Human Reproduction | CBSE Class 12 Biology Notes

This note covers human reproduction: the main reproductive events, the male reproductive system, the female reproductive system and the mammary glands, spermatogenesis and the structure of a sperm, oogenesis, the menstrual cycle and its hormones, fertilisation and the sex of the baby, cleavage and implantation, pregnancy and embryonic development, and parturition and lactation.

What are the main reproductive events in humans?

Humans are sexually reproducing and viviparous. Their reproductive events occur after puberty and take place in the following sequence.

1. Gametogenesis: the formation of gametes, that is, sperms in males and ovum in females.
2. Insemination: the transfer of sperms into the female genital tract.
3. Fertilisation: the fusion of the male and female gametes, which leads to the formation of a zygote.
4. The formation and development of the blastocyst and its attachment to the uterine wall, called implantation.
5. Embryonic development, called gestation.
6. The delivery of the baby, called parturition.

There are remarkable differences between the reproductive events in the male and in the female. For example, sperm formation continues even in old men, but the formation of ovum ceases in women around the age of fifty years.

The primary sex organs are the testis in males and the ovaries in females. They produce the gametes, sperms and ovum respectively, by gametogenesis.

How are the testes organised?

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

The testes are situated outside the abdominal cavity within a pouch called the scrotum. The scrotum helps in maintaining the low temperature of the testes, 2 to 2.5 °C lower than the normal internal body temperature, which is necessary for spermatogenesis.

In adults, each testis is oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm. The testis 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.

What lines a seminiferous tubule?

Each seminiferous tubule is lined on its inside by two types of cells. The male germ cells, or spermatogonia, undergo meiotic divisions that finally lead to sperm formation. The Sertoli cells provide nutrition to the germ cells.

What lies between the tubules?

The regions outside the seminiferous tubules are called interstitial spaces. They contain small blood vessels and interstitial cells, or Leydig cells.

Leydig cells synthesise and secrete testicular hormones called androgens. Other immunologically competent cells are also present in the interstitial spaces.

What are the male accessory ducts, glands and external genitalia?

Accessory ducts

The male sex accessory ducts are the rete testis, vasa efferentia, epididymis and vas deferens. These ducts store and transport the sperms from the testis to the outside through the urethra, along this path.

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

Figure 2.1(a) is a sectional side view of the male pelvis labelling the ureter, urinary bladder, seminal vesicle, vas deferens, prostate, ejaculatory duct, bulbourethral gland, urethra, penis, glans penis, foreskin, testis, scrotum, rectum and anus.

Figure 2.1(b) is a front view in which part of one testis is opened to show the testicular lobules, rete testis, vasa efferentia and epididymis, with the vas deferens rising towards the urinary bladder.

See Figs. 2.1a and 2.1b in your NCERT textbook

Accessory glands

The male accessory glands are the paired seminal vesicles, a prostate and the paired bulbourethral glands. Their secretions make up the seminal plasma, which is rich in fructose, calcium and certain enzymes. The secretions of the bulbourethral glands also help in the lubrication of the penis.

External genitalia

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

What are the internal organs of the female reproductive system?

The female reproductive system consists of a pair of ovaries along with a pair of oviducts, the uterus, cervix, vagina and the external genitalia, located in the pelvic region. With a pair of mammary glands, these parts are integrated structurally and functionally to support ovulation, fertilisation, pregnancy, birth and child care.

Ovaries

The ovaries are the primary female sex organs. They produce the female gamete, the ovum, and several steroid hormones called ovarian hormones.

The ovaries are located one on each side of the lower abdomen. Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments.

Each ovary is covered by a thin epithelium, which encloses the ovarian stroma. The stroma is divided into two zones, a peripheral cortex and an inner medulla.

Oviducts (fallopian tubes)

The oviducts, uterus and vagina constitute the female accessory ducts. Each fallopian tube is about 10 to 12 cm long and extends from the periphery of each ovary to the uterus. Its parts run from the ovary end to the uterus end.

1. The infundibulum is the funnel-shaped part closer to the ovary. Its edges possess finger-like projections called fimbriae, which help in collection of the ovum after ovulation.
2. The infundibulum leads to a wider part of the oviduct called the ampulla.
3. The last part of the oviduct, the isthmus, has a narrow lumen and joins the uterus.

Uterus, cervix and vagina

The uterus is single and is also called the womb. It is shaped like an inverted pear and is supported by ligaments attached to the pelvic wall.

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 of tissue.

Figure 2.3(a) is a sectional side view of the female pelvis labelling the uterus, urinary bladder, pubic symphysis, urethra, clitoris, labium minora, labium majora, vaginal orifice, cervix, vagina, rectum and anus.

Figure 2.3(b) is a front section showing the uterine fundus, uterine cavity, endometrium, myometrium, perimetrium, cervix, cervical canal and vagina. On each side an ovary lies close to the fimbriae, and a bracket marks the fallopian tube with its isthmus, ampulla and infundibulum.

See Figs. 2.3a and 2.3b in your NCERT textbook

What are the female external genitalia and the mammary glands?

External genitalia

The female external genitalia include the mons pubis, labia majora, labia minora, hymen and clitoris.

• Mons pubis: a cushion of fatty tissue covered by skin and pubic hair.
• Labia majora: fleshy folds of tissue that extend down from the mons pubis and surround the vaginal opening.
• Labia minora: paired folds of tissue under the labia majora.
• Hymen: a membrane that often partially covers the opening of the vagina.
• Clitoris: a tiny finger-like structure at the upper junction of the two labia minora, above the urethral opening.

The hymen is often torn during the first coitus (intercourse). However, it can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, or active participation in some sports like horseback riding and cycling.

In some women the hymen persists even after coitus. The presence or absence of the hymen is not a reliable indicator of virginity or sexual experience.

Mammary glands

A functional mammary gland is characteristic of all female mammals. The mammary glands are paired structures (breasts) that contain glandular tissue and a variable amount of fat.

The glandular tissue of each breast is divided into 15 to 20 mammary lobes, containing clusters of cells called alveoli. Milk passes out along the following route.

1. The cells of the alveoli secrete milk, which is stored in the cavities (lumens) of the alveoli.
2. The alveoli open into mammary tubules.
3. The tubules of each lobe join to form a mammary duct.
4. Several mammary ducts join to form a wider mammary ampulla, which is connected to the lactiferous duct. This opens at the nipple, from where milk is expressed during breastfeeding.

A sectional view of the breast against the chest wall, labelling the mammary lobe, mammary alveolus, mammary duct, ampulla, lactiferous duct, nipple, areola and fat, with a rib, the muscles between ribs and the pectoralis major muscle behind.

See Fig. 2.4 in your NCERT textbook

How are sperms formed?

Gametogenesis is the process by which the primary sex organs produce gametes. In the testis, the immature male germ cells produce sperms by spermatogenesis, which begins at puberty.

Stages of spermatogenesis

1. The spermatogonia on the inside wall of the seminiferous tubules multiply by mitotic division and increase in number. Each spermatogonium is diploid and contains 46 chromosomes.
2. Some of the spermatogonia, called primary spermatocytes, periodically undergo meiosis.
3. A primary spermatocyte completes the first meiotic division (reduction division), forming two equal, haploid secondary spermatocytes with only 23 chromosomes each.
4. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids.
5. The spermatids are transformed into spermatozoa (sperms) by spermiogenesis.
6. After spermiogenesis, the sperm heads become embedded in the Sertoli cells. The sperms are finally released from the seminiferous tubules by spermiation.

A wedge of the tubule wall shows sperm formation in layers. A spermatogonium is labelled at the base, next to the tubule wall, with a primary spermatocyte, a secondary spermatocyte and a spermatid above it. Spermatozoa sit at the top with their tails free, and a Sertoli cell is labelled among the germ cells.

See Fig. 2.5 in your NCERT textbook

How do hormones control spermatogenesis?

Spermatogenesis starts at puberty because of a significant increase in the secretion of gonadotropin releasing hormone (GnRH), a hypothalamic hormone.

1. The increased GnRH acts at the anterior pituitary gland.
2. The anterior pituitary secretes two gonadotropins, luteinising hormone (LH) and follicle stimulating hormone (FSH).
3. LH acts at the Leydig cells and stimulates the synthesis and secretion of androgens, which in turn stimulate spermatogenesis.
4. FSH acts on the Sertoli cells and stimulates the secretion of some factors that help in the process of spermiogenesis.

What is the structure of a sperm, and what is semen?

