Human Reproduction Class 12 Notes Biology Chapter 2

Organogenesis

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

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
• 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 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.

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

Phases of the Menstrual Cycle

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

• 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 14^th 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 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.