1. Chapter 3: Meiosis & Development Section 3.4: Prenatal Development

2. What are the steps of prenatal development?
Fertilization
Cleavage
Implantation
Gastrulation
Organogenesis
Embryo development (week 1-8)
Fetus development (week 9-40)

3. What happens during fertilization?
Capacitation- chemicals in female body activate sperm
Oocyte secretes chemical to attract sperm

4. What happens during fertilization?
Sperm contact corona radiata- the covering of follicle cell that is protecting secondary oocyte
Acrosome on sperm bursts and enzymes begin eating through next layer of egg called zona pellucida
Conception occurs when sperm head meets cell membrane of egg
The egg cell membrane changes its charge which prevents other sperm from entering cell.
Fertilization membrane forms- this will hold cells together when they start dividing.

5. What happens during fertilization?
Sperm loses its tail
Egg nuclear membrane degenerates
Sperm & egg chromosomes duplicate
Sperm & egg chromosomes meet, nucleus begins reforming creates zygote

6. What happens during cleavage?
Cleavage- period of frequent cell division (mitosis) that begins about 24 hours after fertilization
Blastomere- 2-4 cells mass
Morula- 16 cell stage. Solid ball of cells
Blastocyst- cell division continues and cells push against outer edge and form a fluid filled cyst in center of blastocyst
A group of cells called the inner cell mass will begin accumulating on one side of the blastocyst- cells are beginning to differentiate
This entire process takes about 6-7 days.

7. What happens during implantation?
Day 7, blatocyst impants into uterine lining
Outer layer of cells (trophoblast) from blastocyst secrete human chorionic gonadotropin (hCG)
hCG prevents menstruation
This is what a pregnancy test detects in urine or blood.

8. What happens during gastrulation?
Amniotic cavity forms between inner cell mass and the trophoblast layer
Cells in the blastocyst and inner cell mass will begin to form layers
Outer layer of cells forms ectoderm
Inner layer of cells forms endoderm
Middle layer of cells forms mesoderm
Cells in each of these layers will differentiate and have different fates.

9. What are the cell fates of the 3 tissue layers?
Ectoderm
Skin
Nervous tissue (brain & spinal cord)
Endoderm
Digestive organs
Liver
pancreas
Mesoderm
Muscle
Bone
Reproductive organs
Kidneys
Gastrulation ends around day 14

10. What supportive structures are forming during the first two weeks?
Chorionic villi- finger-like extensions that extend into uterine wall and come close to mom’s blood stream.
Mom and baby’s blood stream NEVER mixes but anything in mom’s blood stream can diffuse into baby’s (and vice versa)
Baby sends wastes into mom’s blood stream
Yolk sac- makes blood cells
Allantois- makes umbilical blood vessels
Umbilical cord attaches to center of placenta
Placenta- fully formed by 10 weeks; links woman to fetus & secretes hormones to maintain pregnancy & sends nutrients & waste back & forth between mom and baby.

11. What happens during organogenesis?
The ectoderm, endoderm, and mesoderm begin forming organs
Embryo is sensitive to environmental influences such as chemicals and viruses

12. Week 3 Heart begins to forms by day 18
Primitive streak forms along back of embryo. This is a primitive notochord which will become spine
Notochord induces cells of ectoderm to fold and form hollow neural tube which eventually becomes brain & spinal cord
Neural tube seals around day 20.

14. What is a neural tube defect?
If NT does not seal, Neural Tube Defect (NTD) will form.
Ex: Spina bifida
Brain or spine is exposed
Baby paralyzed from point of exposure down
Occurs due to lack of vitamin B or folic acid
Pregnant women encouraged to take folic acid supplements/vitamins during pregnancy to prevent NTD
NTD can be detected around 15 weeks with alpha fetoprotein (AFP) blood test that detects a protein from fetus’ liver that is leaked at a fast rate
If detected and caught early enough can sometimes be corrected with in utero surgery.

15. Week 4 Heart begins beating around day 23
Leg and arm buds begin to form
Blood cells form & fill primitive blood vessels
Immature lungs and kidneys develop

16. Week 5 & 6 Enlarged head Apoptosis sculpts fingers and toes
Eyes open but no eyelids or irises
Gene SRY on y chromosome of a boy will begin forming male hormones that stimulate formation of male organs

17. Week 7 & 8 Skeleton of cartilage is formed
Embryo is about size of a paper clip
After 8 weeks it is now called a fetus.

18. Weeks 9-12 Body proportions equal out Bone begins to replace cartilage
Fetus begins coordinating muscle and nerves and begins to move
Fetus sucks thumb, kicks
Urinates & defecates into amniotic sac
Breathes in amniotic fluid
Week 12 ends 1ST TRIMESTER

19. 2nd Trimester Weeks 13-24 Hair Lanugo- downy hair all over body
Eyelashes
Eyebrows
Nipples
Nails
Week 15- gender determined
Skin appears wrinkled due to lack of fat & pink with new capillary formation
Can feel distinct movement
Baby is about 9 inches long

20. 3rd Trimester Weeks 25-40
Fetal brain cells grow and form numerous connections to organs
Fat forms under skin
Digestive & respiratory systems mature
Premature babies often have respiratory diseases and have difficulty digesting milk

21. How can genetic disorders or birth defects be detected?
Chorionic villi sampling- cells are removed from chorionic villi and tested for abnormalities in number or shape of chromosomes.
Done at 10 weeks
Amniocentesis- fluid from amniotic sac is removed by needle and test for abnormalities
Done at 14 weeks

22. How do multiples form? Monozygotic multiples- Dizygotic multiples-
Identical twins
Fertilized egg splits
Twins are genetically identical
Share placenta
Dizygotic multiples-
Fraternal twins
Two sperm fertilized two different eggs
Twins are genetically different
Different placentas

23. What are conjoined twins?
Separation of egg cell begins to occur while organs are developing.
Attachment point depends on where split was occurring & what organs were developing at the time.
Ex: Brittany & Abigail Hensel

24. Brittany & Abigail Hensel
Dicephalic twins- have two heads
Share- liver, bloodstream, all organs below navel, 3 kidneys
Separate- neck, head, heart, stomach, gallbladder, lungs, nervous system
Parents chose not to separate b/c one child usually dies during surgery