Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

                         Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.
 

Chapter 3

HUMAN REPRODUCTION

Reproduction is the production of young ones by an organism. Humans are sexually reproduction and viviparous.

1. Male Reproductive System

It consists of paired testes, Accessory ducts, Accessory glands and external genitalia.

a. Paired testes

-Primary sex organs that produce sperms & testosterone.

-Testes are formed within the abdomen. Soon after the birth or at the 8th month of pregnancy they descent into the scrotal sac (scrotum) through inguinal canal.

- The low temperature (2-2.5° C less than the body temperature) of scrotum helps for proper functioning of testes and for spermatogenesis.

-Each testis is oval in shape. Length 4-5 cm, width: 2-3cm.

-Each testis has about 250 testicular lobules.

-Each lobule contains 1-3 coiled seminiferous tubules.

- Seminiferous tubule is lined internally with spermatogonia (male germ cells) & Sertoli cells.

-Sertoli cells (supporting cells) give shape and nourishment to developing spermatogonia.

-The regions outside the seminiferous tubules (interstitial spaces) contain small blood vessels, interstitial cells. (Leydig cells) and immunologically competent cells.

-Leydig cells secrete testicular hormones (androgens). Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

b. Accessory ducts (Duct system)

-Include rete testis, vasa efferentia, epididymis & vas deferens. They conduct sperms from testis as follows:

-Seminiferous tubules→ open into rete testis (irregular  cavities) → vasa efferentia (series of fine tubules) → epididymis (stores sperms temporarily) → vas deferens→  join with duct of seminal vesicle to form common ejaculatory duct → urethra→ urethral meatus.

- Urethra receives ducts of prostate and Cowper's glands.Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

c. Accessory glands

- Include a prostate gland, a pair of seminal vesicles and a pair of Cowper's glands (bulbo-urethral glands).

. Their collective secretion (seminal plasma) is rich in fructose, Ca and enzymes.

- Seminal plasma + sperms→semen

-Functions of seminal plasma:

·         -for transporting sperms.  

·         Supplies nutrients to sperms.

·         Provides alkalinity to counteract the acidity of uterus.

·         Secretions of Cowper's glands lubricate the penis.

-Secretions of epididymis, vas deferens, seminal vesicle & prostate help for maturation and motility of sperms. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

d. Penis (external genitalia)

-It is a copulatory organ made of erectile spongy tissue.

-When spongy tissue is filled with blood, the penis erects. It facilitates insemination.

- The cone-shaped tip of the penis is called glans penis. It is covered by prepuce (foreskin).

2. Female Reproductive System

It includes Ovaries, Accessory ducts & External genitalia.

a. Paired ovaries

-Primary sex organs which produce ova (female gamete) & steroid ovarian hormones (estrogen & progesterone).

- Ovaries are located on both side of the lower abdomen.

-Each ovary is 2-4 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 has outer cortex and inner medulla.

-Ovary contains groups of cells (Ovarian follicles). Each follicle carries a centrally placed ovum.

b. Accessory ducts (Duct system)

Include 2 oviducts (Fallopian tubes), a uterus & vagina.

·         Oviducts: Each oviduct (10-12 cm long) has 3 parts

·         Infundibulum: Funnel-shaped opening provided with many finger-like fimbriae. It helps to collect the ovum.

·         Ampulla: Wider part.

·         Isthmus: Narrow part. It joins the uterus.

-The ciliated epithelium lined the lumen of the oviduct drives the ovum towards the uterus.

·         Uterus (womb): It is inverted pear shaped. It is supported by ligaments attached to the pelvic wall. Uterus has 3 parts-Upper fundus, middle body and terminal cervix. Cervix opens to vagina.Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

-The uterine wall has 3 layers such as:

·         External perimetrium (thin membrane).

·         Middle myometrium (thick layer of smooth muscle).

·         Inner glandular and vascular endometrium.

·         Vagina: It opens to the exterior between urethra &anus. The lumen of vagina is lined by a glycogen-rich mucous membrane consisting of sensitive papillae and Bartholin's glands. The secretions of Bartholin's glands lubricate the penis during sexual act.

 c. External genitalia (vulva or pudendum)

- Consist of Mons pubis, vestibule, hymen & clitoris.

