Human Reproduction | ISC Class 12 Biology Notes

Human reproduction includes the male and female reproductive systems, formation of reproductive cells, hormonal control, the menstrual cycle, fertilisation, early embryonic development, implantation, pregnancy, childbirth and milk production.

What are the main events in human reproduction?

Humans reproduce sexually, through the fusion of male and female reproductive cells called gametes. The male gamete is a sperm; the female gamete is an ovum. Humans are viviparous, meaning that they give birth to young ones.

Puberty is the stage at which sexual maturity is attained. Reproduction involves coordinated changes in reproductive organs and in hormones, chemical messengers that regulate the activities of target cells and organs.

How do the events connect?

1. Gametogenesis produces gametes in the primary sex organs: testes in males and ovaries in females.
2. Insemination transfers sperms into the female reproductive tract. Their movement brings them towards the female gamete.
3. Fertilisation is fusion of the male and female gametes, producing a single cell called the zygote.
4. The zygote divides and develops into a blastocyst, an early embryo with an outer cell layer and an inner cell mass.
5. Implantation embeds the blastocyst in the uterine lining. Development during pregnancy is called gestation.
6. Parturition is childbirth. Lactation, the production of milk by mammary glands, supports feeding of the newborn.

These events have different locations and functions. Gamete production occurs in the sex organs, fertilisation occurs in the oviduct, and implantation occurs in the uterus. Keeping the event, its site and its result together makes the whole sequence easier to understand.

Definition: A gamete is a reproductive cell that fuses with another gamete during fertilisation. The resulting zygote begins the development of a new individual, called an embryo during early development and a foetus later in pregnancy.

How are the testes and male reproductive ducts organised?

The male reproductive system includes two testes, accessory ducts, accessory glands and the penis. The testes lie outside the abdominal cavity in the scrotum, a pouch that maintains their temperature 2 to 2.5 degrees Celsius below normal internal body temperature, as required for sperm formation.

Each adult testis is oval, about 4 to 5 centimetres long and about 2 to 3 centimetres wide. A dense covering encloses about 250 compartments called testicular lobules. Each lobule contains one to three highly coiled seminiferous tubules, the tubes in which sperms form.

Which cells occur in and around a seminiferous tubule?

Spermatogonia are the immature male germ cells, which develop into gametes, lining the tubules. Sertoli cells provide nutrition to germ cells. In the spaces between tubules, Leydig cells, also called interstitial cells, produce androgens, the testicular hormones. Small blood vessels also occupy these spaces.

The vas deferens receives a duct from a seminal vesicle, an accessory gland, and opens into the urethra as the ejaculatory duct. The accessory ducts collectively store and transport sperms. Their sequence is seminiferous tubules, rete testis, vasa efferentia, epididymis, vas deferens, ejaculatory duct and urethra.

The sectional pelvic view labels the testis, scrotum, vas deferens, seminal vesicle, prostate, ejaculatory duct and urethra. The second drawing opens part of a testis to show testicular lobules and the connections through rete testis, vasa efferentia and epididymis.

See Fig. 2.1 in your NCERT textbook

The section labels spermatogonia, Sertoli cells and spermatozoa within the tubule, with interstitial cells outside. Distinguish the sperm-producing region from the surrounding hormone-producing cells.

See Fig. 2.2 in your NCERT textbook

How do accessory glands and sperm structure support reproduction?

The male accessory glands are paired seminal vesicles, a prostate and paired bulbourethral glands. Their secretions form seminal plasma, which is rich in fructose, a sugar, calcium and certain enzymes. Enzymes are biological catalysts that help chemical reactions occur.

Semen consists of seminal plasma and sperms. Secretions of the epididymis, vas deferens, seminal vesicles and prostate are essential for sperm maturation and movement. Bulbourethral secretions also lubricate the penis. Androgens maintain the functions of male accessory ducts and glands.

The penis contains special tissue that permits erection, facilitating insemination. Its enlarged end is the glans penis, covered by a loose skin fold called the foreskin.

What are the parts of a sperm?

A sperm has a head, neck, middle piece and tail. Its whole body is enclosed by the plasma membrane, the cell's outer boundary. The head contains a nucleus with chromosomes, the structures carrying hereditary information.

