Fertilization and Development
Step 1: Fertilization
• Process of sperm fusing
(joining) with an egg
• Produces a zygote
(fertilized egg, diploid
cell)
• In humans, this occurs in
the oviduct
External fertilization
• Gametes fuse outside of the female’s body
• Involves hormone/behavior controls
• Involves the release of hundreds of gametes at the
same time.
Internal fertilization
• Gametes fuse inside the female’s body
• Advantage: no longer need to have part of
life cycle attached to water
• Requires specialized organs to deliver
sperm into the female’s body
Step 2: Mitotic Cell Division of
the zygote to form embryo
• During this time, the embryo is receiving
nourishment from a small amount of stored
yolk
Step 3: Implantation
• About 6-7
days after
fertilization,
the embryo
embeds
itself into
the wall of
the uterus
Step 4: Development of the
Placenta and Amnion
Placenta
• Organ made up of both mother’s and embryo’s
blood vessels
• Site of nutrient, oxygen, and waste exchange
• Does NOT allow mother and embryo blood to mix
rather, the nutrients and wastes DIFFUSE between
the blood vessels
• Acts a filter for SOME substances (alcohol,
caffeine, some viruses CAN pass through)
• Umbilical cord – attaches the embryo to the
placenta. Is cut away after birth and the scar
becomes the naval.
Fetal Alcohol Syndrome
• Group of disorders caused
when a pregnant females
consumes alcohol
– Includes brain damage, stunted
growth, learning disabilities,
physical deformities
– Most dangerous during first
trimester (3 months) because the
organs are DEVELOPING the
most during this time period
– Affects approx. 1 in 750 births
(CDC)
Amnion
• Fluid filled sac that
surrounds the embryo
• Acts as a shock
absorber and cushion
to protect the
developing fetus
Step 5: Cellular Differentiation
• As embryo grows, the cells begin to sort
themselves into layers which give rise to various
organs and organ systems
• At specific times, each cell type begins to change
in order to carry out its specific function
After 8 weeks of development, the embryo is now
called a FETUS.
First Trimester
• Up to week 12
• Organogenesis –
development of body
organs
• Heart beats around week 4
• By week 8 all major
structures of the adult are
present in rudimentary
form
• At end, fetus is only 5cm
long
Second Trimester
• Months 4-6
• Fetus grows about
30cm and is very
active
Third Trimester
• Fetal activity
decreases as it fills
available space
• Fetus is ~50 cm long
• Lungs complete
development
Gestation (AKA Pregnancy)
• In humans, this typically lasts 40 weeks
• Premature birth – babies born prior to
completing 37 weeks of development
– Increase chance of lung and brain problems
– Typically, the earliest survivable birth is 24 weeks
Multiple Births
• When more than one fetus is carried in the uterus
in a single pregnancy period
Monozygotic (Identical) Twins
• One egg is fertilized
and resulting zygote
splits into two separate
individuals
• Offspring have
IDENTICAL genetic
information
• Both twins will be the
same gender
Dizygotic (Fraternal) Twins
• Where multiple sperm
fertilized multiple
eggs
• Each offspring is
unique in their genetic
make up (no more
closely related than
any other 2 siblings)
Labor and Delivery
• Labor – series of rhythmic
contractions of the uterus
accompanied by changes in
the cervix
• Averages 8-13 hours, but can
last longer
• After the baby is removed,
contractions continue to expel
the AFTERBIRTH (placenta,
amniotic sac, umbilical cord)
Breech births
• Fetus attempts to descent through the birth
canal any way but head first
– Results in Cesarean section
Miscellaneous
• Breastfeeding – characteristic that all mammals
share
– Controlled by hormones
– Colostrum – first milk produced, contains
ANTIBODIES and WBCs to give newborn limited
immunity
• Abortion – removal of embryo or fetus, reulting or
causing its death
• Miscarriage – natural or accidental termination of
the pregnancy before 20 weeks (AKA
Spontaneous abortion)
• Stillbirth – when a fetus is >20 weeks and dies in
uterus or during labor