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3. human reproduction gametogenesis and menstrual cycle

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Inderjit Singh

- Gametogenesis is the process of forming male and female gametes called spermatogenesis and oogenesis. Spermatogenesis occurs in the testes and forms sperm from spermatogonia. Oogenesis occurs in the ovaries and forms eggs from oogonia. - The menstrual cycle involves the monthly development of an egg, its release from the ovaries (ovulation), and preparation for potential fertilization by the lining of the uterus. If fertilization does not occur, the uterine lining sheds and menstruation begins. - Fertilization is the fusion of an egg and sperm, usually occurring in the fallopian tubes. The sperm enters the egg, and their nuclei fuse to form a

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HUMAN REPRODUCTIONG A M E TO G E N E S I S , M E N S T R U A L C Y C L E A N D F E R T I L I Z AT I O N BY I N D E R J I T S I N G H P G T B I O L O G Y K V S E C TO R 3 1 C H A N D I G A R H
• Gametogenesis: the process of formation of male and female gamete. • Formation of male gamete in human is known as spermatogenesis and formation of female gamete is known as oogenesis. • # Spermatogenesis: - • Formation of spermatozoa in the testis; the process starts at puberty. • In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty. • The spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. • Each spermatogonium is diploid and contains 46 chromosomes.
• Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. • A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. • The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids. • The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. • After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released from the seminiferous tubules by the process called Spermiation.
Schematic representation of SpermatogenesisDiagrammatic sectional view of a seminiferous tubule (enlarged) Spermatogenesis: -
HORMONAL CONTROL OF SPERMATOGENESIS
SPERMIOGENESIS AND SPERMATOZOON STRUCTURE
Structure of a sperm
• # Oogenesis: - • Process of formation of female gametes or ova in ovary. • Oogenesis is initiated during the embryonic development stage when a couple of gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth. • These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. • Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle. • A large number of these follicles degenerate during the phase from birth to puberty.
• Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. • 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. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. • The tertiary follicle further changes into the mature follicle or Graafian follicle.
• The secondary oocyte forms a new membrane called zona pellucida surrounding it. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation. • The secondary oocyte starts its second meiotic division but it is suspended in metaphase II, until a sperms enters it. Diagrammatic Section view of ovary
Schematic representation of Oogenesis
HORMONAL CONTROL OF OOGENESIS
# MENSTRUAL CYCLE: - • The reproductive cycle in the female primates (e.g. monkeys, apes and human beings) is called menstrual cycle. • The first menstruation begins at puberty and is called menarche. • In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. • One ovum is released (ovulation) during the middle of each menstrual cycle. The major events of the menstrual cycle are: - • i) Menstrual Phase: - • Cycle starts with this phase and menstrual flow (menstruation) lasts for 3-5 days.
Diagrammatic presentation of various events during a menstrual cycle
• Results due to break down of endometrial lining of uterus and its blood vessels, along with unfertilized ovum. • ii) Follicular Phase/ Proliferative Phase: - • the primary follicles in the ovary grow to become a fully mature Graafian follicle. • simultaneously the endometrium of uterus regenerates through proliferation of its cells. • These changes are due to an increased level of pituitary hormones, FSH and LH and ovarian hormone, estrogen. • FSH controls the follicular phase; it stimulates the growth of follicles and secretion of estrogen by the growing follicles. • Both LH and FSH attain a peak level in the middle of cycle (about 14th day).
• iii) Ovulatory Phase: - • The peak level of LH (called LH surge) induces the rupture of the mature Graafian follicle and thereby release of secondary oocytes (ovum); this process is called ovulation. • iv) Luteal Phase/ Secretory Phase: - • Ruptured follicle is transformed into corpus luteum. • It secretes large quantities of progesterone’s. • Endometrium thickens further and their glands secrete a fluid in the uterus. • In the absence of the fertilization, corpus luteum degenerates and this causes disintegration of the endometrium leading to menstruation. • Ceasing of the menstrual cycle is known as menopause at the age of about 45-50 years.
Ovum surrounded by few sperms # Fertilization: -
• During copulation (coitus) semen is released by the penis into the vagina (insemination). • The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube. • The ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilisation takes place. • The process of fusion of a sperm with an ovum is called fertilisation. • During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. • The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. • This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). • Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote.
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3. human reproduction gametogenesis and menstrual cycle

