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NCERT Notes on Human Reproduction Class 12 Chapter 2

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NCERT Notes of Class 12 Chapter 2 Human Reproduction: Human reproduction is the biological process by which a new individual offspring is produced from one or two parent organisms. The Human Reproduction process involves the fusion of gametes, which are specialized cells that carry genetic information from each parent, resulting in the formation of a zygote.

Human Reproduction

Human reproduction encompasses the whole process from fertilization to childbirth. Humans are sexually reproducing, internally fertilizing, and viviparous organisms. The reproductive system of humans includes the primary sex organs that produce the gametes, and secondary organs that show secondary sexual characteristics that separate males from females and help in the whole process of reproduction. We humans are sexually dimorphic i.e. there is a remarkable difference between male and female reproductive events, and these events only occur when a person achieves puberty. The events include the following stages;

  1. Gametogenesis i.e. formation of gametes
  2. Insemination i.e. the transfer of gametes from male to female
  3. Fertilization i.e. fusion of the  male and female gamete
  4. Formation and development of the zygote into the blastocyst
  5. Implantation i.e. the attachment of the blastocyst into the uterine wall
  6. Gestation i.e. the embryonic development
  7. Parturition i.e. delivery  of the baby
  8. Lactation i.e. feeding of milk from mammary glands by the mother to the baby

Human Reproductive System

The human Reproductive system is divided into two types i.e., Male and Female Reproductive Systems:

Male Reproductive System

The male reproductive system is mostly located externally in the pelvic region of the body. It consists of the testes (primary sex organ), accessory ducts, accessory glands, and external genitalia. 

Male Reproductive System

 

Scrotum

It is the external pouch in which the testis is situated outside the body to maintain a temperature of 2 to 2.5°C lower than the body’s normal temperatures as the body’s temperature is not suitable for the formation of sperm cells. The scrotum is connected to the abdominal cavity and the pelvis with the help of the inguinal canal through which the spermatic cord passes.

Testis

The male testis is the primary sex organ in the male reproductive system that is also called the male gonad. It is oval, 4-5 cm in length, 2-3 cm in width, and is covered by a dense covering (made of three layers i.e. tunica vaginalis, tunica albuginea, and tunica vasculosa). 
Inside each testis, there are 250 compartments found which are called testicular lobules and each lobule consists of about 1 to 3 highly coiled tubular structures called the seminiferous tubules (site of sperm cell synthesis). The seminiferous tubules inside are lined by;

  1. Spermatogonia: The male germ cells undergo meiosis to form sperm cells.
  2. Sertoli cells: Provides nutrition to the developing sperm cells.

Outside the seminiferous tubules, there is the interstitial space present that consists of the blood vessels, and Leidig cells or the interstitial cells that are responsible for the secretion of male androgen i.e. testosterone. 

Testes

 

Accessory Ducts

The male duct system includes; rete testis, vasa efferentia, epididymis, and vas deferens. The pathway includes;

Seminiferous tubules (inside the testis) –> Rete testis (inside the testis) –> Vasa efferentia (inside the testis) –> Epididymis (present in the posterior surface of testis) –> Vas deferens (leads to the abdomen) –> Seminal duct (from seminal vesicle) + vas deferens + urethra  –> Forms the ejaculatory duct.

The vas deferens form a loop over the urinary bladder. This duct system is responsible for the transport of sperm cells outside through an external opening called the urethral meatus. 

Accessory Glands

It includes a seminal vesicle, a prostate, and Cowper’s glands.

  1. Seminal Vesicles: Pair of sac-like structures present at the base of the bladder that produces the seminal fluid which is alkaline (pH 7.4) and constitutes about 60-70% volume of semen. This fluid is released into the urethra via the seminal duct. This fluid is responsible for neutralizing the acidity of the urethra as well as the vaginal tract, providing nutrition to sperm with its fructose content, stimulating uterine contraction by its prostaglandin content, and coagulating the semen after ejaculation with the help of its clotting proteins.  
  2. Prostate gland: Large, chestnut-shaped gland surrounding the urethra. It produces slightly alkaline fluid that contains citric acids, enzymes, and prostaglandins constituting around 20-30 % volume of semen. This secretion helps in providing nutrition and activates the sperm cells. 
  3. Cowper’s gland: Also called bulbourethral gland. It is a pair of pea-shaped glands present on either side of the urethra and secretes alkaline fluid to neutralize the acidity of the urethra, and helps in the lubrication of the urethra. 

External Genitalia 

It is commonly called the Penis which serves as both the reproductive organ as well as the urinal duct. There are three cylindrical masses of erectile tissue (two corpora cavernosa on the dorsal side, and one corpus spongiosum on the ventral side) that is responsible for the erection of the penis during copulation. The urethra passes through the corpus spongiosum and ends in the enlarged glans penis which is covered by the foreskin or prepuce.

