motile sperms swim rapidly through the cervix, enter the uterus and finally reach the ampullary region of the fallopian tube.
3. The ovum, collected by the fimbriae, is also carried to the ampullary region.
4. A sperm comes in contact with the zona pellucida of the ovum and causes changes in the membrane that block the entry of other sperms. So only one sperm can fertilise an ovum.
5. Secretions of the acrosome help the sperm to enter the cytoplasm of the ovum through the zona pellucida and plasma membrane.
6. This makes the secondary oocyte complete its second meiotic division, which is also unequal. It forms a second polar body and a haploid ovum.
7. The haploid nucleus of the sperm fuses with that of the ovum, forming a diploid zygote.

Fertilisation can occur only if the ovum and sperms reach the ampullary region at the same time. This is why not every act of coitus leads to fertilisation and pregnancy.

Sex is decided at fertilisation

The human female has XX sex chromosomes and the male has XY. So every ovum carries an X chromosome, while 50 per cent of sperms carry X and the other 50 per cent carry Y.

If an X-bearing sperm fertilises the ovum, the zygote is XX and develops into a girl; if a Y-bearing sperm fertilises it, the zygote is XY and develops into a boy. The sex of the baby is therefore determined by the father, not the mother.

Cleavage and implantation

1. As the zygote moves through the isthmus towards the uterus, it divides by mitosis (cleavage) into 2, 4, 8 and 16 cells called blastomeres.
2. The embryo with 8 to 16 blastomeres is called a morula.
3. The morula keeps dividing and changes into a blastocyst as it moves further into the uterus.
4. In the blastocyst, the cells form an outer layer, the trophoblast, and an inner group, the inner cell mass, which becomes the embryo.
5. The trophoblast attaches to the endometrium. The uterine cells then divide rapidly and cover the blastocyst, so that it becomes embedded in the endometrium.

Definition: Implantation is the embedding of the blastocyst in the endometrium of the uterus. It leads to pregnancy.

An ovary, fallopian tube and the uterus, with points A to H along the way. A is the ovum just released near the fimbriae; B is the ovum surrounded by sperms in the ampulla; C, D and E are the two-cell, four-cell and morula stages moving along the tube; F and G are later stages in the uterus; H is the blastocyst implanting in the uterine wall. Enlarged insets (a) to (g) show the stages, with (e) labelled morula and (g) labelled blastocyst.

See Fig. 2.11 in your NCERT textbook

What is the placenta and what are its functions?

After implantation, finger-like projections called chorionic villi appear on the trophoblast. They are surrounded by uterine tissue and maternal blood.

Definition: The placenta is the structural and functional unit between the developing embryo (foetus) and the mother's body, formed by the chorionic villi and uterine tissue fitting closely into each other.

The placenta is a disc embedded in the uterine wall. It has villi on the embryo's side, and on the mother's side there are blood spaces surrounding the villi.

This arrangement gives a very large surface area for exchange between the mother's blood and the embryo. The placenta is joined to the embryo by the umbilical cord, which carries substances to and from the embryo.

Note: The mother's blood and the foetal blood come very close in the placenta, but they normally do not mix. Substances pass between them across thin layers of tissue.

Role in nutrition, respiration and excretion

• Nutrition: nutrients such as glucose pass from the mother's blood into the embryo.
• Respiration: oxygen passes from the mother's blood to the embryo, and carbon dioxide (CO₂) produced by the embryo is removed.
• Excretion: waste materials produced by the embryo are passed into the mother's blood and removed by her body.

Endocrine function

The placenta also acts as an endocrine tissue. It produces several hormones, including human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens and progestogens.

hCG, hPL and relaxin (secreted by the ovary in the later phase of pregnancy) are produced in women only during pregnancy. In early pregnancy, hCG keeps the corpus luteum active, so that progesterone continues to be secreted.

A section of the uterus with the foetus curled inside a sac. The labels are placental villi (at the top, where the placenta joins the uterine wall), cavity of uterus, yolk sac, embryo, umbilical cord with its vessels, and a plug of mucus in the cervix at the bottom.

See Fig. 2.12 in your NCERT textbook

After the baby is born, the placenta is also expelled from the uterus.

