Exercises
Q1. Fill in the blanks
(a) Humans reproduce ________ (asexually/sexually) (b) Humans are ________ (oviparous, viviparous, ovoviviparous) (c) Fertilisation is ________ in humans (external/internal) (d) Male and female gametes are ________ (diploid/haploid) (e) Zygote is ________ (diploid/haploid) (f) The process of release of ovum from a mature follicle is called ________ (g) Ovulation is induced by a hormone called ________ (h) The fusion of male and female gametes is called ________ (i) Fertilisation takes place in ________ (j) Zygote divides to form ________ which is implanted in uterus. (k) The structure which provides vascular connection between foetus and uterus is called ________
Ans. (a) sexually (b) viviparous (c) internal (d) haploid (e) diploid (f) ovulation (g) luteinising hormone (LH) (h) fertilisation (i) the ampullary region of the fallopian tube (oviduct) (j) blastocyst (k) placenta
Draw a labelled diagram of male reproductive system.
Answer
The diagram (side view of the pelvic region) should label: testes inside the scrotum; epididymis along the back of each testis; vas deferens ascending into the abdomen and looping over the urinary bladder; seminal vesicle joining the vas deferens to form the ejaculatory duct; prostate gland below the bladder; paired bulbourethral (Cowper's) glands; urethra running through the penis to the urethral meatus; the glans penis covered by the foreskin; and the urinary bladder and ureter for orientation.
Draw a labelled diagram of female reproductive system.
Answer
The diagram (front sectional view) should label: a pair of ovaries held by ligaments; each fallopian tube (oviduct) with its infundibulum bearing fimbriae, the wider ampulla and the narrow isthmus; the uterus showing its three wall layers – perimetrium, myometrium and endometrium; the uterine cavity; the cervix with the cervical canal; and the vagina.
Write two major functions each of testis and ovary.
Answer
Testis: (i) produces male gametes (sperms) by spermatogenesis in the seminiferous tubules; (ii) its Leydig cells secrete male hormones (androgens, mainly testosterone).
Ovary: (i) produces female gametes (ova) by oogenesis; (ii) secretes female hormones, estrogen and progesterone (steroid hormones).
Describe the structure of a seminiferous tubule.
Answer
Each testis has about 250 compartments called testicular lobules, each containing one to three highly coiled seminiferous tubules, where sperms are made. The inside of a tubule is lined by two types of cells: male germ cells (spermatogonia), which divide by meiosis to form sperms, and Sertoli cells, which provide nutrition to the germ cells. The regions outside the tubules, the interstitial spaces, contain small blood vessels and interstitial (Leydig) cells that synthesise and secrete androgens, along with some immunologically competent cells.
What is spermatogenesis? Briefly describe the process of spermatogenesis.
Answer
Spermatogenesis is the formation of sperms in the seminiferous tubules of the testis; it begins at puberty.
Process: spermatogonia (2n, 46 chromosomes) on the inner wall of the tubules multiply by mitosis. Some of them, the primary spermatocytes (2n), undergo meiosis I to form two equal haploid secondary spermatocytes (n, 23 chromosomes). Each secondary spermatocyte undergoes meiosis II to give two haploid spermatids. So one primary spermatocyte
⇒ 2 secondary spermatocytes
⇒ 4 spermatids. The spermatids then change into spermatozoa (sperms) by spermiogenesis, and the sperm heads stay embedded in Sertoli cells until they are released (spermiation).
Name the hormones involved in regulation of spermatogenesis.
Answer
GnRH (gonadotropin releasing hormone) from the hypothalamus; the pituitary gonadotropins LH (luteinising hormone) and FSH (follicle stimulating hormone); and androgens (testosterone). GnRH stimulates release of LH and FSH. LH acts on Leydig cells to make androgens, which stimulate spermatogenesis. FSH acts on Sertoli cells, making them secrete factors that help spermiogenesis.
Define spermiogenesis and spermiation.
Answer
Spermiogenesis: the transformation of non-motile spermatids into motile spermatozoa (sperms). Spermiation: the release of sperms from the Sertoli cells into the cavity (lumen) of the seminiferous tubule.
Draw a labelled diagram of sperm.
Answer
The sperm diagram shows four regions inside a plasma membrane: the head, with an elongated haploid nucleus capped in front by the acrosome; the short neck; the middle piece, which has many mitochondria; and a long tail.
What are the major components of seminal plasma?
Answer
Seminal plasma is the secretion of the seminal vesicles, prostate and bulbourethral glands. It is rich in fructose, calcium and certain enzymes. The bulbourethral secretion also lubricates the penis.
What are the major functions of male accessory ducts and glands?
Answer
Accessory ducts (rete testis, vasa efferentia, epididymis and vas deferens) store sperms and carry them from the testis to the outside through the urethra. Accessory glands (paired seminal vesicles, a prostate and paired bulbourethral glands) secrete the seminal plasma, which is rich in fructose, calcium and enzymes; it provides a medium and nourishment for the sperms and helps their motility, and the bulbourethral secretion lubricates the penis.
What is oogenesis? Give a brief account of oogenesis.
Answer
Oogenesis is the formation of a mature female gamete (ovum) in the ovary.
It starts during embryonic (foetal) development, when millions of oogonia (egg mother cells) form in each ovary; no new oogonia are added after birth. They enter meiosis I and become primary oocytes, which are temporarily arrested in prophase I. Each primary oocyte is surrounded by a layer of granulosa cells to form a primary follicle. Many follicles degenerate from birth to puberty, so only about 60,000–80,000 primary follicles remain in each ovary at puberty. Primary follicles gain more layers of granulosa cells and a new theca layer, becoming secondary follicles, which then develop a fluid-filled cavity called the antrum, becoming tertiary follicles (theca splits into theca interna and theca externa). At this stage the primary oocyte completes meiosis I by unequal division, forming a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle matures into a Graafian follicle; the secondary oocyte forms a new membrane, the zona pellucida. The Graafian follicle ruptures to release the secondary oocyte (ovulation). Meiosis II is completed only if a sperm enters, forming the ovum and a second polar body.