  • 1. HUMAN REPRODUCTIONG A M E TO G E N E S I S , M E N S T R U A L C Y C L E A N D F E R T I L I Z AT I O N BY I N D E R J I T S I N G H P G T B I O L O G Y K V S E C TO R 3 1 C H A N D I G A R H
  • 2. • Gametogenesis: the process of formation of male and female gamete. • Formation of male gamete in human is known as spermatogenesis and formation of female gamete is known as oogenesis. • # Spermatogenesis: - • Formation of spermatozoa in the testis; the process starts at puberty. • In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty. • The spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. • Each spermatogonium is diploid and contains 46 chromosomes.
  • 3. • Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. • A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. • The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids. • The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. • After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released from the seminiferous tubules by the process called Spermiation.
  • 4. Schematic representation of SpermatogenesisDiagrammatic sectional view of a seminiferous tubule (enlarged) Spermatogenesis: -
  • 5. HORMONAL CONTROL OF SPERMATOGENESIS
  • 8. • # Oogenesis: - • Process of formation of female gametes or ova in ovary. • Oogenesis is initiated during the embryonic development stage when a couple of gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth. • These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. • Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle. • A large number of these follicles degenerate during the phase from birth to puberty.
  • 9. • Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. • 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. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body. • The tertiary follicle further changes into the mature follicle or Graafian follicle.
  • 10. • The secondary oocyte forms a new membrane called zona pellucida surrounding it. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation. • The secondary oocyte starts its second meiotic division but it is suspended in metaphase II, until a sperms enters it. Diagrammatic Section view of ovary
  • 13. # MENSTRUAL CYCLE: - • The reproductive cycle in the female primates (e.g. monkeys, apes and human beings) is called menstrual cycle. • The first menstruation begins at puberty and is called menarche. • In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. • One ovum is released (ovulation) during the middle of each menstrual cycle. The major events of the menstrual cycle are: - • i) Menstrual Phase: - • Cycle starts with this phase and menstrual flow (menstruation) lasts for 3-5 days.
  • 14. Diagrammatic presentation of various events during a menstrual cycle
  • 15. • Results due to break down of endometrial lining of uterus and its blood vessels, along with unfertilized ovum. • ii) Follicular Phase/ Proliferative Phase: - • the primary follicles in the ovary grow to become a fully mature Graafian follicle. • simultaneously the endometrium of uterus regenerates through proliferation of its cells. • These changes are due to an increased level of pituitary hormones, FSH and LH and ovarian hormone, estrogen. • FSH controls the follicular phase; it stimulates the growth of follicles and secretion of estrogen by the growing follicles. • Both LH and FSH attain a peak level in the middle of cycle (about 14th day).
  • 16. • iii) Ovulatory Phase: - • The peak level of LH (called LH surge) induces the rupture of the mature Graafian follicle and thereby release of secondary oocytes (ovum); this process is called ovulation. • iv) Luteal Phase/ Secretory Phase: - • Ruptured follicle is transformed into corpus luteum. • It secretes large quantities of progesterone’s. • Endometrium thickens further and their glands secrete a fluid in the uterus. • In the absence of the fertilization, corpus luteum degenerates and this causes disintegration of the endometrium leading to menstruation. • Ceasing of the menstrual cycle is known as menopause at the age of about 45-50 years.
  • 17. Ovum surrounded by few sperms # Fertilization: -
  • 18. • During copulation (coitus) semen is released by the penis into the vagina (insemination). • The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube. • The ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilisation takes place. • The process of fusion of a sperm with an ovum is called fertilisation. • During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. • The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane. • This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). • Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote.