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Human Reproduction Chapter Notes | Biology Class 12 - NEET PDF Download

Reproduction, one of the most fundamental aspects of life, is a fascinating process in humans. It involves the formation of sperms in males and ovum in females, the transfer of sperms into the female genital tract, the fusion of gametes, the attachment of the fertilized egg to the uterine wall, and the development of the embryo.

 Let's now examine the male and female reproductive systems in humans:

Male Reproductive System

The male reproductive system is a complex and fascinating network of organs and tissues that work together to produce, transport, and deliver sperm for fertilization. Sperm production begins during puberty and continues throughout a man's life.
Male reproductive system is located in the pelvis region. It Consists of – a pair of testis, glands, accessory ducts, external genitalia.

Male Reproductive SystemMale Reproductive System

There are four main parts in the male reproductive system:

  • Testes
  • Accessory ducts
  • Glands
  • External Genitalia

(a) Testes

  • The testes are smooth organs situated outside the abdominal cavity within a pouch called the scrotum.

TestesTestes

  • The scrotum helps maintain a lower temperature (2-2.5°C lower than internal body temperature) necessary for spermatogenesis, the process of sperm production.
  • Each testis measures about 4 to 5 cm in length and 2 to 3 cm in width in adults.
  • Each testis contains about 250 compartments called testicular lobules.

Question for Chapter Notes: Human Reproduction
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Seminiferous Tubules and Spermatogenesis

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • Each testicular lobule contains one to three highly coiled seminiferous tubules, which are the sites where sperms are produced.
  • The wall of each seminiferous tubule is lined by two types of cells - male germ cells (spermatogonia) and Sertoli cells.
  • The male germ cells undergo meiosis, a type of cell division, leading to sperm formation.
  • Sertoli cells provide nutrition and support to the germ cells during spermatogenesis.

Interstitial Spaces and Leydig Cells

  • The regions outside the seminiferous tubules, called interstitial spaces, contain small blood vessels and interstitial cells or Leydig cells.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • Leydig cells are responsible for synthesizing and secreting testicular hormones called androgens, such as testosterone, which are important for male sexual development and function.

(b) Male Accessory Ducts

The accessory ducts facilitate the transportation of the sperms from the testes to the urethra for their release outside the body. The male reproductive system consists of four accessory ducts:

  • Rete Testis
  • Vasa efferentia
  • Epididymis
  • Vas deferens

Rete testis, Vasa efferentia, Epididymis, and Vas deferensRete testis, Vasa efferentia, Epididymis, and Vas deferens

  • The seminiferous tubules of the testis open into the rete testis, which is a network of tubules.
  • The vasa efferentia leave the testis and open into the epididymis, which is located along the posterior surface of each testis.
  • The epididymis leads to the vas deferens, which ascends to the abdomen and loops over the urinary bladder.
  • The vas deferens receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct.

Urethra

  • The urethra originates from the urinary bladder and extends through the penis to its external opening called the urethral meatus.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • The urethra plays an important role in both the urinary and reproductive systems.
  •  In the urinary system, it serves as a passage for urine to be expelled from the body. 
  • In the reproductive system, it serves as a passage for semen to be released during ejaculation. 
  • During sexual arousal, the sperm, along with fluids from the seminal vesicles and prostate gland, travel through the vas deferens and enter the urethra via the ejaculatory duct. The urethra then transports the semen out of the body through the urethral meatus during ejaculation.

Question for Chapter Notes: Human Reproduction
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(c) Male Accessory Glands

  • The male accessory glands include paired seminal vesicles, a prostate, and paired bulbourethral glands.

Male Accessory GlandsMale Accessory Glands

  • These glands produce secretions that contribute to the seminal plasma, which is the fluid that combines with sperm to form semen.
  • The seminal plasma is rich in fructose, calcium, and certain enzymes, which provide energy and nutrients for sperm.
  • The secretions of the bulbourethral glands also help in the lubrication of the penis during sexual activity, facilitating ejaculation.

(i) Seminal Vesicles: Seminal vesicles play a significant role in semen composition, contributing approximately 60-75% of its fluid. This fluid is rich in enzymes, proteins, vitamin C, fructose, prostaglandins, and phosphorylcholine. The high fructose content provides nutrient energy to the spermatozoa.

(ii) Prostate Gland: Located just below the urinary bladder, the prostate gland is a dense structure. Its secretion is slightly alkaline, thin, and milky in color. It aids in the survival of sperm in the acidic vaginal environment and enhances sperm motility.

(iii) Bulbourethral Glands: The secretion of these glands serves to lubricate the penis and neutralize residual acidity in the urethra.

