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Human Reproduction Class 12 Notes Biology Chapter 2

Human Reproduction

Reproduction is the formation of new individuals of their own kind by living organisms.

The Male Reproductive System consists of:Male Reproductive SystemMale Reproductive System

  • Primary sex organs, i.e. a pair of testes suspended in a scrotum.
    Secondary organs, i.e. a pair of ducts each differentiated into an epididymis, a vas deferens and an ejaculatory duct.
  • A male urethra passing through an erectile penis.
  •  Three types of Glands:
    (i) A pair of seminal vesicles.
    (ii) A prostate gland.
    (iii) A pair of Cowper’s glands.

The Female Reproductive system consists of:
Female Reproductive SystemFemale Reproductive System

  • Primary sex organ, i.e. a pair of ovaries.
  • Secondary sex organs, i.e. a pair of fallopian tubes (oviducts), a uterus (womb), a vagina.
  • Ovaries produce female gametes called ova.

Phases of Embryonic Development

Embryonic development involves following dynamic changes and identifiable process.

Gametogenesis

It involves the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary).
It is of two types:
(i) Spermatogenesis: Formation of sperm.
(ii) Oogenesis: Formation of ova.

GametogenesisGametogenesis

Fertilization

  • It involves the fusion of haploid male and female gametes to form a diploid zygote.
  • The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis.

Cleavage

It includes the rapid mitotic division of the zygote to form a single-layered hollow spherical larva called a blastula, and its formation is called blastulation.

Cleavage of zygote Cleavage of zygote 

Implantation

The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.Showing ImplantationShowing Implantation

Gastrulation

It includes the mass and orderly migration of the organ-specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers.Process of GastrulationProcess of Gastrulation

Organogenesis

It includes the formation of a specific organ system from three primary germ layers of gastrula and includes morphogenesis and differentiation.

Question for Revision Notes: Human Reproduction
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What is the site of fertilization in human females?
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Fertilization

  • Definition: Fusion of a haploid male gamete (spermatozoon) and a haploid female gamete (ovum) to form a diploid cell, the zygote, is called fertilization or syngamy.
  • Site of Fertilization: Fertilization in human female is internal as in other mammals. It takes place usually in the ampulla of the fallopian tube.

Steps of Fertilization

Approach of Sperm to Ovum

  • Male discharges semen (3.5 ml) in the female’s vagina close to the cervix during coitus. This is called ejaculation or insemination. This ejaculation contains as many as 400 million sperms. Still, only about 100 sperms reach the fallopian tube because many sperms are either killed by the acidity of the female genital tract or engulfed by the phagocytes of the vaginal epithelium.
  • The sperm swim in the seminal fluid at the rate of 1-4 mm per minute by the aspiratory action of the uterus and peristaltic movement of the fallopian tube.
  • Capacitation is the phenomenon of physiological maturation of sperms by breaking the acrosome membrane inside the female genital tract. It takes about 5-6 hours.
  • Ovum is released on the 14th day of the menstrual cycle trapped by the fimbriae of the ampulla of the fallopian tube and move towards the uterus by peristalsis and ciliary action.
  • At the time of ovulation, the egg is at the secondary oocyte stage.
  • Fertilizability of human sperm in the female genital tract is 12 to 24 hours while its survival value is up to 3 days and of an ovum is only 24 hours though it can live for about 72 hours.

Penetration of Sperm

  • The ovum secretes a chemical substance called fertilizin, which has several spermophillic sites on its surface where the sperm of species-specific type can be bound by their antifertilizin site.Penetration of Sperm
    Penetration of Sperm
  • This fertilizin-antifertilizin interaction causes agglutination (sticking together) of egg and sperm.
  • The sperm generally comes in contact with ovum in the animal pole (side of an ovum with an excentric nucleus) while the opposite side of an ovum is called the vegetal pole.
  • Ovulation in the human female occurs at a secondary oocyte stage in which meiosis-I have been completed. First, the polar body has been released, but the second maturation is yet to complete.
  • Penetration of sperm is a chemical mechanism.
  • In this acrosome of sperm undergoes acrosomal reaction and releases certain sperm lysins which dissolve the egg envelopes locally and make the path for the penetration of sperm.
  • These sperm lysins contain a lysing enzyme hyaluronidase which dissolves the hyaluronic acid polymers in the intercellular spaces which hold the granulosa cells of corona radiata together; corona penetrating enzyme (that dissolves the corona radiata) and acrosin (which dissolves the zona pellucida). Then it dissolves the zona pellucida.
  • Only the sperm nucleus and middle piece enter the ovum. The tail is lost.

