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 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

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 4 th day after conception, the zygote has become a blastocyst –Fluid-filled ball of cells

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

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

Conception and Implantation

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

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

The Placenta and Umbilical Cord

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

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

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

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

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”)

Period of the Fetus (2 nd trimester): Between approximately weeks, mother starts to feel movement of fetus

Period of the Fetus (3 rd 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)

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

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

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

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)

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

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

Fetus exposed to carcinogens contained in tobacco

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

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