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