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
Low Birth Weight and Disabilities
© Allyn & Bacon/Longman 2007
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