Human Reproduction
Welcome to HSLC Guru! In this article, we provide a complete study guide for Class 12 Biology Chapter 3 — Human Reproduction from the ASSEB syllabus. This English-medium resource covers the male and female reproductive systems, gametogenesis, the menstrual cycle, fertilisation, embryonic development, pregnancy, parturition, and lactation. The notes are followed by a comprehensive question bank, MCQs, fill-in-the-blanks, true/false statements, and a glossary table to help you prepare effectively for the ASSEB Class 12 Biology examination.
Summary
Humans are sexually reproducing and viviparous organisms. The male reproductive system consists of a pair of testes, accessory ducts, glands, and the external genitalia. The testes are located in the scrotum, which maintains a temperature 2–2.5 °C lower than body temperature, essential for spermatogenesis. Each testis is divided into about 250 testicular lobules, and each lobule contains 1–3 highly coiled seminiferous tubules in which sperms are produced. The seminiferous tubules are lined by two types of cells — spermatogonia (male germ cells) and Sertoli cells. Sertoli cells provide nutrition to the germ cells. The regions outside the seminiferous tubules contain Leydig cells (interstitial cells) that synthesise and secrete the male sex hormone testosterone. The accessory ducts include rete testis, vasa efferentia, epididymis, and vas deferens. The vas deferens joins the duct of the seminal vesicle to form the ejaculatory duct, which opens into the urethra. The accessory glands include a pair of seminal vesicles, a prostate gland, and a pair of bulbourethral glands (Cowper’s glands). Their secretions, together with sperms, constitute the seminal plasma.
Spermatogenesis is the process of formation of sperms in the seminiferous tubules. Spermatogonia (2n) multiply by mitosis to form primary spermatocytes (2n), which undergo meiosis I to form secondary spermatocytes (n) and meiosis II to form spermatids (n). Spermatids are transformed into spermatozoa (sperms) through spermiogenesis. The release of sperms from seminiferous tubules is called spermiation. Spermatogenesis is initiated at puberty due to a significant increase in GnRH from the hypothalamus, which stimulates the anterior pituitary to secrete FSH and LH. A sperm has a head, neck, middle piece, and tail. The head contains an acrosome (filled with enzymes that help in fertilisation) and an elongated haploid nucleus. The middle piece possesses many mitochondria that produce energy for sperm movement. The female reproductive system consists of a pair of ovaries, a pair of fallopian tubes (oviducts), uterus, cervix, vagina, and external genitalia. The ovaries produce ova and steroid hormones (oestrogen and progesterone). Each ovary is covered by germinal epithelium and contains primordial follicles. The fallopian tube extends from the ovary toward the uterus and is divided into infundibulum, ampulla, and isthmus. The uterus has three layers — perimetrium, myometrium, and endometrium. The accessory glands include a pair of Bartholin’s glands located on either side of the vaginal opening, which secrete lubricating mucus.
Oogenesis is the process of formation of a mature ovum in the ovary. It begins during embryonic development when oogonia (2n) multiply by mitosis. The oogonia enter prophase I of meiosis to form primary oocytes (2n), which get surrounded by granulosa cells to form primary follicles. The primary follicle becomes a secondary follicle and then a tertiary follicle. At this stage, the primary oocyte completes meiosis I to form a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle changes into the mature Graafian follicle, which ruptures to release the secondary oocyte (ovulation). The menstrual cycle in human females begins at menarche (puberty) and stops at menopause. The cycle is about 28 days long and has four phases — menstrual phase (1–5 days, breakdown of endometrium and bleeding), follicular/proliferative phase (6–13 days, follicles grow and endometrium regenerates under the influence of FSH and oestrogen), ovulatory phase (around the 14th day, rapid surge of LH causes the rupture of the Graafian follicle and release of the ovum), and luteal/secretory phase (15–28 days, the ruptured follicle becomes the corpus luteum, which secretes progesterone to maintain the endometrium for implantation).