Question for Chapter Notes: Human Reproduction
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(d) External Genitalia

  • The penis is the male external genitalia and is responsible for facilitating insemination, which is the process of depositing sperm into the female reproductive tract.
  • It is made up of special tissue that allows for erection, which is the stiffening and enlargement of the penis during sexual arousal.
  • The enlarged end of the penis is called the glans penis, and it is covered by a loose fold of skin called the foreskin.

Female Reproductive System

Female reproductive system is located in the pelvic region. It consists of - a pair of ovaries, a pair of oviducts, uterus, cervix, vagina and the external genitalia. A pair of mammary glands is also integrated structurally and functionally with the parts of female reproductive system to support the process of ovulation, fertilization, gestation, parturition and care of the baby after birth. A pair of oviducts, uterus, cervix, vagina constitute the female accessory ducts.

Female Reproductive SystemFemale Reproductive System

(a) Ovaries

  • The primary female sex organs that produce the ovum and several ovarian hormones, steroid in nature.
  • Located one on each side of the lower abdomen.
  • Each ovary is covered by a thin epithelium which encloses the ovarian stroma.
  • The ovarian stroma is divided into two zones – a peripheral cortex and an inner medulla.

(b) Oviduct (fallopian tube)

  • 10-12 cm in length.
  • Extends from the periphery of each ovary to the uterus.
  • Part closer to the ovary is the funnel shaped
  • Fimbriae are the finger like projections located on the edges of the infundibulum.
  • Fimbriae help in collection of the ovum after ovulation.
  • The infundibulum leads to the ampulla which is the wider part of the oviduct.
  • The last part of the oviduct is isthmus which has a narrow lumen and it joins the uterus.

(c) Uterus

  • Uterus is also called womb.
  • The shape of the uterus is like an inverted pear.
  • Ligaments attached to the pelvic wall support the uterus.
  • The narrow cervix opens the uterus into the vagina.
  • Cervical canal is the cavity of the cervix which forms birth canal along with vagina.
  • Three layers of tissues are present in the uterus wall- the outer thin membrane bound perimetrium, middle thick layer of smooth muscle called myometrium, inner glandular layer called endometrium.
  • Endometrium lines the uterine cavity.
  • During menstrual cycle, endometrium undergoes cyclical changes but the myometrium exhibits strong contraction during parturition.

(d) External Genitalia

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • Vagina is the female external genitalia.
  • Vagina includes mons pubis, labia majora (labia majus), labia minora (labia minora), hymen and clitoris.
  • Mons pubis is a cushion of fatty tissue covered by skin and pubic hair.
  • The labia majora are folds of tissue extend down from the mons pubis and surround the vaginal opening.
  • Under the labia majora, there are paired tissue folded to form labia minora.
  • Hymen is membrane covering the opening of the vagina.
  • A tiny finger-like structure which lies at the upper junction of the two labia minora above the urethral opening is called clitoris.

Mammary Glands

  • Paired structures containing glandular tissues and fats, the amount of fat varies from person to person.
  • The glandular tissue of each breast is divided into 15-20 mammary lobes containing clusters of cells called alveoli.
  • The cells of alveoli secrete milk, which is stored in the cavities called as lumens of alveoli.
  • The alveoli open into mammary tubules and the tubules of each lobe join to form a mammary duct which joins to form a wider mammary ampulla.
  • Mammary ampulla is connected to lactiferous duct through which milk is sucked out.

Gametogenesis


The process of formation of gametes in primary sex organs is called Gametogenesis.

Gametogenesis includes

  • Spermatogenesis and Spermiogenesis in males
  • Oogenesis in females.

(a) Spermatogenesis

The process of formation of sperms is called spermatogenesis. It involves 3 phases- multiplication phase, growth phase, maturation phase.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • In multiplication phase, male germ cells also called as spermatogonia undergo mitotic divisions to form large number of spermatogonia.
  • In growth phase, spermatogonia increases their size by accumulation of nutrition in the cytoplasm and are ready for meitoic division and the spermatocytes are called as primary spermatocytes with 46 chromosomes.
  • In maturation phase- A primary spermatocyte completes the first meiotic division leading formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each and the secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids.

Spermiogenesis

  • The spermatids are transformed into sperms, also called as spermatozoa by the process called spermiogenesis.
  • After spermiogenesis, sperm heads become embedded in the Sertoli cells and are released from the seminiferous tubules by the process called spermiation.

Hormonal control of spermatogenesis

  • Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH) from hypothalamus.
  • The increases level of gonadotropin releasing hormone stimulates the anterior pituitary to secrete luteinizing hormone(LH) and follicle stimulating hormone(FSH).

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • LH acts at the Leydig cells and stimulates synthesis and secretion of androgens.
  • Androgens stimulate the process of spermatogenesis.
  • FSH acts on the Sertoli cells and secrete two factors- androgen binding protein (ABP) and inhibin which helps in spermiogenesis.

Question for Chapter Notes: Human Reproduction
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Structure of a Sperm

  • It is a microscopic, motile structure composed of a head, neck, a middle piece and a tail.
  • Whole body is covered by plasma membrane.
  • The sperm head contains an elongated haploid nucleus and the anterior portion is covered by a cap-like structure acrosome
  • The middle piece contains numerous mitochondria which produce energy for sperm motility needed for fertilization.
  • Tail helps the sperm cell to swim to reach the egg cell.
  • Seminal plasma along with sperm constitutes the semen.

(b) Oogenesis

Structure of Human OvumStructure of Human Ovum

  • The process of formation of a mature female gamete is called oogenesis.
  • Some of the germinal epithelial cells divide by mitosis to produce a large number of gamete mother cells or oogonia
  • Oogonia Multiply by mitosis and form primary oocytes.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

Growth Phase

  • Each primary oocyte then gets surrounded by a layer of granulosa cells and called primary follicle.
  • The primary oocytes enlarge and mature by obtaining food from follicle cells.
  • The primary follicles get surrounded by more layers of granulosa cells and a new theca and called secondary follicles.
  • The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum
  • The theca layer is organised into an inner theca interna and an outer theca externa.
  • The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division which is an unequal division and forms a large secondary oocyte and tiny first polar body.
  • The tertiary follicle changes into the mature follicle or Graafian follicle.
  • The secondary oocyte forms a new membrane called zona pellucida
  • The Graafian follicle then ruptures to release the secondary oocyte from the ovary by the process called ovulation
  • If a sperm can enter the secondary oocyte through zona pellucida layer, the secondary oocyte completes meiosis II and thus results in the formation of second polar body and an ovum.

OogenesisOogenesis

Menstrual Cycle


The reproductive cycle starting from the one menstruation till the next one in the female primates is called menstrual cycle. The first menstruation which begins at puberty and is called menarche . The cycle is repeated at an interval of 28-29 days. Menstrual cycle involve three phases- menstrual phase, follicular phase and luteal phase.

(i) Menstrual Phase

  • Menstrual flow occurs and lasts for about 3-5 days.
  • The endometrial lining of the uterus breaks along with the blood vessels which forms a red fluid and results in menstrual flow.
  • If the ovum is fertilized by a sperm menstrual flow does not occur and hence indicates pregnancy.

(ii) Follicular phase

  • In this phase, the primary follicles in the ovary grow to become a fully matured graafian follicle.
  • Endometrium regenerates through proliferation.
  • Changes in Pituitary hormone and ovarian hormones induce the formation of graafian follicle and regeneration of endometrium.
  • The secretion of gonadotropins like luteinizing hormone and follicular stimulating hormone increases gradually during this phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.
  • Both LH and FSH attain a peak level in the middle of cycle about 14th day.
  • Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum known as ovulation

Human Reproduction Chapter Notes | Biology Class 12 - NEET

(iii) Luteal phase

  • In this phase, the ruptured part of Graafian follicle transforms into yellow body called Corpus luteum.
  • The corpus luteum secretes large amounts of progesterone hormone which maintains the endometrium for implantation of the fertilized ovum.
  • During pregnancy all events of the menstrual cycle stop and there is no menstruation.
  • In the absence of fertilization, the corpus luteum degenerates hence causes disintegration of the endometrium leading to menstruation and a new cycle begins.

In human beings, menstrual cycles ceases around 50 years of age and known as menopause.

Fertilization

  • The fusion of haploid male gamete, sperm and haploid female gamete, ovum is called fertilization.
  • During coitus, sperm is released by male partner into the vagina of the female partner is called as insemination
  • The motile sperms swim and pass the cervix to enter into the uterus and finally to reach the ovum released by the ovary in the ampulla-isthmic junction.
  • Fertilization takes place in the ampulla-isthmic junction.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • All copulations do not lead to fertilization because fertilization can only occur if the ovum and sperms are transported simultaneously to the ampulla-isthmic junction.
  • The sperm after reaches the ovary in the ampulla-isthmic junction comes in contact with the zona-pellucida layer of the ovum and block the entry of the additional sperms thus only one sperm fertilizes the ovum.
  • The secretions of acrosome help the sperm to enter into the ovum through zona pellucida and the plasma membrane and thus secondary oocyte completes meiosis II and results in the formation of a second polar body and haploid ovum.
  • The haploid nucleus of the sperm and ovum fuse together to form a zygote which develops into new individual.

