Early Human Development

Question 1

What Takes Part in Pre-Embryonic Period? (First 2 Weeks)

Answer 1

1. Zygote (1 cell, diploid)
2. Morula
3. Blastocyst

Question 2

Zygote

Answer 2

• Forms in uterine tube
• Undergoes 4-5 mitotic divisions called cleavage divisions that will form the morula

Question 3

Morula

Answer 3

• Solid ball of 16-32 cells
• SAME size as Ovum/zygote
• Cells continue to divide and begin to differentiate into different cell types with specific form and function to form the blastocyst

Question 4

Blastocyst

Answer 4

• Features:
  
  • Fluid filled cavity develops = blastocoel/blastocyst cavity
  • Embryoblast (inner cell mass)
    
    • Cells will later divide and differentiate to form the embryo
  • Trophoblast (made of trophoblast cells)
    
    • Cells will divide and differentiate to form chorion - provides nutrients to developing embryo
• Blastocyst moves from uterine tube and attaches to endometrium of uterine wall = implantation

Question 5

What Happens After Implantation?

Answer 5

• The embryoblast (inner cell mass) differentiates into a bilayered (2 cell layers) embryonic disc made up of:
  
  • Epiblast
  • Hypoblast
• Extraembryonic membranes start to form

Question 6

What is Epiblast?

Answer 6

• will undergo mitosis to form:
  
  • amnion - with a fluid filled amniotic cavity
  • 3 germ layers of the embryo

Question 7

What is Hypoblast?

Answer 7

• Will undergo mitosis to form the yolk sac

Question 8

What gets formed in the Embryonic Period? (3rd week to end of the 8th week)

Answer 8

• 3 germ layers are now present forming the embryo (each layer was formed from mitosis and differentiation of epiblast cells):
  
  1. Ectoderm (Outside layer)
  2. Mesoderm (Middle layer)
  3. Endoderm (Inside layer)
• Major organ systems almost completely formed
• Heart begins to beat
• limb buds differentiate
• Extraembryonic membranes continue forming:
  
  1. Chorion
  2. Amnion
  3. Yolk sac
  4. Allantois

Question 9

Ectoderm (Germ Layer)

Answer 9

• Outside Layer
• Will form the nervous system and the epidermis of the skin

Question 10

Mesoderm (Germ Layer)

Answer 10

• Middle Layer
• Will form muscles, cardiovascular system, dermis, and most bones

Question 11

Endoderm (Germ Layer)

Answer 11

• Inside Layer
• Will form epithelial linings of digestive, respiratory, urinary and reproductive systems and their associated glands

Question 12

Chorion (Extraembryonic Membrane)

Answer 12

• Derieved from trophoblast cells
• Surrounds all embryonic membranes
• Chorionic villi = projections of the chorion containing the blood vessels that will form fetal part of placenta
  
  • Where nutrients, wastes, oxygen/carbon dioxide move between fetal and maternal blood

Question 13

Amnion (Extraembryonic Membrane)

Answer 13

• Derived from epiblast
• Forms amniotic cavity - surrounds embryo / fetus with amniotic fluid
  
  • Function of fluid - shock absorption, temperature regulation

Question 14

Yolk Sac (Extraembryonic Membrane)

Answer 14

• Derived from hypoblast
• Forms part of digestive tract
• Source of reproductive germ cells & embryonic blood cells

Question 15

Allantois (Extraembryonic Membrane)

Answer 15

• Projection off of the yolk sac
• Forms the umbilical cord and the urinary bladder

Question 16

What happens in the Fetal Period? (9th week to end of 40th week)

Answer 16

• Growth and maturation of organs