Lecture Outline

• Periods of Prenatal Development– Period of the zygote– Period of the embryo– Period of the fetus

• Teratogens– General Principles – Cigarette Smoking and Alcohol (examples)

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

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 (2nd trimester):

• Between approximately 16-20 weeks, mother starts to feel movement of fetus

 

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)

 

Period of the Fetus (3rd trimester) con’t:

• Near the end of pregnancy, fetus is awake more often than earlier in pregnancy

– But still spends most of its time sleeping (like newborns)

• REM sleep is present

• Fetal activity level is correlated with infant activity level

• 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)

Cigarette Smoking:

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

© Allyn & Bacon/Longman 2007

Low Birth Weight and Disabilities

Mechanisms (Smoking):

• 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

Mechanisms (Alcohol):

• Interferes with brain development

• A woman’s body requires a lot of oxygen to metabolize alcohol

– Fetus receives less oxygen