The nucleus is haploid, containing one chromosome set, or 23 chromosomes in humans. Diploid cells contain two sets, or 46 chromosomes in humans. This distinction is essential when tracing the formation and fusion of gametes.

The head bears an acrosome, an enzyme-filled cap whose secretions help fertilisation. The middle piece contains many mitochondria, cellular structures that produce energy for tail movement. The tail enables motility, the ability of the sperm to move.

The drawing labels the head, neck, middle piece and tail. It marks the acrosome above the nucleus, the enclosing plasma membrane and mitochondria in the middle piece as an energy source for swimming.

See Fig. 2.6 in your NCERT textbook

How are the female reproductive organs arranged?

The female reproductive system includes two ovaries, two oviducts or fallopian tubes, the uterus, cervix, vagina and external genitalia. Mammary glands, the milk-producing glands, function with this system to support reproduction and care of the newborn.

Each ovary lies on one side of the lower abdomen and is about 2 to 4 centimetres long. Ligaments, supporting bands of tissue, connect it to the pelvic wall and uterus. A thin epithelium, a covering layer of cells, encloses the ovarian stroma, the internal tissue containing developing follicles.

The stroma has an outer cortex and an inner medulla. A follicle is an ovarian structure containing a developing female germ cell and its surrounding cells. The ovaries produce ova and ovarian hormones.

What are the functions of the ducts and uterus?

Each oviduct is about 10 to 12 centimetres long. The uterine wall has an outer membranous perimetrium, a thick muscular myometrium and an inner glandular, or gland-containing, endometrium. The endometrium changes during the menstrual cycle; the myometrium contracts strongly during childbirth.

The frontal sectional drawing shows the ovaries beside the fimbriae. The fallopian tube is labelled with infundibulum, ampulla and isthmus. The uterus shows its cavity and three wall layers, continuing below into the cervix, cervical canal and vagina.

See Fig. 2.3 in your NCERT textbook

What forms the external genitalia?

The mons pubis is a fatty cushion covered by skin and pubic hair. The labia majora are fleshy folds surrounding the vaginal opening; the labia minora are paired folds beneath them. The clitoris lies at their upper junction, above the urethral opening.

The hymen is a membrane that often partly covers the vaginal opening. It can break through several activities and may persist after intercourse. Its presence or absence is not a reliable indicator of virginity or sexual experience.

How does spermatogenesis occur and how is it controlled?

Spermatogenesis is the formation of sperms from spermatogonia in the testes. It begins at puberty. The process combines cell multiplication, chromosome reduction and changes in cell shape that produce the specialised sperm.

Mitosis is cell division that preserves the chromosome number. Meiosis involves two divisions and reduces the chromosome number from diploid to haploid. Its first division separates the two chromosome sets; its second division produces the next generation of haploid cells.

What is the sequence of cell stages?

1. Diploid spermatogonia, each with 46 chromosomes, multiply by mitosis along the inner wall of seminiferous tubules.
2. Some develop into primary spermatocytes, the diploid cells that enter meiosis.
3. Each primary spermatocyte completes the first meiotic division to form two equal secondary spermatocytes, each with 23 chromosomes.
4. The secondary spermatocytes complete the second meiotic division. One primary spermatocyte therefore produces four haploid spermatids, the immature cells that develop into sperms.
5. Spermiogenesis transforms spermatids into spermatozoa, another name for sperms. Their heads become embedded in Sertoli cells.
6. Spermiation releases the sperms from the seminiferous tubules after spermiogenesis.

Which hormones act on which cells?

At puberty, secretion of gonadotropin-releasing hormone (GnRH) increases. GnRH comes from the hypothalamus, a brain region involved in hormonal regulation. It stimulates the anterior pituitary, the front region of the pituitary gland, to release two hormones acting on the sex organs.

These are luteinising hormone (LH) and follicle-stimulating hormone (FSH), collectively called gonadotropins. LH stimulates Leydig cells to synthesise and secrete androgens. These androgens stimulate spermatogenesis.

FSH acts on Sertoli cells, stimulating secretion of factors that help spermiogenesis. Hormonal control therefore depends on both the hormone and its target cell. Assigning LH to Leydig cells and FSH to Sertoli cells keeps their roles distinct.

How do oogenesis and follicle development produce the female gamete?