- Mons pubis: A cushion of fatty tissue covered by pubic hair.

 - Vestibule: A median channel. It includes

·         Labia majora: Large, fleshy, fatty and hairy outer folds. Surrounds vaginal opening.

·         Labia minora: Smail, thin and hairless inner folds.

-Hymen (Maiden head): A membrane which partially cover the vaginal opening. It is often torn during the first coitus. It may also be broken by a sudden fall or joit, insertion of a vaginal tampon, active participation in some sports items etc. In some women, hymen persists after coitus. So the hymen is not a reliable indicator of virginity. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

-Clitoris: A highly sensitive organ lying just in front of the urethral opening.

 

Mammary glands (breasts)

- A pair of mammary glands contains glandular tissue & fat. .

- Glandular tissue of each breast has 15-20 mammary lobes containing clusters of cells (mammary alveoli).

- Cells of alveoli secrete milk. It is stored in lumen of alveoli.

-The alveoli open into mammary tubules.

-The tubules of each lobe join to form a mammary duct.

- Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out.Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

 GAMETOGENESIS

 - It is the process of formation of gametes in the gonads.

- It includes Spermatogenesis and Oogenesis.

1.Spermatogenesis

 The It is the process of formation of sperms (spermatozoa) in seminiferous tubules of testis.

It has 2 stages: -

a.       Formation of spermatids: In this, Spermatogonia (Sperm mother cells or immature male germ cells) produce spermatids.

b.      Spermiogenesis: Spermatids transform into sperm.

 Schematic representation of spermatogenesis

Spermatogonia -2n (46 chromosomes)  

Mitosis differentiation

Primary spermatocytes (2n)               

 1^st meiotic division

Sec. spermatocytes -n (23)      

2^nd meiotic division

 Spermatids (n)           

Differentiation

Spermatozoa (n)  

-Each Primary spermatocyte produces 2 secondary spermatocytes. Each secondary spermatocyte produces 2 haploid spermatids. Thus 4 spermatids are formed from each primary spermatocyte.

-After spermiogenesis, sperm heads become embedded in the Sertoli cells. Then they are released to lumen of seminiferous tubules. It is called spermiation. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

Role of Hormones in Spermatogenesis

- Hypothalamus releases Gonadotropin releasing hormone (GnRH).

- GnRH stimulates the anterior pituitary gland to secrete 2 gonadotropins such as Luteinizing hormone (LH) & follicle stimulating hormone (FSH).

 - LH acts on the Leydig cells and stimulates secretion of androgens. Androgens stimulate the spermatogenesis.

-FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.

Structure of spermatozoa (Sperm)

- A mature sperm is about 60 µ (0.06 mm) long.

 - A plasma membrane envelops the whole body of sperin.

- Sperm has 3 regions:

a.       Head: Oval shaped. Formed of nucleus and acrosome. Acrosome is formed from Golgi complex. It contains lytic enzymes. Behind the head is a neck containing proximal and distal centrioles.

b.      Middle piece: Composed of axial filament surrounded by mitochondria and cvtoplasm. Mitochondria produce energy for the sperm motility.

c.       Tails: Consists of a central axial filament. The sperm moves in fluid medium and female genital tract by the undulating movement of the tail.

 - The human male ejaculates about 200-300 million sperms during a coitus.

-For normal fertility at least 60% sperms must have normal shape and size. 40% of them must show vigorous motility. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

2. Oogenesis

-It is the process of formation and maturation of ovum.

- It takes place in Graafian follicles.

 -Oogenesis is initiated in embryonic stage when millions of egg mother cells (oogonia) are formed within each ovary.

-No more oogonia are formed and added after birth.

-Oogonia multiply to form primary oocytes. They enter into prophase-I of the meiosis and get temporarily arrested at that stage.

 - Each primary oocyte gets surrounded by a layer of granulosa cells to form primary follicle.

- A large number of primary follicles degenerate during the phase from birth to puberty. Therefore at puberty only 60,000-80,000 primary follicles are left in each ovary.

- Primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.

- The secondary follicles transform into a tertiary follicle. It has a fluid filled cavity (antrum). The theca layer forms an inner theca interna and an outer theca externa.

