NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET

NEET: NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET

The document NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET

Introduction
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)

Male Reproductive System
Male reproductive system is located in the pelvis region. It Consists of – a pair of testis, glands, accessory ducts, external genitalia.

Testes

  • Smooth organ situated outside the abdominal cavity within a pouch called
  • The scrotum helps in maintaining the low temperature of the testis which is 2-2.5O C which is below than the normal internal body temperature.
  • Each testis is 4 to 5 cm in length and 2 to 3 cm in width in adults.
  • Each testis contains about 250 compartments called testicular lobules.
  • Each testicular lobules contain one to three highly coiled seminiferous tubules, in which sperms are produced.
  • The wall of each seminiferous tubule is lined by two types of cells called male germ cells (spermatogonia) and Sertoli cells.
  • The male germ cells undergo meiosis leading to sperm formation and Sertoli cells provide nutrition to the germ cells.
  • The regions outside the seminiferous tubules called interstitial spaces contain small blood vessels and interstitial cells or Leydig cells.
  • Leydig cells synthesize and secrete testicular hormones called androgens.

Accessory ducts

  • The male accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens.
  • 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.
  • Vas deferens receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct.
  • The urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus.

Accessory glands

  • The male accessory glands include paired seminal vesicles, prostate gland and paired bulbourethral glands.
  • Accessory glands secrete seminal plasma which is rich in fructose, calcium and some enzymes
  • Secretion of the bulbourethral gland also helps in lubricating the penis.

External genitalia

  • The penis is the male external genitalia.
  • Some special tissues make up the penis which helps in the erection of the penis.
  • The enlarged end of penis called the glans penis.
  • Foreskin, a loose fold of tissue covers the glans penis.
    Fig. male reproductive system
    Fig. male reproductive system

Fig. seminiferous tubule with accessory ductsFig. seminiferous tubule with accessory ducts

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.

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.

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.

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.
    Fig. female reproductive system
    Fig. female reproductive system

External genitalia

  • 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.
    Fig. external genitalia with other parts in female reproductive system
    Fig. external genitalia with other parts in female reproductive system

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
  • The cells of alveoli secrete milk, which is storedin 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.
    Fig. mammary gland
    Fig. mammary gland

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

Gametogenesis includes

  • spermatogenesis and spermiogenesis in males
  • oogenesis in females.

The process of formation of sperms is called spermatogenesis. It involves 3 phases-

  • multiplication phase
  • growth phase
  • maturation phase.

The conversion of spermatids into sperms is called spermiogenesis.
The process of formation of a mature female gamete is called oogenesis. 

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

  • 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
    Fig. Spermatogenesis
    Fig. Spermatogenesis

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).
  • 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.

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.
    Fig. structure of a sperm
    Fig. structure of a sperm

Oogenesis

  • 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.

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.
    Fig. Oogenesis
    Fig. Oogenesis

Fig. structure of human ovumFig. structure of human ovum

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.

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.

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

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.
  • 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.
    Fig. fusion of sperm and egg
    Fig. fusion of sperm and egg

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.
  • 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.
    Fig. blastocyst 
    Fig. blastocyst 

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.
  • 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
    Fig. foetus with placenta
    Fig. foetus with 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,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.
    Fig. embryonic development
    Fig. 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 atthe end of pregnancy causes expulsion/delivery of the fetuscalled 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.
  • He milk produced during the first few days of lactation is called colostrum.
  • Colostrum contains antibodies necessary to develop resistance against diseases of the new born baby.
    Fig. parturitionFig. parturition
The document NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET
144 videos|244 docs|241 tests

How to Prepare for NEET

Read our guide to prepare for NEET which is created by Toppers & the best Teachers

Download free EduRev App

Track your progress, build streaks, highlight & save important lessons and more!

Related Searches

Viva Questions

,

Sample Paper

,

Objective type Questions

,

shortcuts and tricks

,

practice quizzes

,

MCQs

,

Extra Questions

,

Previous Year Questions with Solutions

,

ppt

,

video lectures

,

mock tests for examination

,

Semester Notes

,

NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET

,

Summary

,

Important questions

,

NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET

,

Free

,

pdf

,

past year papers

,

Exam

,

study material

,

NCERT Notes: Human Reproduction Notes | Study Biology Class 12 - NEET

;