Cortical Reaction
(a) Immediately after the entry of a sperm into the egg, the later shows a cortical reaction to check the entry of more sperms.
(b) In this reaction, the cortical granules present beneath the egg’s plasma membrane release chemical substance between the ooplasm and the plasma membrane (vitelline membrane).
(c) These substances raise the vitelline membrane above the egg surface. The elevated vitelline membrane is called fertilization membrane.
(d) The increased space between the ooplasm and the fertilization membrane and the chemical present in it effectively check the entry of other sperm.
(e) If polyspermy occurs, that is more than one sperm enter the secondary oocyte, the resulting cell has too much genetic material to develop normally

  • The haploid gametes fuse together to form diploid zygote. As the zygote moves towards the uterus, the mitotic division starts and form cleavage to change into 2, 4,8,16 celled blastomeres.
  • The blastomeres with 8 to 16 cells are called morula. Morula divide to change into blastocysts .The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The outer layer of blastocyst is called trophoblast that attach with endometrium of uterus, called implantation that leads to pregnancy.

Sex Determination in Human

  • Male has two sex chromosomes X and Y hence male produces 50% of sperms carrying X and 50% carrying Y, while female has two X chromosomes.
  • After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilized the ovum.
  • The zygote carrying XX would develop into a female baby and XY would form a male.
    Fig. sex determination
    Fig. sex determination

Cleavage

  • Cleavage is the mitotic division which starts as the zygote moves through the isthmusof the oviducttowards the uterus and forms 2, 4, 8, 16 daughter cells called as blastomeres
  • The embryo with 8 to 16 blastomeres is called a morula
  • The morula divides further as it moves further in to the uterus and transforms into blastocyst
  • The blastomeres in the blastocyst are arranged in to an outer layer called trophoblast and inner mass of cells attached to trophoblast is called as inner cell mass.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • The trophoblast layer then gets attached to the endometrium of the uterus and the inner cell mass divide to cover the blastocyst hence blastocyst becomes embedded in the endometrium of the uterus and the process is called as implantation.

Pregnancy and Embryonic Development

  • After implantation, finger like projections appear on the trophoblast called as chorionic villi.
  • Uterine tissue and maternal blood surrounds the chorionic villi.

Human Reproduction Chapter Notes | Biology Class 12 - NEET

  • The chorionic villi and uterine tissue together form a structural and functional organic structure between developing embryo and tissues of the mother called as placenta. 

Functions of Placenta

  • The placenta facilitates the supply of oxygen and nutrients to the embryo.
  • Help in the removal of carbon dioxide and excretory/waste materials produced by the embryo.
  • The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo.
  • Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogen. etc.
  • A hormone called relaxin is secreted by the ovary in the later phase of pregnancy.
  • hCG, hPL and relaxin are produced in women only during pregnancy.
  • Levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several folds in the maternal blood.
  • Increased production of all the hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.
  • After implantation, the inner cell mass is differentiated into outer layer called ectoderm and an inner layer called endoderm with a middle
  • Three layers give rise to all organs in adults.
  • The cells which have the potency to give rise to any types of cells in the body are called stem cells.
  • The human pregnancy lasts for 9 months, heart develops after one month of pregnancy, limbs develop by the end of second month, and major organ systems are formed by the end of 3 months.
  • First movement and appearance of hairs are during fifth month of pregnancy.
  • By the end of 24 weeks, the body covers with fine hair, eye-lids separate, eyelashes form.
  • By the end on nine month, the fetus fully develops.
    Embryonic Development
    Embryonic Development

Parturition and Lactation

  • The average duration of human pregnancy is about 9 months called as the gestation period.
  • Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the fetus called as parturition.
  • The signals for parturition originate from the fully developed fetus and the placenta which induces mild uterine contraction is called fetal ejection reflex.
  • Fetal ejection reflex releases oxytocin hormone from the pituitary gland of mother which acts on the uterine muscle and causes contraction of uterus which in turn stimulates further oxytocin secretion.
  • Production of milk at the end of pregnancy by the differentiation of mammary glands is called lactation.
  • The milk produced during the first few days of lactation is called colostrum.
  • Colostrum contains antibodies necessary to develop resistance against diseases of the newborn baby.
The document Human Reproduction Chapter Notes | Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
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FAQs on Human Reproduction Chapter Notes - Biology Class 12 - NEET

1. What is the function of the mammary glands in the female reproductive system?
Ans. The mammary glands in the female reproductive system are responsible for producing and secreting milk to nourish newborn babies.
2. How does fertilization occur in human reproduction?
Ans. Fertilization in human reproduction occurs when a sperm cell from a male fuses with an egg cell from a female, forming a zygote which will eventually develop into an embryo.
3. How is sex determined in humans?
Ans. Sex determination in humans is determined by the combination of sex chromosomes inherited from the parents. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
4. What is the process of gametogenesis?
Ans. Gametogenesis is the process by which male and female gametes (sperm and egg cells) are produced through meiosis, resulting in cells with half the normal number of chromosomes.
5. What are the stages of embryonic development during pregnancy?
Ans. During pregnancy, the stages of embryonic development include cleavage, implantation, gastrulation, and organogenesis, leading to the formation of a fully developed fetus.
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