Cortical Reaction

  • Immediately after the entry of sperm into the egg, the latter shows a cortical reaction to check the entry of more sperm.
  • 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).
  • These substances raise the vitelline membrane above the egg surface. The elevated vitelline membrane is called fertilization membrane.
  • The increased space between the ooplasm and the fertilization membrane and the chemical present effectively check the entry of other sperm.
  • If polyspermy occurs, that is more than one sperm enters the secondary oocyte, the resulting cell has too much genetic material to develop normally.

Fusion of Gametic Nuclei

The fusion of the sperm nucleusThe fusion of the sperm nucleus

  • The entrance of spermatozoon serves to acts as a stimulus which causes the second maturation division.
  • As the head and middle piece of the sperm advance into the egg, those parts rotate through an angle of 180° so that the mitochondria and proximal centriole of the associated middle piece assume the leading position.
  • Besides this rotation, the chromatin itself starts swelling by absorbing fluid from the surrounding cytoplasm and becomes vesicular.
  • It is now called the male pronucleus. This direction of movement of male pronucleus is called penetration path.
  • The centriole brought in by the spermatozoon subdivides into two and as the achromatic spindle is established in the centre of the active cytoplasm.
  • With the production of the second polar body, the egg nucleus or female pronucleus is ready for union with the male pronucleus provided by the sperm head.
  • The male pronucleus which has been advancing the penetration path now moves directly toward the female pronucleus. This, in many cases, involves a slight change in the course of sperm.
  • In such cases, the later portion of its course is called the copulation path.
  • The centrioles of the middle piece of sperm form a spindle.
  • The nuclear membrane of the gametic nuclei degenerates. Two sets of chromosomes initially lie on two poles of the spindle, but later these sets of chromosomes mix up, and the process is called amphimixis.
  • The fertilized egg is now called a zygote and the zygote nucleus is called synkaryon.

Significance of Fertilization

  • It provides a stimulus for the egg to complete its maturation.
  • It activates the ovum to develop into a new individual by repeated mitotic division.
  • Fertilization restores the diploid number of chromosomes (46 in man) in the zygote by adding a male’s haploid set of chromosomes.
  • It makes the egg more active metabolically.
  • It combines the character of two parents and introduces variations. So help in evolution.
  • Sex chromosomes of sperm are either X or Y and help in sex determination.
  • Fertilization membrane formed after sperm entry checks the entry of additional sperms.
  • The copulation path sets the axis of division.

Question for Revision Notes: Human Reproduction
Try yourself:
What is the primary function of the ovaries in the female reproductive system?
View Solution

Menstrual Cycle

  • Menstruation occurs in humans, apes and old world monkeys.
  • Menstruation is bleeding from the uterus of adult females at intervals of one lunar month.
  • Beginning of menstruation or first menstruation is called menarche.
  • The beginning of menstruation varies. It usually occurs between 12 and 15 years.
  • The cycle of events starting from one menstruation till the next one is called Menstrual Cycle.
  • In human females, menstruation is repeated at an average interval of about 28/29 days.
  • One ovum is released (ovulation) during the middle of each menstrual cycle.
  • It is regulated by certain hormones, some of which are secreted by the pituitary gland.
  • The pituitary gland is stimulated by releasing factors produced in the hypothalamus.
  • The hormones produced by the pituitary gland influence the ovaries. The hormones secreted by the ovaries affect the walls of the uterus.Menstrual Cycle
    Menstrual Cycle

Phases of the Menstrual Cycle

The menstrual cycle consists of the following four phasesFour phases of Menstrual cycleFour phases of Menstrual cycle

Menstrual Phase
  • In a 28 days menstrual cycle, the menses takes place on cycle days 3-5.
  • The production of LH from the anterior lobe of the pituitary gland is reduced.
  • The withdrawal of this hormone causes degeneration of the corpus luteum and, therefore, progesterone production is reduced.
  • Production of oestrogen is also reduced in this phase.
  • The endometrium of uterus breaks down & menstruation begins.
  • The cells of endometrium secretions, blood & unfertilised ovum constitute the menstrual flow.

Follicular Phase

  • This phase usually includes cycle days 6-13 or 14 in a 28 days cycle.
  • The follicle-stimulating hormone (FSH) secreted by the anterior lobe of the pituitary gland stimulates the ovarian follicle to secrete oestrogens.
  • Oestrogen stimulates the proliferation of the endometrium of the uterine wall.
  • The endometrium becomes thicker by rapid cell multiplication, and this is accompanied by an increase in uterine glands & blood vessels.

