The Pre-embryonic Period

Question 1

Pre-embryonic period

first two weeks of development

Answer 1

cleavage 
- 1st mitotic division 
- formation of morula 
compaction 
- formation of blastocyst 
implantation 
- blastocyst makes contact with the endometrius of the uterus which establishes the pregnancy

Question 2

Fertilisation

Answer 2

• oocyte is released from the ovary
• travels along the Fallopian tube
• fertilised in the AMPULLA (nearest part to ovary)
• fertilised oocyte is the ZYGOTE
• ideal implantation site of blastocyst is the posterior uterine wall

Question 3

Week 1

cleavage

Answer 3

Cleavage (continues to form morula) 
- 30 hrs after fertilisation 
- 2 blast immerse of equal size 
Zona pellucida 
- glycoproteins shell 
Morula 
- cells are TOTIPOTENT (all identical - undifferentiated)

Question 4

Pre-implantation Genetic Diagnosis

PGD

Answer 4

a cell can safely be removed from the morula in vitro and tested for serious heritable conditions prior to transfer of the embryo into the mother

Question 5

Week 1

compaction - the first differentiation

Answer 5

• formation of the first cavity
• BLASTOCYST
  Outer cell mass TROPHOBLAST
• will become the supporting tissue e.g. the placenta
  Inner cell mass EMBRYOBLAST
• cells that become the embryo
  Blastocyst cavity
• fluid filled space
  Zona pellucida

Question 6

Length of pregnancy

Answer 6

Post fertilisation = 38 weeks

But calculated from date of LMP (gives a definitive date) so +2 = 40 weeks

Question 7

Week 1

implantation begins

Answer 7

The conceptus has 107 cells

• 99 trophoblast cells (early stages the placenta has priority)
• 8 embryoblast cells

Question 8

Week 2

“the week of twos”

Answer 8

differentiation continues
From TROPHOBLAST
- syncytiotrophoblast (multinucleated layer - good for transport)
- cytotrophoblast (seed layer - allows the syncytiotrophoblast to continue with growth)
EMBRYOBLAST becomes a bilaminar disk
- epiblast
- hypoblast

Question 9

The end of week 2

Answer 9

The conceptus has implanted 
The embryo has two cavities 
- amniotic cavity 
- yolk sac 
Is suspended by connecting stalk within a supporting sac of the chorionic cavity

Question 10

Implantation complete

days 9-10

Answer 10

Invasive process
- uterine epithelium is breached (can cause bleeding that can be mistaken for a normal menstrual cycle)
- Fibrin plug - for healing of the invasion site
Establishes a maternal blood flow signing the placenta (uteroplacental circulation begins at day 12)
- embryo goes from histiotrophic (relying on tissue support) to haemotrophic (relying on the support of the maternal blood supply)
- establishes the basic structural unit if maternal-fetal exchange

Question 11

Implantation defects (at the wrong site)

Answer 11

Ectopic pregnancy
- somewhere other than the uterine body (commonly the Fallopian tube)
- can rapidly become life threatening due to risk of haemorrhage because of invasive implantation
Placenta praevia
- implantation in the lower uterine segment
- requires C-section delivery as placenta is at risk of bleeding during labour

Question 12

Day 13

Answer 12

Formation of secondary (definitive) yolk sac

- pinches off from primitive yolk sac

Question 13

Early pregnancy loss

Answer 13

• approximately 50% of all zygotes lost in the first 2-3 weeks
• 15% of diagnosed pregnancies will miscarry
• 1% of women suffer from recurrent miscarriage (in 3 consecutive pregnancies)