What are the foetal membranes and the functions of amniotic fluid?

As the embryo develops, protective foetal membranes form around it. They come from the embryo's own cells, not from the mother. There are four foetal membranes:

• Amnion: the innermost membrane. It encloses the embryo in a sac, the amniotic cavity, filled with amniotic fluid.
• Chorion: the outermost membrane, developed from the trophoblast. Its chorionic villi form the embryo's part of the placenta.
• Yolk sac: a small sac attached near the umbilical cord. Human eggs have very little yolk, but the yolk sac is the first site where blood cells form.
• Allantois: a small outgrowth whose blood vessels become the blood vessels of the umbilical cord.

Functions of amniotic fluid

• Shock absorber: it protects the embryo from mechanical shocks and jerks.
• Prevents drying: it keeps the embryo moist.
• Free movement: the foetus can move and grow freely, without sticking to the membranes.
• Even temperature: it helps to keep the temperature around the foetus steady.

Amniotic fluid also contains foetal cells and dissolved substances. In amniocentesis, some of the fluid is taken and analysed to test for certain genetic disorders such as Down syndrome, haemophilia and sickle-cell anaemia.

In India there is a legal ban on using amniocentesis for sex determination, to check female foeticide.

Draw a section of the uterus with the foetus inside. Show the amnion as a thin sac close around the foetus, the amniotic cavity filled with fluid, and the chorion outside it, with villi at one side forming the placenta in the uterine wall. Join the foetus to the placenta with the umbilical cord, and draw a small yolk sac near the cord. Label the amnion, amniotic fluid, chorion, placenta, umbilical cord, yolk sac and uterine wall.

What happens during gestation?

Definition: Gestation is the period of development of the embryo inside the uterus until birth. In humans its average duration is about 9 months.

Immediately after implantation, the inner cell mass (embryo) forms an outer layer, the ectoderm, and an inner layer, the endoderm. A mesoderm soon appears between them.

These three germ layers give rise to all the tissues and organs of the adult. The inner cell mass contains stem cells, which can give rise to all the tissues and organs.

The growth of the foetus month by month is summarised below.

Hormones during pregnancy

During pregnancy, the levels of oestrogens, progestogens, cortisol, prolactin, thyroxine and other hormones rise several times over in the mother's blood. This supports foetal growth, the metabolic changes in the mother and the maintenance of pregnancy.

How do parturition and lactation take place?

Definition: Parturition is the delivery of the fully developed foetus (childbirth) by vigorous contractions of the uterus at the end of pregnancy.

Parturition is set off by a complex neuroendocrine mechanism involving cortisol, oestrogens and oxytocin. It proceeds in these steps:

1. Signals for parturition come from the fully developed foetus and the placenta.
2. These signals cause mild uterine contractions, called the foetal ejection reflex.
3. The reflex triggers the release of oxytocin from the mother's pituitary gland.
4. Oxytocin acts on the uterine muscle and causes stronger contractions, which in turn stimulate further secretion of oxytocin.
5. This reflex between contraction and oxytocin secretion continues, giving stronger and stronger contractions.
6. The baby is pushed out of the uterus through the birth canal. Soon afterwards, the placenta is also expelled.

Because oxytocin causes strong contractions of the uterus, doctors may inject oxytocin to induce delivery.

Lactation

The mammary glands differentiate during pregnancy and start producing milk towards its end. This process is called lactation, and it lets the mother feed the newborn.

The milk produced in the first few days is called colostrum. It contains several antibodies that are essential for developing resistance in newborn babies, so doctors recommend breastfeeding in the early period of growth.

Each breast has 15 to 20 mammary lobes containing clusters of milk-secreting cells called alveoli. Milk passes through mammary tubules and ducts to the lactiferous duct, which opens at the nipple.

What roles do the sex hormones and prolactin play in reproduction?

At puberty, the hypothalamus greatly increases its secretion of gonadotropin releasing hormone (GnRH). GnRH acts on the anterior pituitary, which then secretes two gonadotropins: luteinising hormone (LH) and follicle stimulating hormone (FSH).

