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Lecture Outline Conception Periods of Prenatal Development Period of the zygote Period of the embryo Period of the fetu PowerPoint Presentation
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Lecture Outline Conception Periods of Prenatal Development Period of the zygote Period of the embryo Period of the fetu - PowerPoint PPT Presentation


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Lecture Outline Conception Periods of Prenatal Development Period of the zygote Period of the embryo Period of the fetus. Conception Ovulation: Release of ovum (egg) from one of a woman’s two ovaries Occurs approximately every 28 days

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Presentation Transcript
slide1
Lecture Outline
  • Conception
  • Periods of Prenatal Development
    • Period of the zygote
    • Period of the embryo
    • Period of the fetus
slide2
Conception
    • Ovulation: Release of ovum (egg) from one of a woman’s two ovaries
      • Occurs approximately every 28 days
      • Egg moves through one of the fallopian tubes toward the uterus
      • If sexual intercourse occurs near ovulation, conception can occur
        • A sperm must penetrate the outer membrane of the egg
          • Each sperm and egg cell has only 23 chromosomes
          • When merged, the resulting cell has 46 chromosomes
      • Conception usually occurs in the fallopian tube
      • Fertilized egg is called a zygote
slide3
Period of the Zygote (or Germinal Period)
  • Lasts about 2 weeks, from conception through implantation
  • Zygote undergoes mitosis (cell duplication) as it travels down the fallopian tube to the uterus
  • By approximately the 4th day after conception, the zygote has become a blastocyst
    • Fluid-filled ball of cells
slide4
Two parts of the blastocyst:
  • Inner cell mass: Cells on the inside of the blastocyst
    • Will become the embryo
  • Trophoblast: Cells on the outside of the blastocyst
    • Will develop into tissues that protect and nourish the embryo
slide5
Implantation of the blastocyst into the uterine wall begins approximately 7 to 9 days post-conception
  • Support structures begin to develop from the trophoblast after implantation
slide7
Support Structures:
  • Amnion: Membrane that encloses the embryo in amniotic fluid
    • Cushions organism from injury
    • Helps to keep temperature constant
slide8
Placenta: Organ that is fed by blood vessels from the mother and from the embryo
    • Connected to the embryo by the umbilical cord
    • Allows nutrients and oxygen to reach the organism and waste products and carbon dioxide to be carried away
    • Provides some protection to the embryo by preventing some substances from reaching the embryo’s bloodstream
slide10
By 2 weeks post-conception, inner cell mass has differentiated into three layers of cells:
    • Ectoderm --> nervous system, outer layer of skin, nails, teeth, ears, eyes
    • Mesoderm-->muscles, skeleton, circulatory system, inner layers of skin
    • Endoderm-->digestive system, lungs, urinary tract, glands
slide11
Period of the Embryo
  • Lasts from the 3rd through the 8th week of pregnancy (about 6 weeks)
slide12
Ectoderm folds over to form a neural tube (primitive spinal cord)
    • Top of the neural tube swells to form a brain (about 3.5 weeks post-conception)
  • External body structures (e.g., arms, legs) and internal organs (e.g., heart) begin to develop
  • Rapid brain development occurs
slide13
Period of the Fetus
  • Lasts from the ninth week post-conception until the end of pregnancy (approximately 38 weeks)
slide14
Between approximately 9-12 weeks:
  • Organs, muscles, and nervous system start to become organized and connected
    • By about 12 weeks, fetus engages in most movements that are present at birth
      • Exs: kicking, thumb-sucking, grasping, swallowing
  • Expansion and contraction of lungs (“fetal breathing”)
slide15
Period of the Fetus (2nd trimester):
  • Between approximately 16-20 weeks, mother starts to feel movement of fetus
slide16
Period of the Fetus (3rd trimester):
  • Fetus triples its weight during the last trimester
    • Brain growth also continues
      • Cerebral cortex enlarges
  • Fetus reacts to a variety of sounds
  • Age of viability: The point at which a fetus can first survive on its own
    • Approximately 28 weeks post-conception (without major medical intervention)
slide18
General Principles of Teratogenic Effects:
  • Dose: Larger doses over longer time periods usually have more negative effects
  • Heredity: The genetic makeup of the mother and embryo/fetus influence the effect of a teratogen
slide19
Timing: Effects of a teratogen vary with the age of the organism at the time of exposure
    • Sensitive Period: Time during which basic structures are being formed
      • Each major organ system has its own sensitive period
        • An organ system is most vulnerable to teratogens during its sensitive period
slide21
Cumulative Risk: Effect of a teratogen may be worse if there are other risk factors present (e.g., poor nutrition, lack of medical care, other teratogens)
slide22
Tobacco:
  • Most consistently associated with low birth weight, which is a risk factor for other developmental problems
slide23
Tobacco: Mechanisms
  • Nicotine constricts blood vessels
    • Reduces blood flow to the uterus and causes placenta to grow abnormally
      • Reduces the transfer of nutrients to the fetus
  • Nicotine raises the concentration of carbon monoxide (and decreases oxygen) in mother’s and fetus’ bloodstreams
    • Likely to slow fetal growth and may damage CNS
slide25
Fetal Alcohol Syndrome:
  • Facial abnormalities and small head
  • Slow physical growth and small size
  • Mental retardation and/or other cognitive impairments (in memory, attention, language)
  • Hyperactivity
  • Impaired motor coordination
slide26
Alcohol: Mechanisms
  • Interferes with brain development
  • A woman’s body requires a lot of oxygen to metabolize alcohol
    • Fetus gets less oxygen