Fertilisation takes place in the ampullary–isthmic junction of the fallopian tube. The fusion of a sperm with the ovum forms the zygote (2n). The zygote undergoes mitotic divisions called cleavage while moving through the fallopian tube to form 2-, 4-, 8-, 16-celled blastomeres. The embryo with 8 to 16 blastomeres is called the morula, which divides further to form the blastocyst. The blastocyst gets attached to the endometrium of the uterus — this process is called implantation, leading to pregnancy. After implantation, finger-like projections called chorionic villi appear on the trophoblast and form the structural and functional unit between the developing embryo and the maternal body called the placenta. The placenta facilitates the supply of oxygen and nutrients and the removal of carbon dioxide and excretory wastes. It also acts as an endocrine tissue, producing hormones such as hCG, hPL, oestrogens, and progesterone. Parturition is the process of delivery of the foetus, induced by the foetal ejection reflex involving the release of oxytocin from the maternal pituitary, causing strong uterine contractions. After parturition, the mammary glands produce milk in a process called lactation. The milk produced during the initial few days, called colostrum, contains several antibodies essential for the new-born baby.
Question and Answers
1-Mark Questions
Q1. Where are the testes located in the human male?
Answer: The testes are located in the scrotum, outside the abdominal cavity.
Q2. What is the function of Leydig cells?
Answer: Leydig cells synthesise and secrete the male sex hormone testosterone.
Q3. Name the cells that provide nutrition to the developing sperms.
Answer: Sertoli cells provide nutrition to the developing sperms.
Q4. Where does fertilisation occur in human females?
Answer: Fertilisation occurs in the ampullary–isthmic junction of the fallopian tube.
Q5. Define ovulation.
Answer: Ovulation is the rupture of the Graafian follicle and the release of the secondary oocyte (ovum) from the ovary.
Q6. What is the function of the acrosome in a sperm?
Answer: The acrosome contains hydrolytic enzymes that help the sperm penetrate the egg during fertilisation.
Q7. Name the hormone responsible for the LH surge during the menstrual cycle.
Answer: Luteinising hormone (LH) shows a sudden surge near the middle of the cycle and induces ovulation.
Q8. What is colostrum?
Answer: Colostrum is the yellowish milk produced by mammary glands during the initial few days after parturition; it is rich in antibodies.
Q9. What is implantation?
Answer: Implantation is the attachment of the blastocyst to the endometrium of the uterus.
Q10. Name the hormone secreted by the placenta during pregnancy.
Answer: The placenta secretes human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens, and progesterone.
2–3 Marks Questions
Q1. Differentiate between spermatogenesis and oogenesis.
Answer: Spermatogenesis is the process of formation of sperms in the seminiferous tubules of the testes; it begins at puberty and produces four functional sperms from one primary spermatocyte. Oogenesis is the process of formation of an ovum in the ovaries; it begins during the embryonic stage and produces only one functional ovum and three polar bodies from one primary oocyte.
Q2. Describe the structure of a human sperm.
Answer: A human sperm is a microscopic structure with four parts — head, neck, middle piece, and tail. The head contains an elongated haploid nucleus and an anterior cap-like acrosome filled with enzymes. The neck has proximal and distal centrioles. The middle piece is rich in mitochondria that supply energy for movement. The tail is the locomotory part that propels the sperm.
Q3. What are the accessory glands of the male reproductive system, and what is their role?
Answer: The accessory glands include a pair of seminal vesicles, a prostate gland, and a pair of bulbourethral glands (Cowper’s glands). Their combined secretions form the seminal plasma, which is rich in fructose, calcium, and certain enzymes. These secretions nourish the sperms, neutralise vaginal acidity, and provide lubrication.
Q4. Explain the role of FSH and LH in the menstrual cycle.
Answer: FSH (Follicle Stimulating Hormone) stimulates the growth of follicles in the ovary and the secretion of oestrogen during the follicular phase. LH (Luteinising Hormone) shows a sudden surge near the middle of the cycle on day 14, which induces ovulation. After ovulation, LH stimulates the formation of the corpus luteum, which secretes progesterone to maintain the endometrium.