Oogenesis is formation of a mature female gamete. It starts during embryonic development, when a couple of million oogonia, the female gamete mother cells, form in each foetal ovary. No more oogonia are formed and added after birth.

These cells enter prophase I, the first stage of the first meiotic division, and temporarily stop there as primary oocytes. A primary oocyte surrounded by a layer of granulosa cells, its surrounding follicular cells, forms a primary follicle.

Many follicles degenerate between birth and puberty. At puberty, only 60,000 to 80,000 primary follicles remain in each ovary. A developing follicle acquires more granulosa layers and a theca, an outer covering, becoming a secondary follicle.

How does the mature follicle develop?

1. A secondary follicle becomes a tertiary follicle, distinguished by an antrum, a fluid-filled cavity.
2. The theca differentiates into an inner theca interna and outer theca externa.
3. The primary oocyte grows and completes meiosis I unequally, forming a large haploid secondary oocyte and a tiny first polar body, the smaller cell produced by this division.
4. The secondary oocyte retains most of the nutrient-rich cytoplasm, the cell material outside the nucleus. A surrounding layer called the zona pellucida forms.
5. The follicle becomes a mature Graafian follicle. Its rupture releases the secondary oocyte in ovulation.

The released cell is a secondary oocyte. Completion of its second meiotic division is induced by sperm entry during fertilisation, producing the mature haploid ovum and a second polar body.

Note: It is not very certain whether the first polar body divides further or degenerates. Do not turn oogenesis into a fixed claim that exactly three polar bodies must form.

The drawing labels a primary follicle, a tertiary follicle with an antrum, a Graafian follicle, a released secondary oocyte and a corpus luteum, the structure formed from the follicle after ovulation. Blood vessels are shown entering the ovary.

See Fig. 2.7 in your NCERT textbook

How do the two forms of gametogenesis differ?

How do hormones coordinate the menstrual cycle?

The menstrual cycle is the reproductive cycle of female primates, including humans, monkeys and apes. In humans, menstruation repeats at an average interval of about 28 or 29 days. One cycle extends from the beginning of one menstruation to the next.

Menarche is the first menstruation at puberty. Menopause is cessation of menstrual cycles around 50 years of age. The reproductive phase with cyclic menstruation extends between these events. These ages and intervals should not be treated as identical for every individual.

What happens during each phase?

1. Menstrual phase: Menstrual flow lasts 3 to 5 days. Breakdown of the endometrial lining and its blood vessels produces material discharged through the vagina.
2. Follicular phase: Ovarian follicles develop and the endometrium rebuilds by cell proliferation, an increase in cell number. LH and FSH secretion gradually increases, stimulating follicular development and secretion of oestrogens, ovarian hormones produced by growing follicles.
3. Ovulatory phase: LH and FSH reach peak levels around the middle of the cycle, about day 14. A rapid rise of LH, the LH surge, causes the Graafian follicle to rupture and release the secondary oocyte.
4. Luteal phase: The remaining follicle becomes the corpus luteum. It secretes much progesterone, a hormone essential for maintaining the endometrium needed for implantation and pregnancy.

Without fertilisation, the corpus luteum degenerates, followed by breakdown of the endometrium and a new menstruation. During pregnancy, the events of the menstrual cycle stop and menstruation does not occur.

Note: Lack of menstruation may indicate pregnancy, but it may also result from stress, poor health or other underlying causes. Absence of a period alone does not establish pregnancy.

How does an oestrous cycle differ?

An oestrous cycle is the reproductive cycle found in many non-primate mammals. Oestrus is its period of sexual receptivity, commonly called heat. Sexual receptivity is generally restricted to this period, whereas it is not confined to a comparable heat period in a menstrual cycle.

A menstrual cycle involves shedding of the uterine lining when pregnancy does not occur. In an oestrous cycle, the lining is generally reabsorbed rather than discharged as menstrual bleeding. Both cycles coordinate ovarian activity with changes in the reproductive tract.

What happens during fertilisation and how is the mature ovum organised?

Coitus, also called copulation or sexual intercourse, transfers semen into the vagina. Motile sperms pass through the cervix and uterus into the ampullary region of the fallopian tube. The released secondary oocyte is also transported there.

Fertilisation can occur only when both gametes reach the ampullary region at the same time. This requirement explains why every act of intercourse does not lead to fertilisation or pregnancy.