Ovulatory Phase

  • Both LH & FSH attain a peak level in the middle of the cycle (about 14th day).
  • Oestrogen concentration in blood increases.
  • Rapid secretion of LH induces rupturing of graffian follicle and thereby the release of an ovum.
  • In fact, LH causes ovulation.

Luteal Phase

  • Includes cycle days 15 to 28.
  • Corpus luteum secretes progestrone.
  • Endometrium thickens.
  • Uterine glands become secretory.Phases of Menstrual cyclePhases of Menstrual cycle

Hormonal Control of MC

  • FSH stimulates the ovarian follicles to produce oestrogens.
  • LH stimulates corpus luteum to secrete progestrone.
  • The menstrual phase is caused by the increased production of oestrogens.
  • LH causes ovulation.
  • The proliferative phase is caused by the increased production of oestrogens.
  • The secretory phase is caused by the increased production of progesterone.
The document Human Reproduction Class 12 Notes Biology Chapter 2 is a part of the NEET Course Biology Class 12.
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FAQs on Human Reproduction Class 12 Notes Biology Chapter 2

1. What is the process of fertilization in human reproduction?
Ans. Fertilization is the process in human reproduction where the sperm and egg cells unite to form a zygote. It occurs in the fallopian tubes when the sperm penetrates the egg, leading to the fusion of their genetic material. This fusion forms a zygote, which then undergoes cell division and development to form an embryo.
2. What are the phases of embryonic development?
Ans. The phases of embryonic development are: 1. Cleavage: After fertilization, the zygote undergoes rapid cell division, called cleavage. During this phase, the number of cells increases, but the overall size remains the same. 2. Blastulation: Cleavage leads to the formation of a blastocyst, which is a hollow ball of cells. The blastocyst consists of an outer layer called the trophoblast and an inner cell mass. 3. Implantation: The blastocyst implants itself into the uterine lining during this phase. The trophoblast cells release enzymes that help in the penetration and attachment of the blastocyst to the uterus. 4. Gastrulation: The inner cell mass of the blastocyst differentiates into three germ layers: ectoderm, mesoderm, and endoderm. This process is known as gastrulation. 5. Organogenesis: During organogenesis, the three germ layers further differentiate and form the various organs and tissues of the developing embryo.
3. What is the menstrual cycle and its importance in human reproduction?
Ans. The menstrual cycle is a monthly physiological process that occurs in females of reproductive age. It involves the shedding of the uterine lining and prepares the body for potential pregnancy. The menstrual cycle is controlled by hormonal changes and consists of three main phases: the follicular phase, ovulation, and the luteal phase. The menstrual cycle is important in human reproduction as it regulates the release of eggs from the ovaries and prepares the uterus for the implantation of a fertilized egg. If fertilization does not occur, the uterine lining is shed during menstruation. It also provides a regular indication of a woman's fertility and helps in family planning.
4. What is the importance of the zygote in human reproduction?
Ans. The zygote is the first cell that forms after fertilization and marks the beginning of a new individual's life. It is formed by the fusion of the genetic material from the sperm and egg cells. The zygote contains all the genetic information necessary for the development of a complete human being. The zygote undergoes cell division and differentiation to form an embryo, which further develops into a fetus. It is during the early stages of development that the zygote establishes the basic body plan and starts the process of organ formation. Without the zygote, human reproduction and the development of a new individual would not be possible.
5. What are the factors that affect human reproductive health?
Ans. Several factors can affect human reproductive health, including: 1. Age: Both male and female fertility decline with age, with women experiencing a more significant decline after the age of 35. Advanced age can increase the risk of infertility and pregnancy complications. 2. Lifestyle factors: Smoking, excessive alcohol consumption, drug abuse, and poor nutrition can negatively impact reproductive health in both men and women. Maintaining a healthy lifestyle is crucial for optimal reproductive function. 3. Hormonal imbalances: Hormonal disorders such as polycystic ovary syndrome (PCOS) and thyroid disorders can affect fertility and menstrual regularity in women. Similarly, hormonal imbalances in men can lead to reduced sperm production and quality. 4. Sexually transmitted infections (STIs): STIs, such as chlamydia and gonorrhea, can cause infertility if left untreated. It is important to practice safe sex and get regular check-ups to prevent and treat STIs. 5. Environmental factors: Exposure to certain chemicals, toxins, and pollutants in the environment can have detrimental effects on reproductive health. Prolonged exposure to these substances can lead to infertility, hormonal imbalances, and birth defects.
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