In the male, LH acts on the Leydig cells and stimulates them to secrete androgens. FSH acts on the Sertoli cells, which secrete factors that help sperm formation. In the female, LH and FSH control the growth of follicles and ovulation.

Testosterone

Testosterone, the main androgen, is secreted by the Leydig cells. It stimulates spermatogenesis, controls the development and functions of the male accessory organs (epididymis, vas deferens, seminal vesicles, prostate, urethra), brings about the male secondary sexual characters and influences male sexual behaviour.

Oestrogen

Oestrogen (also spelt estrogen) is secreted mainly by the growing ovarian follicles. It stimulates the female accessory sex organs and the development of follicles, and brings about the female secondary sexual characters, including development of the mammary glands.

Progesterone

Progesterone is secreted mainly by the corpus luteum. It maintains the endometrium for implantation and supports pregnancy. It also stimulates the formation of milk-storing alveoli in the mammary glands, and milk secretion.

Prolactin

Prolactin is secreted by the anterior pituitary gland. It regulates the growth of the mammary glands and the formation of milk in them. Its level in the mother's blood rises during pregnancy.

How do identical and fraternal twins differ?

Usually one ovum is released in each menstrual cycle and one baby develops. Sometimes two babies, called twins, develop in the same pregnancy. There are two kinds.

Identical twins

Identical (monozygotic) twins develop from a single ovum fertilised by a single sperm. The one zygote divides, and at an early stage the embryo splits into two, each of which grows into a separate baby.

Because both come from the same zygote, identical twins have the same genetic material. They are of the same sex, since sex is fixed at fertilisation, and they look very much alike. Their DNA fingerprints are also identical.

Fraternal twins

Fraternal (dizygotic) twins develop when two ova are released in the same cycle and each is fertilised by a different sperm. Two separate zygotes form and implant.

Fraternal twins are no more alike genetically than ordinary brothers and sisters. They may be of the same sex or of different sexes, and they may or may not look alike.

Glossary

• Scrotum — The pouch outside the abdominal cavity that holds the testes and keeps them 2 to 2.5 °C cooler than body temperature, as sperm formation requires.
• Leydig cells — Interstitial cells lying between the seminiferous tubules that secrete androgens, mainly testosterone.
• Semen — The fluid ejaculated by the male, made of sperms and seminal plasma from the seminal vesicles, prostate and bulbourethral glands.
• Acrosome — The cap-like structure over the front of the sperm head, filled with enzymes that help the sperm enter the ovum.
• Endometrium — The inner glandular lining of the uterus, which changes cyclically during the menstrual cycle and in which the blastocyst implants.
• Graafian follicle — The mature ovarian follicle containing the secondary oocyte, which ruptures to release the ovum at ovulation.
• Ovulation — The release of the ovum from a ruptured Graafian follicle, induced by the LH surge at about the middle of the cycle.
• Corpus luteum — The structure formed from the remains of the Graafian follicle after ovulation; it secretes progesterone and degenerates if fertilisation does not occur.
• Menarche — The first menstruation, which begins at puberty.
• Menopause — The stopping of menstrual cycles in women at around 50 years of age.
• Implantation — The embedding of the blastocyst in the endometrium of the uterus, which leads to pregnancy.
• Placenta — The structural and functional unit between the foetus and the mother, formed by chorionic villi and uterine tissue, for exchange and hormone secretion.
• Parturition — Childbirth: the delivery of the fully developed foetus by strong uterine contractions, controlled by a neuroendocrine mechanism involving oxytocin.

Common errors and misconceptions

• Misconception: The mother decides whether the baby is a boy or a girl. Correct: Every ovum carries an X chromosome, while a sperm carries X or Y, so the father's sperm determines the sex of the baby.
• Misconception: Fertilisation takes place in the uterus. Correct: It takes place in the ampulla of the fallopian tube; the dividing zygote then moves to the uterus, where the blastocyst implants.
• Misconception: Androgens are secreted by the Sertoli cells. Correct: Androgens are secreted by the Leydig (interstitial) cells; Sertoli cells nourish the developing germ cells.
• Misconception: The mother's blood flows through the umbilical cord into the foetus. Correct: The two bloods normally do not mix; substances are exchanged across the placenta, and the cord carries the foetus's own blood vessels.
• Misconception: A missed period always means pregnancy. Correct: Lack of menstruation may indicate pregnancy, but it can also be caused by stress, poor health or other causes.
• Misconception: Identical twins come from two eggs fertilised at the same time. Correct: Identical twins come from one zygote that splits; fraternal twins come from two ova fertilised by two sperms.
• Misconception: The egg released at ovulation has already completed meiosis. Correct: It is a secondary oocyte; its second meiotic division is completed only when a sperm enters it.