Which physical and chemical events occur?

1. A sperm contacts the zona pellucida surrounding the female gamete. Changes induced in its membrane block entry of additional sperms.
2. Enzymes released from the acrosome help the sperm pass through the zona pellucida and plasma membrane into the cytoplasm.
3. Sperm entry induces completion of the secondary oocyte's second meiotic division. This unequal division produces a second polar body and a haploid ovum, also called an ootid.
4. The haploid sperm nucleus and ovum nucleus fuse, producing a diploid zygote with 46 chromosomes.

The female gamete contains a nucleus and nutrient-rich cytoplasm enclosed by its plasma membrane. Outside it lies the zona pellucida, with a surrounding layer of follicular cells called the corona radiata. The perivitelline space is the space between the ovum surface and zona pellucida.

The drawing shows the ovum surrounded by a zona pellucida and an outer ring of corona radiata cells. The perivitelline space is labelled inside the zona pellucida, and several sperms are drawn around the outer cells.

See Fig. 2.10 in your NCERT textbook

How do sex chromosomes combine?

X and Y are names of sex chromosomes. In the human chromosome pattern described here, females have XX, two X chromosomes, and males have XY, one X and one Y chromosome. Each ovum carries X; a sperm carries either X or Y.

An X-bearing sperm gives an XX zygote and a Y-bearing sperm gives an XY zygote. Thus the sperm supplies the varying sex chromosome. Blaming the mother for the chromosomal sex of a baby has no scientific basis.

How does the zygote become an implanted blastocyst?

Cleavage is the series of mitotic divisions that starts as the zygote travels through the isthmus of the oviduct towards the uterus. The daughter cells produced are called blastomeres. Successive divisions produce stages with 2, 4, 8 and 16 cells.

An embryo with 8 to 16 blastomeres is a morula. It continues dividing and develops into a blastocyst as it moves into the uterus. Development changes both the number of cells and their arrangement.

Which parts of the blastocyst have different roles?

The blastocyst has an outer cell layer called the trophoblast. An inner cell mass, a group of cells attached to this layer internally, develops into the embryo. The outer layer and inner group should be labelled separately.

1. The zygote divides by mitosis while travelling along the oviduct towards the uterus.
2. The 8 to 16 cell embryo forms the morula, which undergoes further division.
3. The blastocyst develops an outer trophoblast and an inner cell mass.
4. The trophoblast attaches to the endometrium, the inner uterine lining.
5. Uterine cells divide rapidly and cover the blastocyst, embedding it in the endometrium. This is implantation and leads to pregnancy.

Fertilisation, cleavage and implantation are consecutive but distinct events. Fertilisation combines two gametes into a zygote. Cleavage produces the early multicellular embryo. Implantation establishes that embryo within the uterine lining.

The cell mass inside the blastocyst contains stem cells, cells capable of giving rise to all the body's tissues and organs. This developmental capacity allows a small initial group of cells to produce the many specialised tissues of the developing individual.

How do the placenta and pregnancy hormones support development?

The placenta is a structural and functional connection between the developing embryo and the maternal body. After implantation, finger-like projections called chorionic villi develop on the trophoblast. They become closely interlocked with uterine tissue, together forming this connection.

The placenta supplies oxygen and nutrients to the embryo and removes carbon dioxide and other waste materials. An umbilical cord connects the embryo with the placenta and transports substances to and from the embryo.

Why is the placenta also an endocrine tissue?

Endocrine tissue produces hormones. Placental hormones include human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens and progestogens, hormones belonging to the progesterone group. These are part of the hormonal environment supporting pregnancy.

The ovary secretes relaxin, another pregnancy-associated hormone, in the later phase of pregnancy. Levels of several maternal hormones rise during pregnancy. Their increased production supports foetal growth, changes in the mother's metabolism and maintenance of pregnancy.

How does the embryo begin to organise its tissues?

After implantation, the inner cell mass forms an outer ectoderm and inner endoderm. A third layer, the mesoderm, then appears between them. These are the three embryonic tissue layers from which adult tissues and organs develop.

Placental exchange and embryonic development have complementary roles. The placenta supports the supply and removal of substances, while the embryo's cells divide and become specialised. Pregnancy therefore requires both support from the maternal body and organised development within the embryo.