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

The head is bracketed and covered by the plasma membrane, with a cap-shaped acrosome at its tip over the nucleus containing chromosomal material. A short neck joins the head to the middle piece, whose mitochondria are labelled as the energy source for swimming. A long tail follows.

See Fig. 2.6 in your NCERT textbook

The human male ejaculates about 200 to 300 million sperms during a coitus. For normal fertility, at least 60 per cent of these sperms must have normal shape and size, and at least 40 per cent of them must show vigorous motility.

Semen

Sperms released from the seminiferous tubules are transported by the accessory ducts. Secretions of the epididymis, vas deferens, seminal vesicle and prostate are essential for the maturation and motility of sperms.

The seminal plasma along with the sperms constitutes the semen. The functions of the male sex accessory ducts and glands are maintained by the testicular hormones, the androgens.

How is the ovum formed?

The process of formation of a mature female gamete is called oogenesis. It is markedly different from spermatogenesis, and it is initiated during the embryonic development stage.

Stages of oogenesis

1. A couple of million gamete mother cells, the oogonia, are formed within each fetal ovary. No more oogonia are formed and added after birth.
2. These cells start division, enter prophase I of the meiotic division and get temporarily arrested at that stage. They are now called primary oocytes.
3. Each primary oocyte gets surrounded by a layer of granulosa cells and is called a primary follicle. A large number of these follicles degenerate from birth to puberty, so at puberty only 60,000 to 80,000 primary follicles are left in each ovary.
4. The primary follicles get surrounded by more layers of granulosa cells and a new theca, and are called secondary follicles.
5. A secondary follicle soon transforms into a tertiary follicle, characterised by a fluid-filled cavity called the antrum. The theca is organised into an inner theca interna and an outer theca externa.
6. Within the tertiary follicle, the primary oocyte grows in size and completes its first meiotic division. This unequal division forms a large haploid secondary oocyte and a tiny first polar body.
7. The tertiary follicle changes into the mature follicle, or Graafian follicle. The secondary oocyte forms a new membrane called the zona pellucida around it.
8. The Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary. This is ovulation.

The secondary oocyte retains the bulk of the nutrient-rich cytoplasm of the primary oocyte. Whether the first polar body divides further or degenerates is not yet certain.

Follicles at different stages are drawn in the ovary: small primary follicles, a tertiary follicle showing an antrum, and a large Graafian follicle. A secondary oocyte is shown released at the surface, a corpus luteum is labelled in the lower part, and blood vessels enter at one end.

See Fig. 2.7 in your NCERT textbook

Panel (a) runs from spermatogonia at puberty through primary spermatocytes, secondary spermatocytes and spermatids to spermatozoa. A central column gives the chromosome number per cell as 46 at the top and 23 at the secondary spermatocyte and spermatid levels.

Panel (b) shows oogonia forming a primary oocyte in fetal life. After puberty, the first meiotic division, marked as completed prior to ovulation, gives a secondary oocyte and a first polar body, and the next division gives the ovum and a second polar body.

See Fig. 2.8 in your NCERT textbook

How does oogenesis differ from spermatogenesis?

What happens during the menstrual cycle?

The reproductive cycle in female primates, for example monkeys, apes and human beings, is called the menstrual cycle. The first menstruation begins at puberty and is called menarche.

In human females, menstruation is repeated at an average interval of about 28/29 days. The cycle of events from one menstruation to the next is the menstrual cycle. One ovum is released during the middle of each cycle.

Phases of the cycle

The changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.

1. Menstrual phase. The cycle starts with menstrual flow, which lasts for 3 to 5 days. It results from the breakdown of the endometrial lining of the uterus and its blood vessels, which forms a liquid that comes out through the vagina.
2. Follicular phase. The primary follicles grow to become a fully mature Graafian follicle, and the endometrium regenerates through proliferation. The secretion of LH and FSH increases gradually and stimulates follicular development as well as secretion of estrogens by the growing follicles.
3. Ovulatory phase. LH and FSH attain a peak level in the middle of the cycle, about the 14th day. The rapid secretion of LH to its maximum, the LH surge, induces rupture of the Graafian follicle and release of the ovum.
4. Luteal phase. The remaining parts of the Graafian follicle transform into the corpus luteum, which secretes large amounts of progesterone. Progesterone is essential for maintenance of the endometrium, which is necessary for implantation and other events of pregnancy.