Draw a labelled diagram of a section through ovary.
Answer
The section should show, around the ovarian stroma, follicles at successive stages: primary follicle (oocyte with one granulosa layer), secondary follicle, tertiary follicle with an antrum, a mature Graafian follicle, the ovum being released at ovulation, a corpus luteum formed from the ruptured follicle, and a degenerating corpus albicans; also blood vessels in the stroma.
Draw a labelled diagram of a Graafian follicle?
Answer
Labels: outer theca externa and inner theca interna; layers of granulosa cells; a large fluid-filled antrum; the secondary oocyte surrounded by the zona pellucida and a cluster of granulosa cells (corona radiata), attached to the follicle wall.
Name the functions of the following: (a) Corpus luteum (b) Endometrium (c) Acrosome (d) Sperm tail (e) Fimbriae
Answer
(a) Corpus luteum secretes large amounts of progesterone, which is essential for maintaining the endometrium. (b) Endometrium undergoes cyclical changes during the menstrual cycle and becomes the site of implantation of the blastocyst. (c) Acrosome contains enzymes that help the sperm to penetrate the ovum during fertilisation. (d) Sperm tail gives the sperm motility needed to swim to the egg. (e) Fimbriae help in collecting the ovum after ovulation and guiding it into the fallopian tube.
Identify True/False statements. Correct each false statement to make it true. (a) Androgens are produced by Sertoli cells. (b) Spermatozoa get nutrition from Sertoli cells. (c) Leydig cells are found in ovary. (d) Leydig cells synthesise androgens. (e) Oogenesis takes place in corpus luteum. (f) Menstrual cycle ceases during pregnancy. (g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience.
Answer
(a) False – androgens are produced by Leydig (interstitial) cells. (b) True. (c) False – Leydig cells are found in the testis (interstitial spaces). (d) True. (e) False – oogenesis takes place in the ovary (in ovarian follicles). (f) True. (g) True.
What is menstrual cycle? Which hormones regulate menstrual cycle?
Answer
The reproductive cycle of female primates (monkeys, apes, humans) is called the menstrual cycle. It begins at puberty (menarche), repeats on average every 28/29 days and continues till menopause at about 50 years. It has a menstrual phase (days 1–5, shedding of the endometrium), a follicular phase (follicle grows and endometrium regenerates), ovulation (around day 14) and a luteal phase (corpus luteum forms and secretes progesterone).
Hormones: pituitary LH and FSH (both rise during the follicular phase and peak mid-cycle; the LH surge causes ovulation), and the ovarian hormones estrogen (from growing follicles) and progesterone (from corpus luteum).
What is parturition? Which hormones are involved in induction of parturition?
Answer
Parturition is the process of delivery of the foetus (childbirth) through vigorous contractions of the uterine muscles at the end of pregnancy (about 9 months in humans). It is set off by a complex neuroendocrine mechanism: signals from the fully developed foetus and placenta cause mild uterine contractions (foetal ejection reflex), which trigger release of oxytocin from the maternal pituitary. Oxytocin causes stronger uterine contractions, which stimulate still more oxytocin release, until the baby is expelled. Hormones involved: oxytocin, together with signals from the foetus and placenta (e.g. cortisol from the foetus and placental estrogens).
In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?
Answer
Sex of the baby is decided by the father's sperm, not the mother. A human female has XX chromosomes, so all her ova carry an X. A male has XY, so half of his sperms carry X and half carry Y. If an X-bearing sperm fertilises the ovum, the zygote is XX and a girl develops; if a Y-bearing sperm fertilises it, the zygote is XY and a boy develops. Since the mother always contributes X, blaming women for daughters is scientifically wrong.
How many eggs are released by a human ovary in a month? How many eggs do you think would have been released if the mother gave birth to identical twins? Would your answer change if the twins born were fraternal?
Answer
Normally only one egg (ovum) is released in each menstrual cycle (by one of the two ovaries alternately). Identical twins develop from one fertilised egg whose embryo splits into two, so only one egg was released. Yes, the answer changes for fraternal twins: they develop from two separate eggs fertilised by two different sperms, so two eggs were released.
How many eggs do you think were released by the ovary of a female dog which gave birth to 6 puppies?
Answer
Dogs are polyovulatory and each puppy usually develops from a separate fertilised egg, so at least 6 eggs were released (assuming no identical twins among them).
Important Extra Questions
What is the function of the zona pellucida during fertilisation?
Answer
When one sperm contacts it, changes are induced in the membrane that block entry of additional sperms, ensuring only one sperm fertilises the ovum.
Why are testes located outside the abdominal cavity?
Answer
In the scrotum they are kept at a temperature about 2–2.5 °C lower than normal body temperature, which is necessary for spermatogenesis.
Name the hormones produced only during pregnancy by the placenta.
Answer
Human chorionic gonadotropin (hCG), human placental lactogen (hPL) and relaxin (relaxin is also secreted by the ovary in later pregnancy). The placenta also makes estrogens and progestogens.
What is colostrum and why is it important?
Answer
The milk produced in the first few days after birth; it contains antibodies (IgA) that give passive immunity to the newborn.
Differentiate between trophoblast and inner cell mass.
Answer
In the blastocyst, the trophoblast is the outer layer that attaches to the endometrium; the inner cell mass is the group of cells inside it that differentiates into the embryo.