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NCERT Solutions · Class 12 Biology Human Reproduction

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Exercises 2.11–2.21 (part 2 of 2)

  1. Exercise 2.11

    What are the major functions of male accessory ducts and glands?

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    Accessory ducts (rete testis, vasa efferentia, epididymis, vas deferens)
    Store and transport the sperms from the testis to the outside through the urethra.
    The secretions of the epididymis and vas deferens (with those of the seminal vesicle and prostate) are essential for the maturation and motility of sperms.
    Accessory glands (seminal vesicles, prostate, bulbourethral glands)
    Their secretions form the seminal plasma, rich in fructose, calcium and enzymes — the fluid medium that nourishes and carries the sperms.
    The bulbourethral secretion lubricates the penis.
    The functions of both the ducts and the glands are maintained by the testicular hormones, the androgens.

  2. Exercise 2.12

    What is oogenesis? Give a brief account of oogenesis.

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    Oogenesis is the process of formation of a mature female gamete (ovum) in the ovary.
    It is initiated during embryonic development, when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no oogonia are formed or added after birth.
    These cells begin division, enter prophase-I of meiosis and are temporarily arrested there — they are now the primary oocytes.
    Each primary oocyte is surrounded by a layer of granulosa cells, forming the primary follicle.
    Many primary follicles degenerate between birth and puberty, so that only $\displaystyle 60,000$–$\displaystyle 80,000$ are left in each ovary at puberty.
    A primary follicle acquires more layers of granulosa cells and a new theca to become a secondary follicle.
    The secondary follicle transforms into a tertiary follicle, marked by a fluid-filled cavity, the antrum; the theca is now organised into an inner theca interna and an outer theca externa.
    At this stage the primary oocyte grows in size and completes its first meiotic division. The division is unequal, giving a large haploid secondary oocyte and a tiny first polar body; the secondary oocyte retains the bulk of the nutrient-rich cytoplasm.
    The tertiary follicle becomes the mature or Graafian follicle, and the secondary oocyte forms a new membrane around itself, the zona pellucida.
    The Graafian follicle ruptures to release the secondary oocyte from the ovary — ovulation.
    The second meiotic division is completed only when a sperm enters, and it too is unequal, giving a second polar body and the haploid ovum (ootid).

  3. Exercise 2.13

    Draw a labelled diagram of a section through ovary.

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    NCERT_Solution_Class12_Biology_Ch2_Q2-13
    A section through the ovary shows, from the outside in: a thin germinal epithelium, the ovarian stroma it encloses, and the stroma divided into a peripheral cortex and an inner medulla.
    The ovarian follicles, at different stages of development, are embedded in the stroma of the cortex.
    The follicle stages to show, in order outward from the centre of the cortex: primary follicle, secondary follicle, tertiary follicle (which develops a fluid-filled cavity, the antrum, and a theca organised into an inner theca interna and an outer theca externa), and the mature Graafian follicle.
    Inside the tertiary and Graafian follicle, show the secondary oocyte surrounded by the zona pellucida.
    After ovulation releases the oocyte, show the ruptured follicle becoming the corpus luteum.
    The ovary is attached to the pelvic wall and uterus by ligaments; a stalk enters at the hilum.
    What your diagram must show:
    Draw an oval, about twice as wide as it is tall, and outline it with a thin line for the germinal epithelium.
    Divide the interior into an outer band (cortex) and an inner region (medulla), and label both.
    Around the cortex, arrange the follicle stages in a clockwise sequence so the diagram reads as a life cycle: primary, secondary, tertiary (with antrum and theca), Graafian, ovulation, corpus luteum.
    Label the secondary oocyte and zona pellucida inside the Graafian follicle, and the antrum, theca interna and theca externa on the tertiary one.
    Mark the hilum where the ligament enters.

  4. Exercise 2.14

    Draw a labelled diagram of a Graafian follicle?

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    Draw the Graafian follicle as a large rounded follicle in which a big fluid-filled cavity, the antrum, takes up most of the space.
    Place the secondary oocyte to one side, sitting on a mound of granulosa cells and pushed against the follicle wall.
    Draw the new membrane around the oocyte and label it the zona pellucida.
    Line the antrum with several layers of granulosa cells.
    Outside the granulosa layer draw the theca in two layers — an inner theca interna and an outer theca externa.
    Show the follicle bulging at the surface of the ovary, ready to rupture and release the oocyte at ovulation.
    Labels required: antrum, secondary oocyte, zona pellucida, granulosa cells, theca interna, theca externa.

  5. Exercise 2.15

    Name the functions of the following:
    (a)
    Corpus luteum
    (b)
    Endometrium
    (c)
    Acrosome
    (d)
    Sperm tail
    (e)
    Fimbriae

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    (a) Corpus luteum — secretes large amounts of progesterone, which maintains the endometrium in the state needed for implantation of the fertilised ovum and for the other events of pregnancy.
    (b) Endometrium — the inner glandular lining of the uterine cavity; it undergoes cyclical changes during the menstrual cycle, regenerates by proliferation in the follicular phase, and receives the blastocyst at implantation. Its breakdown produces the menstrual flow.
    (c) Acrosome — the cap-like structure over the front of the sperm head, filled with enzymes; its secretions help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
    (d) Sperm tail — its movement gives the sperm motility, which is essential for fertilisation.
    (e) Fimbriae — the finger-like projections at the edge of the infundibulum; they help in collection of the ovum after ovulation.

