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Chapter 17 Reproduction in Humans Lesson 2 - The Menstrual Cycle
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Chapter 17 Reproduction in Humans Lesson 2 - The Menstrual Cycle

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Chapter 17 Reproduction in Humans Lesson 2 - The Menstrual Cycle Chapter 17 Reproduction in Humans Lesson 2 - The Menstrual Cycle Presentation Transcript

  • Menstrual cycle
  • Menstruation
    • First sign of puberty for girls
    • Monthly discharge of blood/menses from the uterus via the vagina
    • Length of menstrual period (22-33 days) + amt. of blood lost varies with individuals
    • Periods could be delayed/stopped due to
    • i) emotional disturbances
    • ii) mental fatigue
    • iii) stress
    • iv) illness
    • v) unbalanced diet/malnutrition
  • Menstrual (oestrous) cycle
    • Many developing follicles in the ovary
    • Young follicles = primary follicles
    • Consists of potential egg cell surrounded by a layer of follicle cells
  • Menstrual (oestrous) cycle
    • Maturation of primary follicles = Graafian follicle (matured primary follicle)
    • Contains an egg surrounded by follicle cells and a fluid-filled space
    • Egg has haploid number of chromosomes due to meiosis
    • Egg is ready to be released by the ovary
    primary follicle Graafian follicle
  • Menstrual (oestrous) cycle
    • At about 14 th day from the beginning of menstruation, Graafian follicle ruptures and releases egg into the oviduct funnel ( ovulation )
    • After ovulation, follicle develops into corpus luteum (secretes progesterone + some oestrogen )
    • Progesterone causes uterine lining to remain thick and soft and to become well supplied with blood vessels
    • Uterine lining is now ready for embryo to be implanted
  • Menstrual (oestrous) cycle
    • If no fertilization occurs, the corpus luteum will persist for some time
    • Eventually it breaks down and stops producing hormones (progesterone + oestrogen)
    • This is the start of menstruation (~28 th day from onset of previous menstruation)
    primary follicle Graafian follicle (secretes progesterone + oestrogen)
  • Follicle developement
  • Hormonal control of menstrual cycle
    • Onset of menstruation
    • Anterior pituitary lobe secretes FSH ( follicle stimulating hormone )
    • FSH stimulates
    • i) development of follicles in ovaries (usually one egg ripens to become Graafian follicle in one of the ovaries during each cycle)
    • ii) stimulates follicles in the ovary to secrete oestrogen
  • Hormonal control of menstrual cycle
    • Effects of oestrogen
    • 1) After menstruation, it causes the repair and growth of the uterine lining ( endometrium )
    • 2) A high concentration of oestrogen in blood inhibits FSH production , preventing the ripening and growth of more follicles
    • 3) It stimulates the pituitary gland to secrete LH ( luteinising hormone )
  • Hormonal control of menstrual cycle
    • Effects of LH
    • 1) causes ovulation
    • 2) causes the formation of corpus luteum
    • Functions of progesterone (secreted by corpus luteum)
    • 1) keeps the uterine lining thick and well supplied with blood, preparing it for implantation of the embryo
    • 2) inhibits both FSH and LH production
  • The Menstrual Cycle primary follicle Graafian follicle (secretes progesterone + oestrogen) (causes ovulation + formation of corpus luteum) (keeps the uterine lining thick and well supplied with blood, preparing it for implantation of the embryo + inhibits both FSH and LH production) Ostrogen causes the repair and growth of the uterine lining + stimulates the pituitary gland to secrete LH + high concentration of oestrogen in blood inhibits FSH production FSH stimulates development of follicles in ovaries + follicles in the ovary to secrete oestrogen oestrogen
  • If fertilization
    • Does not take place…
    • Menstruation occurs (drop in LH -> corpus luteum degenerates -> progesterone production stops -> uterine lining breaks down -> menstruation)
  • Qn: From your knowledge of the menstrual cycle, how do you think oral contraceptives work?
    • Oral contraceptives prevent pregnancy by
      • inhibiting FSH and LH production
      • hence preventing ovulation (as no follicles develop in ovaries)