What developmental changes occur during pregnancy?

The average human gestation period, the duration of pregnancy, is about nine months. The developing baby is described as the foetus during its development in the uterus. A trimester is one of the three broad divisions of pregnancy.

Developmental milestones describe the appearance of particular structures or activities. They should be retained with their time qualifications: “most”, “usually” and “about” matter when interpreting the sequence.

Which milestones belong to which time periods?

The heart's formation precedes the later milestones of limb development and observable movement. First movements and hair on the head belong together in the fifth-month description. Eyelid separation and eyelashes belong to the later description at about 24 weeks.

These milestones connect formation of organs with preparation for birth. “Most major organ systems” at 12 weeks does not mean that every feature is complete then. Development continues during the remaining pregnancy until the foetus is ready for delivery.

How are childbirth and lactation controlled?

Parturition results from vigorous uterine contractions at the end of pregnancy. Its control is neuroendocrine, involving both nervous signals and hormones. Signals from the fully developed foetus and placenta begin the process.

How do uterine contractions become stronger?

1. Signals from the foetus and placenta induce mild contractions called the foetal ejection reflex.
2. This triggers release of oxytocin, a hormone that stimulates contraction of smooth muscle, involuntary muscle in organs, from the maternal pituitary.
3. Oxytocin acts on uterine muscle and produces stronger contractions.
4. Stronger contractions stimulate further oxytocin secretion, which produces still stronger contractions.
5. The baby is expelled through the birth canal. Soon afterwards, the placenta is also expelled.

This mutually reinforcing cycle is positive feedback: an initial response stimulates processes that strengthen that response. Here, uterine contraction and oxytocin release reinforce one another until delivery.

How is milk produced and released?

Each mammary gland contains glandular tissue and a variable amount of fat. Its 15 to 20 lobes contain alveoli, clusters of milk-secreting cells. Milk collects in the alveolar cavities, also called lumens.

Alveoli open into mammary tubules, which join into mammary ducts. Several ducts form a wider mammary ampulla, connected to a lactiferous duct, the passage opening at the nipple through which milk is expressed.

Prolactin, an anterior pituitary hormone, regulates mammary gland growth and milk formation. Oxytocin stimulates milk ejection. Oxytocin is synthesised in the hypothalamus and stored and released by the posterior pituitary, the rear part of the gland.

The mammary glands differentiate during pregnancy and start producing milk towards its end. Milk produced during the first few days is colostrum. It contains several antibodies, protective proteins that help the newborn develop resistance to disease.

Milk production and milk ejection are different activities. Prolactin supports making milk; oxytocin supports releasing it. Breastfeeding during early infant growth supplies milk to the newborn, and colostrum provides important early immune protection.

Glossary

• Gametogenesis — Formation of male and female reproductive cells in the primary sex organs.
• Seminiferous tubule — Highly coiled tube inside a testicular lobule where sperm formation occurs.
• Sertoli cell — Cell inside a seminiferous tubule that provides nutrition to developing germ cells.
• Leydig cell — Interstitial cell outside seminiferous tubules that synthesises and secretes testicular androgens.
• Spermiogenesis — Transformation of haploid spermatids into spermatozoa after completion of meiotic divisions.
• Spermiation — Release of formed sperms from the seminiferous tubules after spermiogenesis.
• Ovulation — Release of a secondary oocyte following rupture of a mature Graafian follicle.
• Corpus luteum — Structure formed from the remaining follicle after ovulation that secretes progesterone.
• Zona pellucida — Layer surrounding the female gamete that a sperm encounters during fertilisation.
• Morula — Early embryo consisting of eight to sixteen blastomeres formed through cleavage.
• Blastocyst — Early embryonic stage with an outer trophoblast and an inner cell mass.
• Implantation — Embedding of the blastocyst in the uterine endometrium after attachment.
• Placenta — Maternal and embryonic connection supporting exchange of substances and producing pregnancy hormones.
• Parturition — Delivery of the baby through vigorous uterine contractions at the end of pregnancy.
• Colostrum — Milk produced during the first few days of lactation, containing protective antibodies.

