NCERT Book Class 12 Biology Human Reproduction

HUMAN REPRODUCTION

Introduction

As you are aware, humans are sexually reproducing and viviparous. The reproductive events in humans include formation of gametes (gametogenesis), i.e., sperms in males and ovum in females, transfer of sperms into the female genital tract (insemination) and fusion of male and female gametes (fertilisation) leading to formation of zygote. This is followed by formation and development of blastocyst and its attachment to the uterine wall (implantation), embryonic development (gestation) and delivery of the baby (parturition). You have learnt that these reproductive events occur after puberty. There are remarkable differences between the reproductive events in the maleand in the female, for example, sperm formation continues even in old men, but formation of ovum ceases in womenaround the age of fifty years. Let us examine the male and female reproductive systems in human.

3.1 THE MALE REPRODUCTIVE SYSTEM

The male reproductive system is located in the pelvis region  (Figure 3.1a). It includes a pair of testes alongwithaccessory ducts, glands and the external genitalia.The testes are situated outside the abdominal cavity within a pouch called scrotum. The scrotum helps in maintaining the low temperature of the testes (2–2.5o C lower than the normal internal body temperature) 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 compartmentscalled testicular lobules (Figure 3.1b).

Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced. Each seminiferous tubule is lined on its inside by two types of cells called male germ cells  (spermatogonia) and Sertoli cells (Figure 3.2 ). The male germ cellsundergo meiotic divisions finally leading to sperm formation, while Sertoli cells provide nutrition to thegerm cells. The regions outside th seminiferous tubules called  interstitial spaces, contain smallblood vessels and interstitial cells or Leydig cells (Figure 3.2). Leydig cells synthesise and secrete testicular hormones calledandrogens. Other immunologically competent cells are also present.

The male sex accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens (Figure 3.1b). The seminiferous tubules of the testis open into the vasa efferentia through rete testis. The vasa efferentia leave the testis and open into epididymis located along the posterior surface of each testis. The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder. It receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct (Figure 3.1a). These ducts store and transport the sperms from the testis to the outside through urethra. The urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus.

3.2 THE FEMALE REPRODUCTIVE SYSTEM 

The female reproductive system consists of a pair of ovaries alongwith a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region (Figure 3.3a). These parts of the system alongwith a pair of the mammary glands are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care. Ovaries are the primary female sex organs that produce the female gamete (ovum) and several steroid hormones (ovarian hormones). The ovaries are located one on each side of the lower abdomen (Figure 3.3b). 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 intotwo zones – a peripheral cortex and an inner medulla.

EXERCISES

1. 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 fetus and uterus is called _____________

2. Draw a labelled diagram of male reproductive system.

3. Draw a labelled diagram of female reproductive system.

4. Write two major functions each of testis and ovary.

5. Describe the structure of a seminiferous tubule.

6. What is spermatogenesis? Briefly describe the process of spermatogenesis.

7. Name the hormones involved in regulation of spermatogenesis.

8. Define spermiogenesis and spermiation.

9. Draw a labelled diagram of sperm.

10. What are the major components of seminal plasma?

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

12. What is oogenesis? Give a brief account of oogenesis.

13. Draw a labelled diagram of a section through ovary.

14. Draw a labelled diagram of a Graafian follicle?

15. Name the functions of the following:

 (a) Corpus luteum

 (b) Endometrium

 (c) Acrosome

 (d) Sperm tail

 (e) Fimbriae

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)

17. What is menstrual cycle? Which hormones regulate menstrual cycle?

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

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

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?

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

Please refer to attached file for NCERT Class 12 Biology Human Reproduction