Lecture Outline Conception Periods of Prenatal Development Period of the zygote Period of the embryo Period of the fetu

1. Lecture Outline • Conception • Periods of Prenatal Development • Period of the zygote • Period of the embryo • Period of the fetus

2. 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

3. 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

4. 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

5. 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

6. Conception and Implantation

7. Support Structures: • Amnion: Membrane that encloses the embryo in amniotic fluid • Cushions organism from injury • Helps to keep temperature constant

8. 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

9. The Placenta and Umbilical Cord

10. 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

11. Period of the Embryo • Lasts from the 3rd through the 8th week of pregnancy (about 6 weeks)

12. 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

13. Period of the Fetus • Lasts from the ninth week post-conception until the end of pregnancy (approximately 38 weeks)

14. 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”)

15. Period of the Fetus (2nd trimester): • Between approximately 16-20 weeks, mother starts to feel movement of fetus

16. 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)

17. Teratogen: Any environmental agent that can cause damage during the prenatal period

18. 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

19. 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

21. 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)

22. Tobacco: • Most consistently associated with low birth weight, which is a risk factor for other developmental problems

23. 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

24. Fetus exposed to carcinogens contained in tobacco

25. 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

26. Alcohol: Mechanisms • Interferes with brain development • A woman’s body requires a lot of oxygen to metabolize alcohol • Fetus gets less oxygen