Exam-style questions with model answers

Q1. Name the exact site of (a) fertilisation and (b) implantation in the human female. [2 marks]

1. (a) Fertilisation takes place in the ampulla (ampullary region) of the fallopian tube.
2. (b) Implantation takes place in the endometrium, the inner lining of the uterus, where the blastocyst becomes embedded.

Q2. Why are the testes placed in the scrotum outside the abdominal cavity? [2 marks]

1. Sperm formation needs a temperature lower than the normal internal body temperature.
2. The scrotum keeps the testes about 2 to 2.5 °C below body temperature, so sperms can be produced; it also protects the testes from injury.

Q3. A mother gave birth to two baby boys who looked very much alike. The doctor said they had formed from a single ovum. (a) What kind of twins are they? (b) Why are they of the same sex? [2 marks]

1. (a) They are identical (monozygotic) twins.
2. (b) They developed from a single zygote formed by one ovum and one sperm, which later split into two embryos. Sex is fixed at fertilisation, so both have the same sex chromosomes (XY) and are boys.

Q4. Give the function of each of the following: (a) acrosome, (b) middle piece of the sperm, (c) fimbriae, (d) corpus luteum. [4 marks]

1. (a) The acrosome contains enzymes whose secretions help the sperm pass through the zona pellucida and plasma membrane into the ovum.
2. (b) The middle piece has numerous mitochondria, which produce energy for the movement of the tail, making the sperm motile.
3. (c) The fimbriae, at the edge of the infundibulum, help to collect the ovum after ovulation.
4. (d) The corpus luteum secretes large amounts of progesterone, which maintains the endometrium for implantation and supports pregnancy; if there is no fertilisation, it degenerates and menstruation follows.

Q5. Describe briefly the role of the placenta in the nutrition, respiration and excretion of the embryo, and state its endocrine function. [4 marks]

1. Nutrition: glucose and other nutrients pass from the mother's blood across the placental villi into the embryo's blood.
2. Respiration: oxygen passes from the mother's blood to the embryo, and carbon dioxide passes from the embryo to the mother's blood.
3. Excretion: waste materials produced by the embryo are transferred into the mother's blood and removed by her body.
4. Endocrine function: the placenta secretes hormones such as hCG, hPL, oestrogens and progestogens, which support the growth of the foetus and maintain pregnancy.

Q6. Name the four foetal membranes and give three functions of amniotic fluid. [3 marks]

1. The foetal membranes are the amnion, chorion, yolk sac and allantois.
2. Amniotic fluid protects the foetus from mechanical shocks and jerks.
3. It keeps the foetus moist and prevents it from drying.
4. It allows the foetus to move freely without sticking to the membranes, and it helps to keep the temperature around the foetus steady.

Q7. Describe the menstrual cycle, naming its phases and the role of the hormones that control it. [5 marks]

1. The menstrual cycle is the reproductive cycle of female primates; in humans it repeats about every 28 or 29 days, and one ovum is released in its middle.
2. Menstrual phase: lasting 3 to 5 days, the endometrium and its blood vessels break down and the liquid flows out through the vagina.
3. Follicular (proliferative) phase: LH and FSH rise gradually; primary follicles grow into a Graafian follicle, which secretes oestrogen, and the endometrium regenerates.
4. Ovulatory phase: LH and FSH peak at about day 14; the LH surge makes the Graafian follicle rupture and release the ovum.
5. Luteal (secretory) phase: the remains of the follicle form the corpus luteum, which secretes progesterone to maintain the endometrium.
6. If fertilisation does not occur, the corpus luteum degenerates, progesterone falls, the endometrium breaks down and menstruation begins a new cycle; if it does occur, the cycle stops during pregnancy.

