Topic 5: Early Human Development - Pre-Embryonic Period

Question 1

What period does the pre-embryonic period last from?

Answer 1

2 weeks

Question 2

What are the 3 stages of the pre-embryonic period?

Answer 2

1. zygote
2. morula
3. blastocyst

Question 3

What occurs to the zygote?

Answer 3

• 1 cell, diploid
• undergoes “rapid” mitotic divisions called cleavage divisions

Question 4

What is a morula? (2)

Answer 4

• solid ball of 16-32 cells called blastomeres
• same size as ovum/zygote (has not increased in size, even if dividing)

Question 5

What is the time period of the morula stage?

Answer 5

72 hours

Question 6

What is the time period of the blastocyst stage?

Answer 6

5 days

Question 7

What are the features of a blastocyst?

Answer 7

1. fluid filled cavity develops called blastocoel/blastocyst cavity
2. Embryoblast (Inner cell mass) - cells will divide and differentiate to form the embryo
3. Trophoblast (made of trophoblast cells) - cells will divide and differentiate to form the chorion
4. Blastocyst moves from uterine tube and attaches to endometrium of uterine wall = implantation

Question 8

What does it mean for a cell to differentiate?

Answer 8

undergo changes to become more specialized

Question 9

What is the function of the chorion?

Answer 9

• provides nutrients to developing embryo

Question 10

When does implantation occur?

Answer 10

6 days after fertilization

Question 11

After implantation, what does an embryoblast do? (2)

Answer 11

• remember that embryoblasts are an inner cell mass
• they will form a 2-layered (bilayered) embryonic disc composed of epiblast and hypoblast

Question 12

What will an epiblast do? (2)

Answer 12

undergo mitosis to form:
amnion - with a fluid fillied amoniotic cavity
3 germ layers of the embryo

Question 13

What does hypoblast undergo?

Answer 13

• mitosis to form the yolk sac