IB DP Biology: SL复习笔记6.6.5 Reproduction: Sex Determination in Females

Oestrogen & Progesterone

  • During early development an embryo develops embryonic gonads that will either become ovaries in females or testes in males
  • The factor that determines whether the embryonic gonads will develop into ovaries or testes is the presence or absence of a single gene known as the SRY gene
  • The SRY gene is on the Y chromosome, so if the embryo has two X chromosomes the embryonic gonads will develop into ovaries
  • This means testosterone will not be secreted by the developing embryo
  • The two female hormones oestrogen and progesterone are present throughout pregnancy
    • These hormones are secreted by the mother's ovaries and the placenta
  • The absence of foetal testosterone and presence of maternal oestrogen and progesterone causes female reproductive organs to develop
  • During female puberty, oestrogen and progesterone secretions increase.
  • This leads to:
    • The start of the menstrual cycle
    • The development of female secondary sexual characteristics e.g. breast development

Human-female-secondary-sexual-characteristics

Secondar sexual characteristics of a human female

Menstrual Cycle

  • The menstrual cycle is the series of changes that take place in the female body leading up to and following the release of an egg from the ovaries
    • It starts in early adolescence in girls and is controlled by hormones
    • The average menstrual cycle is 28 days long
  • The uterus lining, or endometrium, thickens from day 7 through to day 28 of the cycle in preparation for receiving a fertilised egg
  • The release of an egg, or ovulation, occurs about halfway through the cycle on day 14, and the egg then travels down the oviduct to the uterus
    • Eggs develop inside fluid-filled sacs known as egg follicles inside the ovary
    • The follicle releases the egg at ovulation and becomes an empty follicle known as a corpus luteum
  • Failure to fertilise the egg leads to menstruation, commonly known as a period
    • Menstruation involves the loss of menstrual blood via the vagina
    • This is caused by the breakdown of the endometrium
  • Menstruation takes place roughly between days 1-7 of the cycle
    • The number of days during which menstruation occurs can vary
  • After menstruation finishes, the endometrium starts to thicken again in preparation for the possible implantation of a fertilised egg in the next cycle

Changes-in-the-lining-of-the-uterus-during-the-menstrual-cycle

Changes in the endometrium during the menstrual cycle

How ovarian and pituitary hormones control the menstrual cycle

  • Four hormones control the events that occur during the menstrual cycle:
    • Two of these hormones are produced by the pituitary gland in the brain
      • Follicle-stimulating hormone (FSH)
      • Luteinising hormone (LH)
    • The other two hormones are produced in the ovaries
      • Oestrogen; produced by the egg follicle, and by the corpus luteum after ovulation
      • Progesterone; produced by the corpus luteum
  • The roles of FSH and LH:
    • FSH is secreted by the pituitary gland and stimulates the development of several immature egg cells in follicles in the ovary
    • FSH also stimulates the secretion of oestrogen by the follicle wall
    • The pituitary gland is stimulated to release LH when oestrogen levels have reached their peak
    • LH causes ovulation to occur; the shedding of the mature egg cell from the follicle and its release from the ovary
      • The shedding of the mature egg cell leaves behind an empty egg follicle called the corpus luteum
    • LH also stimulates the production of progesterone from the corpus luteum

Pituitary-hormones_1

Changes in the levels of the pituitary hormones FSH and LH in the blood during the menstrual cycle

  • The roles of oestrogen and progesterone:
    • Oestrogen levels rise from day 1 to peak just before day 14
    • This causes the endometrium to start thickening and the egg cell to mature
    • The peak in oestrogen occurs just before the egg is released
    • Progesterone stays low from day 1-14 and starts to rise once ovulation has occurred
      • Progesterone is produced by the corpus luteum
    • The increasing levels of progesterone cause the endometrium to continue to thicken
    • A fall in progesterone levels as the corpus luteum deteriorates causes the endometrium to break down, resulting in menstruation

Ovarian-hormones

Changes in the levels of oestrogen and progesterone in the blood during the menstrual cycle

Negative and positive feedback mechanisms controlling the menstrual cycle

  • The four hormones all interact to control the menstrual cycle via both negative and positive feedback
    • FSH and oestrogen
      • FSH stimulates the development of a follicle, and the follicle wall produces the hormone oestrogen; it can be said that FSH stimulates the production of oestrogen
      • As well as causing growth and repair of the endometrium, oestrogen also causes an increase in FSH receptors; this makes the follicles more receptive to FSH which, in turn, stimulates more oestrogen production
        • This is positive feedback
      • When oestrogen levels are high enough, it inhibits the secretion of FSH
        • This is negative feedback
    • LH and oestrogen
      • When oestrogen rises to a high enough level, it stimulates the release of LH from the pituitary gland, causing ovulation on around day 14 of the cycle
      • After ovulation, LH causes the wall of the follicle to develop into the corpus luteum, which secretes more oestrogen
        • This is positive feedback
    • LH and progesterone
      • LH stimulates the wall of the follicle to develop into the corpus luteum, which secretes progesterone
      • Progesterone thickens and maintains the endometrium but also inhibits the secretion of FSH and LH from the pituitary gland
        • This is negative feedback

Hormones-in-the-menstrual-cycle

Where hormones involved in the menstrual cycle are made and act - remember that hormones travel around the body in the bloodstream but only have an effect on a target organ

Hormones & IVF

  • A couple may find it difficult to conceive a baby naturally
  • This can be due to insufficient levels of reproductive hormones affecting the development of egg and sperm cells, or as a result of issues with the reproductive system of the male or female
  • One possible treatment is for eggs to be fertilised by sperm outside the body in carefully controlled laboratory conditions
    • This is known as in vitro fertilisation, or IVF
  • Although the process can vary, it normally follows the same main steps:
    • The first step involves stopping the normal secretion of hormones; the woman takes a drug to inhibit the secretion of FSH and LH from the pituitary gland
      • This also causes oestrogen and progesterone secretions to stop
    • This temporarily halts the menstrual cycle, allowing doctors to control the timing and quantity of egg production in the woman's ovaries
    • The woman is then given injections of FSH and LH to stimulate the development of follicles; as the injection gives a much higher FSH concentration than is present during a normal menstrual cycle, 'superovulation' occurs
      • Many more follicles than normal begin to mature
    • The eggs are then collected from the woman and fertilised by sperm from the man in sterile conditions in the laboratory
    • The fertilised eggs develop into embryos
    • At the stage when they are tiny balls of cells, about 48 hours after fertilisation, one or more embryos are inserted into the mother’s uterus
    • Finally, extra progesterone is normally given to the woman to ensure the endometrium is maintained
  • The success rate of IVF is low (~30%) but there have been many improvements and advancements in medical technologies which are helping to increase the success rate

 

 

 

 

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