-The primary oocyte in the tertiary follicle grows in size and undergoes first unequal meiotic division to form a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains the nutrient rich cytoplasm of the primary oocyte.

-It is unknown that whether the first polar body divides further or degenerates.

-The tertiary follicle further changes into the mature follicle (Graafian follicle).

- Secondary oocyte forms a new membrane (zona pellucida).

- Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary. This is called ovulation. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

Schematic representation of oogenesis

Oogonia-2n (46 chromosomes)                  

 Mitosis differentiation (at foetal stage)

Primary oocyte-2n (grow in size)  

1^st  meiotic division (prior 10 ovulation)

 Seco. oocyte (n) & first polar body (n)

2^nd meiotic division (during fertilization)

Ovum (n) & second Poia body (n)

Structure of ovum (egg)

 - Spherical and non-motile.

- About 0.2 mm in diameter.

-Ovum has 4 membranes:

a.       Plasma membrane: Innermost layer.

b.      Vitelline membrane: Attached to plasma membrane.

c.       Zona pellucida: Outer to the Vitelline membrane.

d.      Corona radiata: Outer layer formed of follicle cells.

 Spermatogenesis & Oogenesis- A comparison

Spermatogenesis	Oogenesis
·         Occurs in testis	·         Occurs in ovary
·         Limited growth phase	·         Elaborated growth phase
·         Each primary spermatocyte gives 4 sperms	·         Each primary oocyte gives one ovum
·         No polar body formation	·         Polar bodies are formed
·         Begins at puberty and extends up to senility	·         Begins at embryonic stage but suspends up to puberty. It ceases around the age of fifty.

 

 Menstrual cycle (REPRODUCTIVE CYCLE)

-It is the cyclic events starting from one menstruation till the next that take place during the reproductive period (from puberty to menopause) of a woman's life.

-Its duration is 28 or 29 days.

-Menstrual cycle is also seen in other primates.

-Menstrual cycle includes :

a.       Ovarian cycle: Changes in ovary.

b.      Uterine cycle: Changes in uterus, oviduct and vagina.

-Menstrual cycle has the following phases

                                                                                                                                 I.            Menstrual phase: 1-5^TH day

-The cycle starts with menstrual flow (bleeding).

-It lasts for 3-5 days.

-It is due to breakdown of endometrial lining and blood vessels of the uterus that comes out through vagina.

-Menstruation occurs if the released ovum is not fertilized.

-Lack of menstruation indicates pregnancy. It may also becaused due to stress, poor health etc.

-Menarche: The first menstruation during puberty

                                                                                                II.            Follicular (Proliferative) phase: 5-13th day

- It starts from 5^th day after menstruation and completed within 8-12 days.

-In this phase, the action of gonadotropins (FSH &LH) from pituitary occurs. FSH stimulates

Ø  Development of primary follicles into Graafian follicles.

Ø  Secretion of oestrogens by Graafian follicles.

-Oestrogens stimulate

Ø  Proliferation of ruptured uterine endometrium and mucus lining of oviduct & vagina.

Ø  Development of secondary sexual characters.

Ø  Suppression of FSH secretion.

Ø  Secretion of LH (Luteinizing hormone).

                                                                                                                              III.            Ovulatory phase: 14^TH day

-LH & FSH attain a peak level in the middle of cycle.

-Rapid secretion of LH (LH surge) induces rupture of Graafian follicle and thereby ovulation (on 14 day). Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

                                                                                                       IV.            Secretory (Luteal) phase: 15-28^TH day

-After ovulation, Graafian follicle is transformed into a yellow endocrine mass called Corpus luteum. It secretes progesterone. By the action of progesterone,

Ø  Endometrium attains maximum vascularity, thickness and softness. Thus the uterus gets ready for implantation

Ø  FSH secretion is inhibited to prevent development of a second ovarian follicle.

-During pregnancy all events of menstrual cycle stop and there is no menstruation.

-If fertilization does not occur, corpus luteum degenerates It causes disintegration of endometrium. It leads to next menstruation and new cycle.

Menstrual cycle ceases around 50 years of age. It is called Menopause.

 

FERTILIZATION AND IMPLANTATION

-During copulation, semen is released by the penis into the vagina. It is called insemination.