In the absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium, leading to menstruation and marking a new cycle.

Four stacked panels share a time axis of days 1 to 29/1. FSH and LH stay fairly level, then peak sharply at about day 14, the LH peak being taller. Developing follicles grow into a mature follicle, ovulation follows at about day 14, and then a corpus luteum develops and later regresses.

Estrogen peaks just before ovulation, falls and rises again in the luteal phase. Progesterone stays low until ovulation, then peaks in the luteal phase and falls. The endometrium, thin after menses, thickens through the follicular phase (labelled proliferative phase) and is thickest in the luteal phase (labelled secretory phase).

See Fig. 2.9 in your NCERT textbook

Menarche, menopause and pregnancy

Menstruation only occurs if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy. However, it may also be caused by other underlying causes such as stress and poor health. During pregnancy, all events of the menstrual cycle stop and there is no menstruation.

In human beings, menstrual cycles cease around 50 years of age; this is termed menopause. Cyclic menstruation is an indicator of the normal reproductive phase, which extends between menarche and menopause.

Note: Hygiene and sanitation during menstruation are very important. Bathe regularly, use sanitary napkins or clean homemade pads, and change them every 4 to 5 hours as required. Wrap used napkins in used paper before disposal, do not throw them in toilet drainpipes or in the open, and wash hands with soap afterwards.

How does fertilisation take place?

Fertilisation is the process of fusion of a sperm with an ovum. In humans it takes place in the ampullary region of the fallopian tube. The events run in this order.

1. During copulation (coitus), semen is released by the penis into the vagina. This is insemination.
2. The motile sperms swim rapidly, pass through the cervix, enter the uterus and finally reach the ampullary region of the fallopian tube. The ovum released by the ovary is also transported there.
3. A sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. This ensures that only one sperm can fertilise an ovum.
4. The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
5. Sperm entry induces the completion of the meiotic division of the secondary oocyte. This second meiotic division is also unequal and forms a second polar body and a haploid ovum (ootid).
6. The haploid nucleus of the sperm and that of the ovum fuse together to form a diploid zygote.

Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary region. This is why not all copulations lead to fertilisation and pregnancy.

How is the sex of the baby decided?

The sex of the baby is decided at fertilisation itself. The chromosome pattern in the human female is XX and in the male is XY.

So all the ova carry the sex chromosome X. In sperms, the sex chromosome could be either X or Y: 50 per cent of sperms carry X and the other 50 per cent carry Y.

The zygote carries either XX or XY, depending on whether a sperm carrying X or Y fertilised the ovum. The XX zygote develops into a female baby and the XY zygote into a male. Scientifically, therefore, the sex of the baby is determined by the father and not by the mother.

How do cleavage and implantation take place?

After fertilisation, the zygote begins to divide as it moves towards the uterus. The steps up to implantation are as follows.

1. Mitotic division, called cleavage, starts as the zygote moves through the isthmus of the oviduct towards the uterus. It forms 2, 4, 8 and 16 daughter cells called blastomeres.
2. The embryo with 8 to 16 blastomeres is called a morula.
3. The morula continues to divide and transforms into a blastocyst as it moves further into the uterus.
4. In the blastocyst, the blastomeres are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast, called the inner cell mass.
5. The trophoblast layer gets attached to the endometrium, and the inner cell mass gets differentiated as the embryo.
6. After attachment, the uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus.

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

The ovary, fallopian tube and uterus are drawn with stages lettered A to H: the ovum next to the ovary (A), sperms reaching it in the tube (B), the dividing embryo moving along the tube (C to E), and the embryo in the uterus (F to H), with H labelled "Blastocyst Implantation" at the uterine wall.

Enlarged insets show the ovum, sperms at the ovum, a two-celled stage, a later cleavage stage, the morula, and the blastocyst: a hollow ball with an outer layer of cells and an inner cluster of cells on one side.

See Fig. 2.11 in your NCERT textbook

What happens during pregnancy?

How does the placenta form?

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

The chorionic villi and uterine tissue become interdigitated with each other. Together they form a structural and functional unit between the developing embryo (foetus) and the maternal body, called the placenta.

What does the placenta do?

• It facilitates the supply of oxygen and nutrients to the embryo, and the removal of carbon dioxide and the excretory or waste materials produced by the embryo.
• It is connected to the embryo through the umbilical cord, which helps in the transport of substances to and from the embryo.
• It acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens and progestogens.