Q5. What is cleavage? Describe its outcome.
Answer: Cleavage is a series of rapid mitotic cell divisions that the zygote undergoes while moving through the fallopian tube toward the uterus. It produces 2, 4, 8, and 16 daughter cells called blastomeres. The 8–16 celled stage is called the morula, which further develops into a blastocyst with an inner cell mass and a trophoblast layer.
Q6. What is the placenta? Mention any two of its functions.
Answer: The placenta is a structural and functional connection between the developing embryo and the maternal body, formed by chorionic villi and uterine tissue. Its functions include: (i) supplying oxygen and nutrients to the foetus and removing carbon dioxide and excretory wastes; (ii) acting as an endocrine tissue secreting hormones like hCG, hPL, oestrogens, and progesterone to maintain pregnancy.
5–7 Marks Questions
Q1. Describe the structure and functions of the male reproductive system in humans.
Answer: The male reproductive system consists of primary sex organs, accessory ducts, accessory glands, and external genitalia.
(i) Testes — A pair of testes is located in the scrotum, which maintains a lower temperature than the body for spermatogenesis. Each testis contains seminiferous tubules lined with spermatogonia and Sertoli cells. Leydig cells outside the tubules secrete testosterone.
(ii) Accessory ducts — These include rete testis, vasa efferentia, epididymis, vas deferens, and ejaculatory duct. The epididymis stores and matures sperms; vas deferens carries sperms; ejaculatory duct opens into the urethra.
(iii) Urethra — Originates from the urinary bladder, passes through the penis, and serves as a common passage for urine and semen.
(iv) Accessory glands — A pair of seminal vesicles, a prostate gland, and a pair of bulbourethral glands secrete fluids that nourish and protect sperms. Together with sperms, these secretions form semen.
(v) Penis — The external genital organ that delivers sperms into the female reproductive tract during copulation.
Q2. Describe the structure of the female reproductive system in humans.
Answer: The female reproductive system consists of:
(i) Ovaries — A pair of ovaries lies in the lower abdomen on either side of the uterus. Each ovary is covered by germinal epithelium and contains primordial follicles. Ovaries produce ova as well as oestrogen and progesterone.
(ii) Fallopian tubes (oviducts) — A pair of tubes about 10–12 cm long, extending from the ovary to the uterus. Each tube has a funnel-shaped infundibulum with fimbriae, the ampulla (widest part where fertilisation occurs), and the isthmus (narrow part connecting to the uterus).
(iii) Uterus — A pear-shaped, single, hollow organ supported by ligaments. Its wall has three layers — perimetrium (outer), myometrium (middle smooth muscle layer responsible for contractions during parturition), and endometrium (inner glandular layer that undergoes cyclical changes during the menstrual cycle).
(iv) Cervix and Vagina — The uterus opens into the vagina through a narrow cervix. The cavity of the cervix is called the cervical canal, which along with the vagina forms the birth canal.
(v) External genitalia — Includes mons pubis, labia majora, labia minora, hymen, and clitoris. A pair of Bartholin’s glands on either side of the vagina secretes lubricating mucus.
Q3. Explain the menstrual cycle in human females with reference to its phases and hormones.
Answer: The menstrual cycle is a cyclic event of about 28 days that occurs in the reproductive life of human females from menarche to menopause. It has four phases:
(i) Menstrual phase (Day 1–5) — Marked by the breakdown of the endometrial lining of the uterus, leading to bleeding. It occurs only when the ovum is not fertilised. Levels of oestrogen and progesterone fall.
(ii) Follicular/Proliferative phase (Day 6–13) — Primary follicles in the ovary grow into Graafian follicles. Endometrium regenerates by proliferation. FSH and LH are gradually secreted, and oestrogen levels rise.
(iii) Ovulatory phase (Day 14) — A rapid surge of LH (LH surge) and a smaller surge of FSH cause the rupture of the Graafian follicle, releasing the secondary oocyte. This is called ovulation.
