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 to 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 sphere of about 60-70 cells

Two parts of the blastocyst:

• Inner cell mass: Cells on the inside of the blastocyst

– Will become the embryo

• Trophoblasts: 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 occurs approximately between 7 and 9 days post-conception

• Support structures begin to develop from the trophoblasts 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

• Chorion: Membrane surrounding the amnion– Develops by the end of the second week post-

conception

• Tiny blood vessels (villi) emerge from the chorion and burrow into the uterine wall– Placenta begins to develop from these blood

vessels

• Placenta is fed by blood vessels from the mother and from the embryo

• Placenta is connected to the embryo by the umbilical cord

• Placenta is semi-permeable

– Some substances can pass through, but not all

• Prevents blood of mother and embryo from mixing directly

• Placenta allows nutrients and oxygen to reach the organism and waste products and carbon dioxide to be carried away

The Placenta

and Umbilical

Cord

Period of the Embryo

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

• Inner cell mass differentiates into three layers (about 3rd week post-conception):

– 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

• 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”)

– Sexual differentiation has begun• Sex of the fetus can be detected with ultrasound by

approximately 12 weeks

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 outside the womb

– Approximately 22-26 weeks (with 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)

Article: Brown et al. (2004)

1. Users would differ from non-users• Users: Lower socioeconomic status; greater obstetric

risk

2. Children prenatally exposed to cocaine would differ from children without exposure in social development, but not in physical or cognitive development

Users only:

3. Mothers who did not keep custody of their children after birth would differ from mothers who did keep custody

• Heavier prenatal drug use, lower SES, greater obstetric risk in mothers who did not have custody

• Their newborns would also show more birth complications (e.g., prematurity)

4. At 2 years of age, children in non-parental care would differ from children in parental care:

• Non-parental care would be higher-quality than parental care

• Children in non-parental care would show better cognitive and social development than children in parental care

Findings:

1. Users differed from non-users

2. Children with prenatal cocaine exposure (PCE) did not differ from children without exposure in terms of physical, social, or cognitive development

3. Mothers who retained custody did differ from mothers who did not retain custody

4. Children in non-parental care:

• Received higher-quality care than children in parental care

• Showed better social and cognitive development than children in parental care