Semen

It is the collection of secretions from the seminal vesicle, prostate, Cowper’s gland, and sperm cells.  Its pH is around 7.3 to 7.5 i.e. slightly alkaline. In one ejaculate it consists of around 200 to 300 million sperm cells. 

Female Reproductive System

Female Reproductive System is located internally in the pelvic region of the lower abdomen. It consists of a pair of ovaries (primary sex organ), a pair of oviducts, a uterus, a cervix, a vagina, and external genitalia. Along with these structures, there is a pair of mammary glands that acts in a pattern to maintain the process of ovulation, fertilization, pregnancy, parturition, and lactation.

Female Reproductive System

 

Ovaries

Ovaries are a pair of almond-shaped female gonads that is 2 to 4 cm in length, 1.5 cm in width, and 1 cm in thickness. It is attached to the uterine wall with the help of ovarian ligaments and is covered by a layer of cuboidal epithelium called the germinal epithelium which forms the oogonia. Below the epithelium is a layer of connective tissue called the tunica albuginea which covers the ovarian stroma. The stroma is divided into two regions- the outer cortex and the inner medulla

Accessory Ducts

It comprises a pair of fallopian tubes or oviducts, a uterus, and a vagina. Each oviduct is a 10 to 12-cm long, hollow tubular ciliated structure that extends from the periphery of the ovary to the uterus and is divided into three regions;

  1. Infundibulum: The funnel-shaped part near the periphery of the ovary and consists of finger-like projections called the fimbriae which collect the ovum from the ovary after ovulation.
  2. Ampulla: The wider part after the infundibulum.
  3. Isthmus: The last part has a narrow lumen that connects to the uterus. 

The uterus or the womb is a single, inverted pear-like structure that is attached to the pelvic wall with the help of ligaments. Its wall is made up of three layers of tissue; 

  1. Perimetrium: The external thin layer.
  2. Myometrium: The middle muscular layer is made up of smooth muscles that exhibit strong contractions during childbirth.
  3. Endometrium: The inner glandular and vascular layer that undergoes cyclic changes during the menstrual cycle. 

The uterus opens through a narrow cervix into the vagina. The cervical canal of the cervix along with the vagina is called the birth canal. The vagina is about 10 cm long that extends from the cervix to the external genitalia. It is responsible for acting as the birth canal, allowing the menstrual flow, and accepting the male penis during copulation. The opening of the vagina is called the vaginal orifice which is covered partially by a membrane called the hymen

Accessory Glands

There are mainly two categories of glands in the female reproductive system; the vestibular gland and the mammary gland. The vestibular gland is of two types;

  1. Paraurethral glands: Also known as lesser vestibular glands or glands of Skene. These glands are homologous to the male prostate, present on both sides of the urethral opening, and secret mucus.
  2. Bartholin’s glands: Also known as the greater vestibular glands. They are paired glands, homologous to the male Cowper’s gland, and secrete viscid fluid for lubrication only during copulation. 

The mammary glands or breasts are a pair of glandular tissue-containing structures that have a variable amount of fat in them. The glandular tissue of each breast is divided into 15 to 20 mammary lobes that contain a cluster of milk-secreting cells called alveoli. The milk is then stored in the cavities of alveoli which open into the mammary tubules that join to form the mammary duct. And several mammary ducts join to form the mammary ampulla that is connected to the lactiferous duct through which the milk comes out. 

Gametogenesis

The process of formation of gametes (sperm and ovum) by meiosis in the primary sex organs i.e. testis in males and ovaries in females is called gametogenesis. The formation of sperm by the testis is called spermatogenesis and the formation of the ovum by ovaries is called oogenesis. 

Spermatogenesis

The meiotic process by which the four haploid spermatids having 23 chromosomes are produced from the diploid male germ cells having 46 chromosomes inside the seminiferous tubules of the testis is called spermatogenesis. It begins at the time of puberty. It has the following phases;

  1. Multiplication Phase: The germ cells present on the inside wall of seminiferous tubules multiply by mitotic division to form a large number of spermatogonia (diploid-46 chromosomes).
  2. Growth Phase: Spermatogonia grow and increase in size to form a large number of primary spermatocytes (diploid-46 chromosomes).
  3. Maturation Phase: Primary spermatocytes undergo 1st meiotic division (reduction division) leading to the formation of 2 secondary spermatocytes (haploid). These secondary spermatocytes undergo the 2nd meiotic division (equational division) to produce 4 haploid spermatids.

The spermatids will then undergo spermiogenesis in which the transformation of spermatids into spermatozoa (sperms) takes place.  After spermiogenesis, sperm heads get embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation. The human male ejaculates about 200 to 300 million sperms during coitus in which, for normal fertility, at least 60 % of sperm cells must have normal shape and size and at least 40 % of them must show vigorous motility.