  6. Exercise 2.16

    Identify True/False statements. Correct each false statement to make it true.
    (a)
    Androgens are produced by Sertoli cells. (True/False)
    (b)
    Spermatozoa get nutrition from Sertoli cells. (True/False)
    (c)
    Leydig cells are found in ovary. (True/False)
    (d)
    Leydig cells synthesise androgens. (True/False)
    (e)
    Oogenesis takes place in corpus luteum. (True/False)
    (f)
    Menstrual cycle ceases during pregnancy. (True/False)
    (g)
    Presence or absence of hymen is not a reliable indicator of virginity or sexual experience. (True/False)

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    (a) False. Androgens are produced by the Leydig (interstitial) cells, not by the Sertoli cells.
    (b) True. Sertoli cells provide nutrition to the germ cells, and the sperm heads remain embedded in them until spermiation.
    (c) False. Leydig cells are found in the testis, in the interstitial spaces outside the seminiferous tubules.
    (d) True. Leydig cells synthesise and secrete the testicular hormones called androgens.
    (e) False. Oogenesis takes place in the ovary, within the ovarian follicles; the corpus luteum is only the remnant of the ruptured Graafian follicle and secretes progesterone.
    (f) True. During pregnancy all events of the menstrual cycle stop and there is no menstruation.
    (g) True. The hymen can be torn by a fall or jolt, a tampon, or sports such as cycling or horse riding, and in some women it persists even after coitus.

  7. Exercise 2.17

    What is menstrual cycle? Which hormones regulate menstrual cycle?

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    The menstrual cycle is the reproductive cycle of female primates (monkeys, apes and human beings) — the whole cycle of events from one menstruation until the next, repeated on average every $\displaystyle 28$/$\displaystyle 29$ days.
    It begins at puberty with the first menstruation, called menarche, and ceases around $\displaystyle 50$ years of age at menopause; one ovum is released in the middle of each cycle.
    Menstrual phase ($\displaystyle 3$–$\displaystyle 5$ days): the endometrial lining of the uterus and its blood vessels break down and pass out through the vagina. It occurs only if the released ovum is not fertilised.
    Follicular phase: the primary follicles grow into a fully mature Graafian follicle, and the endometrium regenerates by proliferation.
    Ovulatory phase: LH and FSH reach a peak about the 14th day; the LH surge ruptures the Graafian follicle and releases the ovum.
    Luteal phase: the remaining parts of the Graafian follicle become the corpus luteum, whose progesterone maintains the endometrium; in the absence of fertilisation it degenerates, the endometrium disintegrates and a new cycle begins.
    Hormones that regulate it
    The pituitary gonadotropins LH and FSH.
    The ovarian hormones — estrogens, secreted by the growing follicles, and progesterone, secreted by the corpus luteum.

  8. Exercise 2.18

    What is parturition? Which hormones are involved in induction of parturition?

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    Parturition is the process of delivery of the foetus (childbirth) at the end of the roughly nine-month gestation period, brought about by vigorous contraction of the uterus.
    It is induced by a complex neuroendocrine mechanism:
    Signals originate from the fully developed foetus and the placenta and induce mild uterine contractions, the foetal ejection reflex.
    This triggers the release of oxytocin from the maternal pituitary.
    Oxytocin acts on the uterine muscle and causes stronger contractions, which in turn stimulate still more oxytocin secretion.
    This stimulatory reflex builds until the baby is expelled through the birth canal; the placenta is expelled soon after.
    Hormones involved in inducing parturition: oxytocin above all, together with cortisol and estrogens; relaxin, secreted by the ovary in the later phase of pregnancy, is also present at this time.

  9. Exercise 2.19

    In our society the women are often blamed for giving birth to daughters. Can you explain why this is not correct?

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    Blaming the woman is scientifically wrong, because the sex of the baby is determined by the father, not by the mother.
    The chromosome pattern in the human female is XX, so every ovum she produces carries an X chromosome — she has no other kind to give.
    The chromosome pattern in the male is XY, so $\displaystyle 50$ per cent of his sperms carry X and the other $\displaystyle 50$ per cent carry Y.
    If an X-bearing sperm fertilises the ovum, the zygote is XX and a female baby develops; if a Y-bearing sperm does, the zygote is XY and a male baby develops.
    The mother's contribution is exactly the same in both cases, so she cannot influence the sex of the child; it is decided at the moment of fertilisation by which sperm gets there.

  10. Exercise 2.20

    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?

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    One egg — a human ovary releases a single ovum at ovulation in the middle of each menstrual cycle, so one egg a month.
    Identical twins: still one egg. They arise from a single ovum fertilised by a single sperm; the resulting zygote later divides into two, so both babies carry the same genetic material.
    Fraternal twins: two eggs. Yes, the answer changes — two ova are released and each is fertilised by a different sperm, giving two separate zygotes.

  11. Exercise 2.21

    How many eggs do you think were released by the ovary of a female dog which gave birth to 6\displaystyle 6 puppies?

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    At least six eggs (six ova) must have been released by the ovary.
    Each puppy develops from its own zygote, and a zygote is formed by the fusion of one sperm with one ovum.
    Six puppies therefore require six separate ova, each fertilised by a separate sperm.
    Unlike the human female, who releases a single ovum per cycle, the dog releases several ova at one ovulation.