Lecture 5 - Early Embryology 1

Question 1

How long is human gestation?

Define the pre-embryonic, embryonic and foetal periods?

Answer 1

• 38 +/- 2 weeks (fertilisation to birth)
• Pre-embryonic period = 0-2 weeks
• Embryonic period = 3-8 weeks
• Foetal period = 9-38 weeks
  NB: Clinically pregnancy is calculated from time of last menstruation so is 40 weeks.

Question 2

What are the key events occurring during the pre-embyronic period?

Answer 2

• Cleavage of fertilised oocyte to form morula
• Followed by compaction to form blastocyst
• Blastocyst implantation into uterus

Question 3

Where does fertilisation of oocyte occur? What is the oocyte then called?
Where does zygote then need to implant?
How long are oocytes and sperm viable for?

Answer 3

• In the ampulla of the fallopian tube
• Zygote
• The posterior uterine wall
• oocytes = 1 day, sperm = 3 days

Question 4

What is the first developmental process that occurs to the zygote 30 hours after fertilisation?
What does this result in?
What is this structure surrounded by?

Answer 4

• Cleavage occurs (splitting) to form blastomeres
• This results in formation of 2 blastomeres of equal size
• Blastomeres are surrounded by zona pellucia - a glycoprotein shell that prevents further sperm from penetrating the oocyte.

Question 5

As mitotic divisions continue to occur after cleavage, what is formed?

Answer 5

• A morula, a solid ball consisting of 16 cells.

Question 6

What is the next step in week one after formation of the morula?
What are the 2 distinct populations of cells in this structure?

Answer 6

• Compaction, which is where a blastocyst (fluid filled cyst)/the first compartment is formed.
• Embryoblasts (inner cell mass) + trophoblasts (outer cell mass)

Question 7

What kind of cells are in present before and then after compaction?

Answer 7

Before = totipotent 
After = pluripotent

Question 8

What occurs after compaction at the end of week 1 gestation?

Answer 8

• The blastocyst hatches from the zona pellucida

- Is now free and able to implant (day 6-7) in the posterior uterine wall.

Question 9

At the start of week 2, further differentiation occurs. What 2 distinct cell layers emerge from outer cellular layers?
What does the inner cell mass become and what are its 2 groups of cells? Which of these groups will form the embryo?

Answer 9

• Outer cell mass becomes 1) syncytiotrophoblast + 2) cytotrophoblast
• Inner cell mass becomes the bilaminar disc. The 2 groups of cells are the epiblasts and hypoblasts
• Epiblasts will go onto form the embryo

Question 10

What has formed by the end of the 2nd week?

Answer 10

• Embryo with 2 cavities (amniotic sac + yolk sac) suspended by connecting stalk within a large supporting sac called the chorionic cavity.

Question 11

What is the bilaminar disc?

What sits on the inside of the blastocyst cavity and what sits on the outside?

Answer 11

• A 2 layered embryonic disc formed from hypoblasts and epiblasts
• Bilaminar disc sits on inside - syncytiotrophoblast and cytotrophoblast sit on outside

Question 12

Name 2 pregnancy disease that are associated with implantation + explain what they are

Answer 12

1) Ectopic pregnancy - when implantation occurs in areas elsewhere from the uterine wall (most commonly fallopian tube), which can be life threatening.
2) Placenta praevia - when implantation occurs close to the outlet of the uterus - can cause haemorrhage in pregnancy and requires C-section.

Question 13

What % of zygotes are lost within first 2-3 weeks?
How many pregnancies miscarry?
How many women suffer from recurrent miscarriage?

Answer 13

• 50%
• 15%
• 1%