Spermatogenesis

 

Structure of Sperm

A mature sperm is a 0.06mm long, microscopic structure whose whole body is enveloped by a layer of the plasma membrane. Its body is composed of a head, neck, middle piece, and tail. 

  1. Head: It contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome that is filled with enzymes (Hyaluronidase) that help in the fertilization of the ovum. 
  2. Middle piece: It possesses numerous mitochondria, which produce energy for the movement of the tail that facilitates sperm motility. 
  3. Tail: It helps in sperm movement.
Sperm

 

Hormonal Control of Spermatogenesis or the Male Reproductive System

When a male reaches puberty the process of spermatogenesis starts due to the significant increase in the secretion of the Gonadotropin-releasing hormone (GnRH), which then acts on the anterior pituitary gland and stimulates the secretion of two gonadotropins; luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

  1. LH (luteinizing hormone): It acts at the Leydig cells and stimulates the synthesis and secretion of androgens; the testosterone. Androgens, in turn, stimulate the process of spermatogenesis.
  2. FSH (follicle stimulating hormone): It acts on the Sertoli cells and stimulates the secretion of androgen binding factors and inhibin. Androgen binding factor binds with androgen which helps in the process of spermiogenesis. Whereas inhibin inhibits the secretion of LH and FSH when not required. 

Oogenesis

The process by which a mature female gamete or ovum is formed is called Oogenesis. Unlike making gametogenesis this process gets initiated during the embryonic development stage and includes the following phases;

  1. Multiplication Phase: At the fetal stage, about 2 million diploid gamete mother cells (oogonia) are formed within each ovary and then no more oogonia are formed after birth.
  2. Growth Phase: Oogonia undergo meiotic division and enter into prophase-I where it gets temporarily arrested and is called the primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty leaving only 60000 to 80000 primary follicles in each ovary. The primary follicles develop into secondary follicles by getting surrounded by more layers of granulosa cells and a new theca. The secondary follicle then develops a fluid-filled cavity called an antrum thus becoming the tertiary follicle. Then, the primary oocyte within the tertiary follicle grows in size.
  3. Maturation Phase: The primary oocyte then completes its 1st meiotic division to form a large haploid secondary oocyte and a tiny first polar body. The tertiary follicle changes into the mature Graafian follicle and the secondary oocyte form a new membrane called zona pellucida surrounding it. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation
Oogenesis

 

Structure of Ovum/human egg

The ovum is a round, non-motile, and haploid cell that has a centrally located nucleus and dense cytoplasm. The cytoplasm stores food material required for the entire process of development. The ovum has four layers of covering; the plasma membrane, the Vitelline membrane, the zona pellucida, and the corona radiata.

Ovum

 

Hormonal Control of Oogenesis

The hypothalamus secretes the GnRH to stimulate the pituitary gland for the secretion of luteinizing hormone and follicle-stimulating hormone. LH stimulates the corpus luteum to secrete progesterone which when rises to undesirable levels inhibits the release of GnRH thus stopping the whole process. FSH stimulates the formation of estrogen and the development of the primary oocyte to form a secondary oocyte. 

Menstrual Cycle

It is the reproductive cycle in female primates e.g. monkeys, apes, and human beings. The first menstruation begins at puberty and is called menarche. Menstrual cycles cease around 50 years of age; that is termed menopause. In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation to the next one is called the menstrual cycle. It has four phases;

  1. Menstrual / Bleeding Phase (1st -5th Day): This phase lasts for 3-5 days. The menstrual flow results due to the breakdown of the endometrial lining of the uterus and its blood vessels which releases blood, mucus, and tissue fluid pass out and are called menstruation. It only occurs if the released ovum is not fertilized which results in lowered levels of LH causing degeneration of the corpus luteum, and a drop in progesterone levels. 
  2. Proliferative / Follicular Phase (6th -13th Day): It lasts for about 8-12 days. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of the uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones. The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as the secretion of estrogens by the growing follicles.
  3. Ovulatory Phase: (14th day): The shortest phase in the ovarian cycle in which LH and FSH attain a peak level. Rapid secretion of LH (LH surge) ruptures the Graafian follicle to release the ovum (ovulation).
  4. Secretory / Luteal Phase (15th -28th day): It lasts for about 14 days. In this phase after the ovulation, the Graafian follicle is transformed into a yellow glandular mass called Corpus luteum that secretes a large amount of progesterone which is essential for the growth and maintenance of the endometrium making the uterus ready for implantation (of the fertilized ovum) and other events of pregnancy. If fertilization does not occur, the corpus luteum degenerates. This causes the disintegration of the endometrium leading to menstruation, and marking a new cycle.
Menstrual Cycle

 

Fertilization and Implantation

During copulation (coitus) insemination takes place i.e. semen containing sperm cells is released by the penis into the vagina. The motile sperms swim rapidly, pass through the cervix, enter into the uterus, and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube. On the other hand, the ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilization takes place. For the process of fertilization to take place the sperm cells first have to undergo capacitation in which the sperm cells get activated by the secretion of the female genital tract for the release of enzymes stored in the acrosome. 