The uterus is drawn in section with the embryo inside a rounded sac. Labels mark the placental villi at the upper uterine wall, the umbilical cord with its vessels, the yolk sac, the embryo, the cavity of the uterus and a plug of mucus in the cervix.

See Fig. 2.12 in your NCERT textbook

Which hormones rise in pregnancy?

In the later phase of pregnancy, a hormone called relaxin is also secreted by the ovary. hCG, hPL and relaxin are produced in women only during pregnancy.

The levels of other hormones like estrogens, progestogens, cortisol, prolactin and thyroxine are increased several-fold in the maternal blood. This is essential for supporting fetal growth, metabolic changes in the mother and maintenance of pregnancy.

How do the germ layers form?

Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between them. These three layers give rise to all tissues (organs) in adults.

The inner cell mass also contains certain cells called stem cells, which have the potency to give rise to all the tissues and organs.

How does the foetus develop month by month?

Human pregnancy lasts 9 months. The major features of embryonic development at various stages are as follows.

What are parturition and lactation?

The average duration of human pregnancy, about 9 months, is called the gestation period. At the end of pregnancy, vigorous contraction of the uterus causes delivery of the foetus.

This process of delivery of the foetus (childbirth) is called parturition. It is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin.

How is parturition triggered?

1. The signals for parturition originate from the fully developed foetus and the placenta.
2. These signals induce mild uterine contractions called the foetal ejection reflex.
3. The reflex triggers the release of oxytocin from the maternal pituitary.
4. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulate further secretion of oxytocin.
5. This stimulatory reflex continues, resulting in stronger and stronger contractions that expel the baby out of the uterus through the birth canal. Soon after, the placenta is also expelled.

Lactation

The mammary glands differentiate during pregnancy and start producing milk towards the end of pregnancy by the process called lactation. This helps the mother in feeding the newborn.

The milk produced during the initial few days of lactation is called colostrum. It contains several antibodies absolutely essential to develop resistance in newborn babies. Doctors recommend breastfeeding during the initial period of infant growth for bringing up a healthy baby.

Glossary

• Gametogenesis — The formation of gametes by the primary sex organs: sperms in the testis and ovum in the ovary.
• Seminiferous tubule — A highly coiled tubule in a testicular lobule, lined by spermatogonia and Sertoli cells, in which sperms are produced.
• Sertoli cells — Cells lining the seminiferous tubules that provide nutrition to the germ cells and, under FSH, help spermiogenesis.
• Leydig cells — Interstitial cells outside the seminiferous tubules that synthesise and secrete androgens when stimulated by LH.
• Spermiogenesis — The process by which haploid spermatids are transformed into spermatozoa (sperms).
• Spermiation — The release of sperms from the seminiferous tubules after their heads have been embedded in Sertoli cells.
• Acrosome — A cap-like structure over the front of the sperm nucleus, filled with enzymes that help fertilisation.
• Graafian follicle — The mature ovarian follicle, formed from the tertiary follicle, which ruptures to release the secondary oocyte.
• Zona pellucida — The new membrane formed by the secondary oocyte around itself, which the sperm contacts at fertilisation.
• Corpus luteum — The structure formed from the remaining parts of the Graafian follicle after ovulation, secreting large amounts of progesterone.
• Menarche — The first menstruation, which begins at puberty in human females.
• Menopause — The ceasing of menstrual cycles in human beings, around 50 years of age.
• Blastocyst — The stage formed from the morula, with an outer trophoblast and an inner cell mass, which implants in the endometrium.
• Placenta — The structural and functional unit between foetus and mother, formed by interdigitated chorionic villi and uterine tissue.
• Colostrum — The milk of the initial few days of lactation, containing antibodies essential for resistance in newborn babies.