(iv) Luteal/Secretory phase (Day 15–28) — The ruptured Graafian follicle transforms into the corpus luteum, which secretes large amounts of progesterone. Progesterone maintains the endometrium and prepares it for implantation. If fertilisation does not occur, the corpus luteum degenerates, and a new menstrual cycle begins.
Q4. Describe spermatogenesis and oogenesis in detail.
Answer: Spermatogenesis is the process of formation of sperms in the seminiferous tubules of the testes. It begins at puberty under the influence of GnRH and gonadotropins (FSH and LH).
Stages: (i) Spermatogonia (2n) → mitosis → primary spermatocytes (2n). (ii) Primary spermatocytes → meiosis I → two secondary spermatocytes (n). (iii) Secondary spermatocytes → meiosis II → four spermatids (n). (iv) Spermiogenesis: spermatids transform into sperms. (v) Spermiation: sperms are released from the seminiferous tubules.
Oogenesis is the formation of an ovum in the ovary. It is initiated during the embryonic stage.
Stages: (i) Oogonia (2n) divide by mitosis to form a few million gamete mother cells. (ii) These cells start meiosis I and stop at prophase I, becoming primary oocytes (2n). (iii) At puberty, primary oocytes complete meiosis I, forming a large secondary oocyte (n) and a small first polar body. (iv) The secondary oocyte begins meiosis II and pauses at metaphase II. It is released from the ovary at this stage (ovulation). (v) Meiosis II is completed only after fertilisation, producing a haploid ovum and a second polar body.
Q5. Describe fertilisation, implantation, and the formation of the placenta in humans.
Answer: Fertilisation: The fusion of a sperm with an ovum is called fertilisation. It occurs in the ampulla of the fallopian tube. Sperms reach the ovum through the cervix and uterus. The acrosomal enzymes of the sperm dissolve the zona pellucida, allowing one sperm to enter. The cortical reaction prevents polyspermy. The haploid nuclei of sperm and ovum fuse to form a diploid zygote.
Cleavage and blastocyst formation: The zygote undergoes cleavage (rapid mitotic divisions) while moving through the fallopian tube. It forms 2-, 4-, 8-, 16-celled blastomeres, then the morula. The morula divides further to form a blastocyst with an outer trophoblast and an inner cell mass.
Implantation: The blastocyst reaches the uterus, where the trophoblast attaches to the endometrium. The inner cell mass differentiates into the embryo. This attachment of the blastocyst to the endometrium is called implantation, which leads to pregnancy.
Placenta: After implantation, finger-like projections called chorionic villi appear on the trophoblast and are surrounded by uterine tissue and maternal blood. These villi together with the uterine tissue form the placenta. The placenta is connected to the embryo through the umbilical cord. It supplies oxygen and nutrients, removes waste products, and acts as an endocrine tissue producing hCG, hPL, oestrogen, and progesterone to maintain pregnancy.
Multiple Choice Questions (MCQs)
Q1. The site of fertilisation in human females is:
a) Uterus
b) Vagina
c) Ampulla of fallopian tube
d) Ovary
Answer: c) Ampulla of fallopian tube
Q2. Leydig cells produce:
a) Oestrogen
b) Testosterone
c) FSH
d) Progesterone
Answer: b) Testosterone
Q3. The number of mitochondria is highest in which part of the sperm?
a) Head
b) Neck
c) Middle piece
d) Tail
Answer: c) Middle piece
Q4. Ovulation in human female is triggered by a sudden surge of:
a) FSH
b) LH
c) Oestrogen
d) Progesterone
Answer: b) LH
Q5. The corpus luteum mainly secretes:
a) Oestrogen
b) FSH
c) Progesterone
d) Testosterone
Answer: c) Progesterone
Q6. The 8–16 celled stage of the embryo is called:
a) Blastocyst
b) Morula
c) Gastrula
d) Zygote
Answer: b) Morula
Q7. Which hormone is detected in the urine to confirm pregnancy?
a) hCG
b) FSH
c) LH
d) Progesterone
Answer: a) hCG
Q8. Spermatogenesis takes place in:
a) Epididymis
b) Vas deferens
c) Seminiferous tubules
d) Prostate gland
Answer: c) Seminiferous tubules
Q9. Bartholin’s glands are present in:
a) Male reproductive system
b) Female reproductive system
c) Both
d) None
Answer: b) Female reproductive system
Q10. The first milk produced after parturition that is rich in antibodies is:
a) Lactose milk
b) Whey
c) Colostrum
d) Casein
Answer: c) Colostrum
Fill in the Blanks
Q1. The male sex hormone testosterone is secreted by ________ cells.