Now fertilization i.e. fusion of male and female gamete will take place. It occurs only when the ovum and sperms are transported simultaneously to the ampullary-isthmic junction failing to do will miss fertilization and pregnancy. During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum, secrets enzymes from the acrosome for its entry while inducing changes in the membrane that block the entry of additional sperms (Prevent polyspermy) thus ensuring only one sperm cell fertilizes an ovum. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a small second polar body and a haploid egg. Soon the haploid nucleus of the sperm and that of the ovum will fuse to form a diploid zygote

Fertilization

 

The mitotic division called the cleavage will start as the zygote moves towards the uterus through the isthmus and forms 2, 4, 8, 16 celled daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula which will continue to divide and transforms into a blastocyst. In the blastocyst, the blastomeres are arranged into an outer layer called trophoblast (later get attached as finger-like projections called chorionic villi to the endometrium) and an inner group of cells called inner cell mass (give rise to the embryo ) attached to trophoblast. After attachment, the uterine cell divides rapidly and covers the blastocyst embedding it in the endometrium of the uterus. This is called implantation and it leads to pregnancy.

Sex Determination in Humans

Fusion of the male and female nuclei will determine the sex of the baby. As the chromosomal pattern in human females is XX and males is XY, the gametes will be X in the case of females whereas male gametes will either be X or Y. When the sperm cell carrying an X chromosome will fertilize the female ovum the zygote will have XX i.e. it will be a female child and when the Y chromosome carrying sperm will fertilize the ovum the zygote will be XY i.e. the baby will be a male. 

Also Read: Sex Determination

Pregnancy and Embryonic Development

Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm, and in between a mesoderm soon appears. These three layers give rise to all tissues (organs) in adults. 

After implantation, the chorionic villi (finger-like projections) get surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between the developing embryo (fetus) and maternal body called the placenta that is connected to the embryo through an umbilical cord and helps in the transport of substances to and from the embryo. The functions of the placenta are;

  1. The placenta facilitates the supply of oxygen and nutrients to the embryo. 
  2. It helps in the removal of carbon dioxide and excretory/waste materials produced by the embryo.
  3. Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.

The duration between fertilization and parturition (9 months) is called gestation. During gestation following stages of development are seen;

  1. 1st month: Heart is formed.
  2. 2nd month: Limbs and digits develop.
  3. 3rd month (1st trimester): Limbs and external genital organs well developed.
  4. 5th month: Shows movements and hair on the head appears.
  5. 6th month (2nd trimester): The body is covered with fine hair, eyelids separate, and eyelashes are formed.
  6. 9th month (3rd trimester): The fetus is fully developed and is ready for delivery.

Parturition and Lactation

Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the fetus is called parturition. It is controlled by a neuroendocrine mechanism that includes the following steps;

  1. The signals for parturition originate from the fully developed fetus and the placenta which induce mild uterine contractions (fetal ejection reflex).
  2. This triggers the release of oxytocin from the maternal pituitary.
  3. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates the further secretion of oxytocin.
  4. Contraction gets stronger, which leads to the expulsion of the baby out of the uterus through the birth canal.

Soon after the infant is delivered, the placenta is also expelled out of the uterus. The production of milk from the mammary glands is called lactation. The mammary glands of the female undergo differentiation during pregnancy and start producing milk towards the end of pregnancy. This helps the mother in feeding the newborn. The milk produced during the initial few days of lactation is called the colostrum which contains several antibodies essential to develop resistance or immunity in newborn babies. 

FAQs on Human Reproduction

Q1: If there are 15 primary spermatocytes, how many sperm cells will be produced?

Answer: 

By the end of the complete male gametogenesis, 1 diploid primary spermatocyte produces 4 sperm cells, therefore 15 primary spermatocytes will produce 60 sperm cells. 

Q2: Name the main hormones that are responsible for the synthesis of milk, ejection of milk, and pregnancy. 

Answer: 

Milk synthesizing hormone is PRL (Prolactin), milk ejecting hormone is OT (Oxytocin), and the pregnancy hormone is progesterone. 

Q3: Colostrum is rich in which antibody?

Answer: 

It is rich in IgA-type antibodies.



Last Updated : 06 Jun, 2023
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