Common errors and misconceptions

• Misconception: Sertoli cells produce androgens. Correct: Androgens are secreted by the Leydig cells in the interstitial spaces; Sertoli cells nourish the germ cells.
• Misconception: Leydig cells are found in the ovary. Correct: Leydig cells lie in the interstitial spaces of the testis, outside the seminiferous tubules.
• Misconception: Fertilisation takes place in the uterus. Correct: It takes place in the ampullary region of the fallopian tube; the blastocyst later implants in the endometrium.
• Misconception: The cell released at ovulation has completed meiosis. Correct: Ovulation releases the secondary oocyte; its second meiotic division is completed when a sperm enters.
• Misconception: Oogenesis takes place in the corpus luteum. Correct: Oogenesis takes place in the ovary; the corpus luteum forms from the Graafian follicle after ovulation and secretes progesterone.
• Misconception: The mother decides the sex of the baby. Correct: Every ovum carries X, while half the sperms carry X and half Y, so the fertilising sperm decides XX or XY.
• Misconception: A missed period always means pregnancy. Correct: Lack of menstruation may indicate pregnancy, but it may also be caused by stress, poor health and other causes.
• Misconception: The hymen shows whether a woman has had intercourse. Correct: It can break in a fall, with a tampon or in sports, and may persist after coitus, so it is not a reliable indicator.

Exam-style questions with model answers

Q1. Name the hormone whose surge induces ovulation, and state when in the cycle it peaks. [1 mark]

1. Luteinising hormone (LH). Its rapid secretion reaches a maximum in the middle of the cycle, about the 14th day, and this LH surge induces rupture of the Graafian follicle.

Q2. Define spermiogenesis and spermiation. [2 marks]

1. Spermiogenesis is the process by which spermatids are transformed into spermatozoa (sperms).
2. Spermiation is the process by which the sperms, whose heads become embedded in the Sertoli cells after spermiogenesis, are finally released from the seminiferous tubules.

Q3. Describe the structure of a seminiferous tubule. [3 marks]

1. Each testis has about 250 testicular lobules, and each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced.
2. Each tubule is lined on its inside by male germ cells (spermatogonia), which undergo meiotic divisions leading to sperm formation, and by Sertoli cells, which provide nutrition to the germ cells.
3. Outside the tubules, the interstitial spaces contain small blood vessels, Leydig cells that synthesise and secrete androgens, and other immunologically competent cells.

Q4. Name the hormones involved in the regulation of spermatogenesis and state the role of each. [3 marks]

1. GnRH, a hypothalamic hormone, increases significantly at puberty and acts on the anterior pituitary to stimulate secretion of LH and FSH.
2. LH acts on the Leydig cells and stimulates synthesis and secretion of androgens, which in turn stimulate spermatogenesis.
3. FSH acts on the Sertoli cells and stimulates secretion of some factors that help in spermiogenesis.

Q5. In our society women are often blamed for giving birth to daughters. Explain why this is not correct. [3 marks]

1. The human female is XX, so every ovum she produces carries an X chromosome.
2. The male is XY, so 50 per cent of his sperms carry X and the other 50 per cent carry Y.
3. If an X-carrying sperm fertilises the ovum, the zygote is XX and develops into a girl; if a Y-carrying sperm does, the zygote is XY and develops into a boy.
4. The sex of the baby is therefore determined by the father and not by the mother.

Q6. What is parturition? Describe how it is induced. [4 marks]

1. Parturition is the delivery of the fully developed foetus (childbirth) by vigorous contraction of the uterus at the end of pregnancy. It is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin.
2. Signals from the fully developed foetus and the placenta induce mild uterine contractions called the foetal ejection reflex.
3. This triggers release of oxytocin from the maternal pituitary. Oxytocin causes stronger uterine contractions, which in turn stimulate further oxytocin secretion.
4. The stimulatory reflex continues, giving stronger and stronger contractions that expel the baby through the birth canal; the placenta is expelled soon after.

Q7. Give a brief account of oogenesis. [5 marks]

1. Oogenesis begins during embryonic development, when a couple of million oogonia are formed within each fetal ovary; no more are added after birth.
2. The oogonia enter prophase I of meiosis and are temporarily arrested as primary oocytes. Each is surrounded by a layer of granulosa cells to form a primary follicle; only 60,000 to 80,000 remain in each ovary at puberty.
3. Primary follicles gain more granulosa layers and a new theca to become secondary follicles, and then tertiary follicles with a fluid-filled antrum and a theca interna and externa.
4. In the tertiary follicle, the primary oocyte completes its first meiotic division. This unequal division gives a large haploid secondary oocyte and a tiny first polar body.
5. The tertiary follicle becomes the Graafian follicle, and the secondary oocyte forms a zona pellucida. The Graafian follicle ruptures to release the secondary oocyte by ovulation.
6. The second meiotic division, completed when a sperm enters, is also unequal and gives a haploid ovum (ootid) and a second polar body.