Q8. Trace the path of sperms from the seminiferous tubules to the outside, and state the roles of the male accessory glands. [5 marks]

1. Sperms pass from the seminiferous tubules into the rete testis and then into the vasa efferentia.
2. The vasa efferentia open into the epididymis on the posterior surface of the testis, which leads to the vas deferens.
3. The vas deferens ascends into the abdomen, loops over the urinary bladder, receives the duct of the seminal vesicle and opens into the urethra as the ejaculatory duct.
4. The urethra carries the semen through the penis to the outside through the urethral meatus.
5. The accessory glands are the paired seminal vesicles, the prostate and the paired bulbourethral glands. Their secretions form the seminal plasma, rich in fructose, calcium and enzymes, which makes transport of sperms easier, nourishes them and helps their maturation and motility. The bulbourethral secretion also lubricates the penis.

Q9. Explain the terms gestation and parturition, and describe the role of oxytocin in parturition. [4 marks]

1. Gestation is the period of development of the embryo in the uterus; in humans it lasts about 9 months.
2. Parturition is the delivery of the fully developed foetus (childbirth) by vigorous contractions of the uterus.
3. Signals from the foetus and placenta cause mild uterine contractions, the foetal ejection reflex, which trigger the release of oxytocin from the mother's pituitary.
4. Oxytocin causes stronger uterine contractions, which stimulate more oxytocin release, so the contractions keep strengthening until the baby is expelled through the birth canal.

Key takeaways

• The testes, held in the scrotum 2 to 2.5 °C below body temperature, produce sperms in the seminiferous tubules and testosterone in the Leydig cells.
• Sperms pass through the rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct and urethra; seminal vesicles, prostate and bulbourethral glands add seminal plasma.
• The ovaries produce ova and hormones; fimbriae collect the ovum, fertilisation occurs in the ampulla, and the blastocyst implants in the endometrium of the uterus.
• A sperm has a head with acrosome and nucleus, a neck, a middle piece full of mitochondria and a tail; the egg is a large, non-motile secondary oocyte.
• The menstrual cycle of about 28 or 29 days has menstrual, follicular, ovulatory and luteal phases, controlled by LH, FSH, oestrogen and progesterone.
• Menarche is the first menstruation at puberty, and menopause is the end of menstrual cycles at around 50 years of age.
• The placenta handles the embryo's nutrition, respiration and excretion and secretes hCG, hPL, oestrogens and progestogens.
• The amnion encloses the foetus in amniotic fluid, which protects it from shocks, keeps it moist and lets it move freely.
• Parturition after about 9 months of gestation is induced by a neuroendocrine mechanism in which oxytocin strengthens the uterine contractions; prolactin and progesterone prepare the mammary glands for milk.
• Identical twins come from one zygote that splits; fraternal twins come from two ova fertilised by two different sperms.

Test yourself

Which cells in the testis nourish the developing sperms?

The Sertoli cells, which line the seminiferous tubules along with the male germ cells, provide nutrition to the germ cells.

What three substances is seminal plasma rich in?

Seminal plasma is rich in fructose, calcium and certain enzymes.

Name the three layers of the uterine wall, from outside in.

The perimetrium (thin outer membrane), the myometrium (thick smooth muscle) and the endometrium (inner glandular lining).

What causes ovulation?

A rapid rise of LH to its maximum level in the middle of the cycle, the LH surge, makes the Graafian follicle rupture and release the ovum.

What is a morula?

A morula is the early embryo with 8 to 16 blastomeres, formed by cleavage of the zygote as it moves along the fallopian tube.

Which hormones are produced in women only during pregnancy?

Human chorionic gonadotropin (hCG), human placental lactogen (hPL) and relaxin are produced only during pregnancy.

What is colostrum and why is it important?

Colostrum is the milk produced in the first few days of lactation. It contains several antibodies that help the newborn baby develop resistance.

What is the role of prolactin?

Prolactin, from the anterior pituitary, regulates the growth of the mammary glands and the formation of milk in them.