Answer: Leydig (interstitial)
Q2. The release of sperms from the seminiferous tubules is known as ________.
Answer: Spermiation
Q3. The widest portion of the fallopian tube is called ________.
Answer: Ampulla
Q4. Implantation is the attachment of the ________ to the endometrium of the uterus.
Answer: Blastocyst
Q5. The hormone responsible for milk ejection during lactation is ________.
Answer: Oxytocin
True or False
Q1. The testes are located inside the abdominal cavity in adult males.
Answer: False (they are located in the scrotum, outside the abdominal cavity).
Q2. Oogenesis begins during the embryonic stage of a female.
Answer: True
Q3. Fertilisation occurs in the uterus.
Answer: False (it occurs in the ampulla of the fallopian tube).
Q4. The corpus luteum secretes large amounts of progesterone.
Answer: True
Q5. The placenta does not produce any hormone.
Answer: False (it secretes hCG, hPL, oestrogen, and progesterone).
Glossary
| Term | Meaning |
|---|---|
| Scrotum | External pouch that houses the testes outside the abdominal cavity. |
| Seminiferous tubules | Highly coiled tubules in the testes where sperms are produced. |
| Sertoli cells | Cells in the seminiferous tubules that nourish developing sperms. |
| Leydig cells | Interstitial cells of testes that secrete testosterone. |
| Epididymis | Coiled tube where sperms mature and are stored. |
| Vas deferens | Duct that transports sperms from the epididymis to the ejaculatory duct. |
| Ejaculatory duct | Short duct formed by the union of the vas deferens and seminal vesicle duct. |
| Seminal vesicles | Paired accessory glands that secrete fluid rich in fructose. |
| Prostate gland | Single gland that contributes alkaline fluid to semen. |
| Bulbourethral glands | Cowper’s glands that secrete lubricating mucus. |
| Spermatogenesis | Process of formation of sperms in the testes. |
| Spermiogenesis | Transformation of spermatids into sperms. |
| Spermiation | Release of sperms from the seminiferous tubules. |
| Acrosome | Cap-like structure on the sperm head containing enzymes for fertilisation. |
| Oogenesis | Process of formation of an ovum in the ovary. |
| Primordial follicle | Earliest stage of follicle in the ovary surrounding the primary oocyte. |
| Graafian follicle | Mature ovarian follicle that releases the ovum during ovulation. |
| Ovulation | Release of the secondary oocyte from the ovary. |
| Bartholin’s glands | Female accessory glands that secrete lubricating mucus near the vagina. |
| Menarche | The first menstrual cycle in a female at puberty. |
| Menopause | The cessation of menstrual cycles in a woman. |
| Corpus luteum | Endocrine structure formed from the ruptured Graafian follicle, secretes progesterone. |
| FSH | Follicle Stimulating Hormone; stimulates follicle development. |
| LH | Luteinising Hormone; surge causes ovulation. |
| Cleavage | Series of mitotic divisions in the zygote. |
| Morula | Solid ball of 8–16 blastomeres. |
| Blastocyst | Stage of embryo with trophoblast and inner cell mass. |
| Implantation | Attachment of the blastocyst to the endometrium. |
| Placenta | Structural and functional unit between mother and foetus. |
| hCG | Human chorionic gonadotropin secreted by the placenta. |
| Parturition | The process of childbirth. |
| Oxytocin | Hormone causing uterine contractions during parturition. |
| Lactation | Production of milk by the mammary glands after parturition. |
| Colostrum | Yellowish first milk rich in antibodies. |