Common errors and misconceptions

• Misconception: Sertoli cells secrete androgens. Correct: Leydig cells secrete androgens; Sertoli cells nourish germ cells and respond to FSH.
• Misconception: Spermiogenesis and spermiation mean the same thing. Correct: Spermiogenesis transforms spermatids into sperms; spermiation releases sperms from the seminiferous tubules.
• Misconception: The released secondary oocyte has completed meiosis. Correct: Sperm entry induces completion of its second meiotic division during fertilisation.
• Misconception: Fertilisation normally occurs in the uterus. Correct: Fertilisation occurs in the ampullary region of the oviduct; implantation occurs in the uterine endometrium.
• Misconception: Every menstrual cycle lasts exactly 28 days and a missed period proves pregnancy. Correct: The average interval is about 28 or 29 days, and stress or poor health can also cause absent menstruation.
• Misconception: The hymen reliably proves sexual history. Correct: It may break through several activities or persist after intercourse; its presence or absence is not a reliable indicator.
• Misconception: All major organs are complete by 12 weeks. Correct: Most major organ systems are formed then, while development continues throughout pregnancy.
• Misconception: Oxytocin produces milk. Correct: Prolactin regulates milk formation, while oxytocin stimulates milk ejection.

Exam-style questions with model answers

Q1. Distinguish spermiogenesis from spermiation in humans. [2 marks]

1. Spermiogenesis is the transformation of haploid spermatids into spermatozoa, or sperms, following meiotic division.
2. Spermiation is the subsequent release of sperms from the seminiferous tubules after their heads have been associated with Sertoli cells.

Q2. Describe the hormonal control of human spermatogenesis, identifying the signal from the hypothalamus and the separate actions of LH and FSH. [3 marks]

1. At puberty, increased gonadotropin-releasing hormone from the hypothalamus stimulates the anterior pituitary to release luteinising hormone and follicle-stimulating hormone.
2. Luteinising hormone acts on Leydig cells, stimulating androgen synthesis and secretion; androgens in turn stimulate spermatogenesis in the testes.
3. Follicle-stimulating hormone acts on Sertoli cells, stimulating the secretion of factors that help spermiogenesis, the transformation of spermatids into sperms.

Q3. Trace human oogenesis from the primary oocyte to release at ovulation in four points. Include the timing of completion of meiosis II. [4 marks]

1. A primary oocyte begins meiosis during embryonic development and becomes temporarily arrested in prophase I, enclosed within a developing ovarian follicle.
2. Within the tertiary follicle, it grows and completes meiosis I unequally, producing a large haploid secondary oocyte and a small first polar body.
3. The secondary oocyte retains most nutrient-rich cytoplasm and acquires a zona pellucida. The tertiary follicle becomes a mature Graafian follicle.
4. Rupture of the Graafian follicle releases the secondary oocyte at ovulation. Its second meiotic division is completed when sperm entry induces it during fertilisation.

Q4. Menstrual flow lasts 3 to 5 days in the human cycle described here. Explain the four phases of this cycle, linking each phase with its main structural or hormonal event. [4 marks]

1. During the menstrual phase, breakdown of the endometrium and its blood vessels produces menstrual flow through the vagina, lasting 3 to 5 days.
2. During the follicular phase, rising LH and FSH stimulate follicular development and oestrogen secretion, while the uterine endometrium regenerates through proliferation.
3. During the ovulatory phase, a mid-cycle LH surge induces rupture of the mature Graafian follicle and release of the secondary oocyte.
4. During the luteal phase, the remaining follicle forms a corpus luteum that secretes progesterone to maintain the endometrium. Without fertilisation, its degeneration leads to another menstruation.

Q5. Each human gamete has 23 chromosomes, and a morula has 8 to 16 cells. Explain human fertilisation and the subsequent development up to implantation in six ordered points. Include the site, block to additional sperm entry, chromosome restoration and blastocyst organisation. [6 marks]

1. Fertilisation occurs in the ampullary region of the fallopian tube when sperms and the female gamete arrive there simultaneously.
2. A sperm contacts the zona pellucida, inducing membrane changes that prevent entry of additional sperms. Acrosomal enzymes help it enter the cytoplasm.
3. Sperm entry induces completion of meiosis II in the secondary oocyte, producing a haploid ovum and a second polar body.
4. The haploid sperm and ovum nuclei fuse, restoring the diploid number of 46 chromosomes in the zygote. Mitotic cleavage then produces blastomeres.
5. The embryo with 8 to 16 blastomeres is a morula. Further division produces a blastocyst with an outer trophoblast and inner cell mass.
6. The trophoblast attaches to the endometrium. Rapidly dividing uterine cells cover the blastocyst, embedding it in the lining; this is implantation.