-Fusion of a sperm with ovum is called fertilization. It occurs in Ampullary-isthmic junction of fallopian tube.

Sperms → vagina → cervical canal → uterus →isthmus

Fertilization →Ampullary-isthmic Junction

                                                 

Ovum (from ovary) →fimbriae →infundibulum ampulla

-Fertilization happens only if ovum & sperms are transported simultaneously. So all copulations do not lead to fertilization & pregnancy.

-A sperm contacts with zona pellucida. It induces changes in the membrane that block entry of additional sperms.

-The secretions of the acrosome help sperm to enter into the egg cytoplasm via zona pellucida & plasma membrane. This causes second meiotic division of secondary cocyte to form an ovum (ootid) and a second polar body. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

-The haploid nuclei of the sperm and ovum fuse together to form a diploid zygote.

-Sex determination (See chapter 3).

- Zygote undergoes mitotic division (cleavage) as it moves through the isthmus towards the uterus and forms 2. 4,8. 16 daughter cells called blastomeres.

-The embryo with 8-16 blastomeres is called a morula.

-Morula continues to divide and transforms into blastocyst.

-In blastocyst, blastomeres are arranged into trophoblast (outer layer) and an inner cell mass attached to trophoblast.

-The trophoblast layer gives nourishment to inner cell mass. Also, it gets attached to endometrium.

-After attachment, uterine cells divide rapidly and cover the blastocyst. Thus, the blastocyst becomes embedded in the endometrium. This is called implantation.

-The inner cell mass gets differentiated to 3 germ layers  (outer ectoderm, middle mesoderm & inner endoderm). This 3-layered structure (gastrula) on the embryo. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.

 

PREGNANCY AND EMBRYONIC DEVELOPMENT

- After implantation, finger-like projections (chorionic villi) appear on the trophoblast which is surrounded by the uterine tissue and maternal blood.

-The chorionic villi & uterine tissue are interdigitated to form placenta. It is a structural and functional unit b/w  embryo (foetus) and maternal body.

- Placenta is connected to the embryo by an umbilical cord. It transports substances to and from the embryo.

Functions of placenta

·        Acts as barrier between the foetus and mother.

·         Supply O2, nutrients etc. from mother to foetus.

·         Remove CO2 and excretory wastes from foetus.

·         Acts as an endocrine gland by secreting Human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens, progesterone and relaxin.

- During pregnancy, levels of estrogens, progestogens, cortisol, prolactin, thyroxin etc. are also increased in maternal blood. They support the fetal growth, metabolic changes in the mother and maintain pregnancy.

 - The germ layers give rise to all tissues (organs). The stem cells in inner cell mass have the potency to give rise to all the tissues and organs.

 - Human pregnancy (gestation period) lasts 9 months (for cats: 2 months dogs: 2 months, elephants: 21 months).

Changes in embryo during pregnancy

·         After one month: Heart is formed.

·         End of second month: Limbs and digits are developed.

·         End of 12 weeks (first trimester): The major organs (limbs, external genital organs etc.) are well developed.

·         During 5 month: First movement of foetus and appearance of hair on the head.

·         End of 24 weeks (second trimester): Body is covered with fine hair, eyelids separate and eye lashes are formed.

·         End of 9 months: Ready for delivery.

PARTURITION AND LACTATION

- Parturition (labour): Process of giving birth to young ones.

-Parturition is induced by neuroendocrine mechanism.

-The signals originating from the foetus and placenta induce mild uterine contractions (fetal ejection reflex). This causes the release of oxytocin from maternal pituitary.

-Oxytocin causes stronger uterine musele contractions which in turn stimulate further secretion of oxytocin  This process is continued leading to expulsion of the baby out out of the uterus through the birth canal.

-After parturition, the umbilical cord is cut off.

- The placenta & remnants of umbilical cord are expelled from the maternal body after parturition. It is called after birth".

- The mammary glands produce milk towards the end of pregnancy. It is called lactation.

- The yellowish milk produced during the initial few days of lactation is called colostrum. It contains several antibodies essential to develop resistance for the new bom babies. Chapter 3:HUMAN REPRODUCTION Class 12 NCERT NOTES PDF.