Q8. What is the menstrual cycle? Describe its phases and the hormones that regulate them. [5 marks]

1. The menstrual cycle is the reproductive cycle of female primates; in human females it repeats about every 28/29 days, with one ovum released in the middle of each cycle.
2. Menstrual phase: flow lasts 3 to 5 days and results from breakdown of the endometrial lining and its blood vessels, when the ovum is not fertilised.
3. Follicular phase: primary follicles grow into a Graafian follicle and the endometrium regenerates by proliferation. LH and FSH rise gradually and stimulate follicular development and estrogen secretion by the growing follicles.
4. Ovulatory phase: LH and FSH peak about the 14th day, and the LH surge induces rupture of the Graafian follicle and release of the ovum.
5. Luteal phase: the corpus luteum forms from the Graafian follicle and secretes large amounts of progesterone, which maintains the endometrium.
6. Without fertilisation, the corpus luteum degenerates, the endometrium disintegrates and menstruation starts a new cycle.

Q9. Describe the events from fertilisation to implantation in humans. [5 marks]

1. Sperms reach the ampullary region of the fallopian tube, where the ovum is also transported; fertilisation can only occur if both arrive simultaneously.
2. A sperm contacts the zona pellucida and induces membrane changes that block additional sperms. Acrosomal secretions help it enter the ovum.
3. Sperm entry induces completion of the second meiotic division, forming a second polar body and a haploid ootid; the haploid nuclei fuse to form a diploid zygote.
4. As the zygote moves through the isthmus towards the uterus, cleavage gives 2, 4, 8 and 16 blastomeres; the embryo with 8 to 16 blastomeres is a morula.
5. The morula becomes a blastocyst with an outer trophoblast and an inner cell mass as it moves into the uterus.
6. The trophoblast attaches to the endometrium, and the uterine cells divide rapidly and cover the blastocyst, which becomes embedded. This implantation leads to pregnancy.

Key takeaways

• The testes lie in the scrotum at 2 to 2.5 °C below body temperature; each has about 250 lobules with one to three seminiferous tubules.
• Sertoli cells nourish the germ cells and respond to FSH, while Leydig cells in the interstitial spaces secrete androgens in response to LH.
• Sperms pass through the rete testis, vasa efferentia, epididymis, vas deferens and ejaculatory duct into the urethra.
• Seminal plasma is rich in fructose, calcium and certain enzymes; together with the sperms it forms semen.
• Spermatogenesis begins at puberty and gives four equal haploid spermatids, while oogenesis begins in the fetal ovary and its unequal divisions form polar bodies.
• The menstrual cycle of about 28/29 days has menstrual, follicular, ovulatory and luteal phases; the LH surge at about day 14 induces ovulation.
• Fertilisation occurs in the ampullary region, and the X or Y chromosome of the sperm decides whether the zygote is XX or XY.
• The trophoblast attaches to the endometrium and the inner cell mass becomes the embryo; chorionic villi and uterine tissue form the placenta.
• Parturition is triggered by the foetal ejection reflex and a stimulatory reflex between oxytocin release and uterine contraction.

Test yourself

Why are the testes placed in the scrotum?

The scrotum keeps the testes 2 to 2.5 °C lower than the normal internal body temperature, which is necessary for spermatogenesis.

What is seminal plasma rich in?

Fructose, calcium and certain enzymes, from the secretions of the male accessory glands.

How many primary follicles are left in each ovary at puberty?

Only 60,000 to 80,000, because a large number of follicles degenerate between birth and puberty.

Which structures help collect the ovum after ovulation?

The fimbriae, finger-like projections on the edges of the funnel-shaped infundibulum of the fallopian tube.

How many chromosomes does a secondary spermatocyte have?

23. It is haploid, formed by the first meiotic (reduction) division of a primary spermatocyte.

What is a morula?

The embryo with 8 to 16 blastomeres, formed by cleavage of the zygote.

Which hormones are produced in women only during pregnancy?

hCG and hPL from the placenta, and relaxin, secreted by the ovary in the later phase of pregnancy.

What is colostrum, and why is it important?

The milk of the initial few days of lactation. It contains several antibodies essential to develop resistance in newborn babies.