Q6. State three functions of the human placenta: one concerning supply to the embryo, one concerning waste removal and one concerning hormones. [3 marks]

1. The placenta facilitates transfer of oxygen and nutrients from the maternal body to the developing embryo, supporting its growth during pregnancy.
2. It facilitates removal of carbon dioxide and other excretory waste materials produced by the embryo, providing an exchange connection with the mother.
3. It acts as an endocrine tissue, producing hormones including human chorionic gonadotropin, human placental lactogen, oestrogens and progestogens as part of the hormonal support for pregnancy.

Q7. Describe hormonal control of childbirth and lactation in five points, including the initiating signal, reinforcing contractions, milk formation and milk ejection. [5 marks]

1. Signals from the fully developed foetus and placenta induce mild uterine contractions, called the foetal ejection reflex, at the end of pregnancy.
2. This reflex triggers oxytocin release from the maternal pituitary. Oxytocin acts on uterine muscle and causes stronger contractions.
3. These contractions stimulate further oxytocin secretion. The mutually reinforcing process produces progressively stronger contractions until the baby is expelled through the birth canal.
4. Prolactin from the anterior pituitary regulates mammary gland growth and milk formation, supporting production of milk for the newborn.
5. Oxytocin stimulates milk ejection from mammary glands. It is synthesised in the hypothalamus and stored and released by the posterior pituitary.

Q8. A human ovum carries an X sex chromosome. A sperm may carry X or Y. Fusion with an X-bearing sperm produces XX, while fusion with a Y-bearing sperm produces XY. Explain in two points why blaming the mother for the baby's chromosomal sex is incorrect. [2 marks]

1. The ovum contributes X in either case, so its contribution does not distinguish between the two stated chromosome combinations.
2. The sperm supplies either X or Y, determining whether the zygote is XX or XY; the mother is therefore not responsible for this difference.

Key takeaways

• The testes produce sperms in seminiferous tubules; Sertoli cells nourish germ cells, while interstitial Leydig cells secrete androgens.
• Spermatogenesis begins at puberty, whereas oogenesis starts before birth and includes unequal meiotic divisions.
• The secondary oocyte is released at ovulation; sperm entry later induces completion of its second meiotic division.
• The LH surge triggers ovulation, and progesterone from the corpus luteum maintains the endometrium during the luteal phase.
• Fertilisation occurs in the ampullary region; cleavage produces a morula and blastocyst before implantation in the uterus.
• The placenta supports exchange of nutrients, gases and wastes and also acts as a hormone-producing tissue.
• Foetal milestones include heart formation after one month and, usually during the fifth month, first movements and head hair.
• Oxytocin reinforces uterine contractions and stimulates milk ejection, while prolactin regulates mammary gland growth and milk formation.

Test yourself

Why are the testes located in the scrotum?

The scrotum maintains the testes 2 to 2.5 degrees Celsius below normal internal body temperature, as required for spermatogenesis.

Which cells respond to LH and FSH in the testes?

LH acts on Leydig cells to stimulate androgen secretion. FSH acts on Sertoli cells to stimulate factors that help spermiogenesis.

What distinguishes a tertiary follicle from earlier follicular stages?

A tertiary follicle has a fluid-filled antrum, and its theca is organised into an inner theca interna and outer theca externa.

What are the three layers of the uterine wall?

They are the outer perimetrium, middle muscular myometrium and inner glandular endometrium, which undergoes changes during the menstrual cycle.

How does a morula differ from a blastocyst?

A morula is an embryo with 8 to 16 blastomeres. A blastocyst has an outer trophoblast and an inner cell mass.

Which features appear by the end of about 24 weeks?

The foetal body is covered with fine hair, the eyelids separate and eyelashes are formed by this stage.

Why is colostrum important for the newborn?

Colostrum, produced during the first few days of lactation, contains several antibodies that help the newborn develop resistance to disease.

What is the difference between prolactin and oxytocin in lactation?

Prolactin regulates mammary gland growth and milk formation. Oxytocin stimulates the ejection of milk from the mammary glands.