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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