Development of Embryo

Question 1

What is the structure of the blastocyst?

Answer 1

Inner cell mass - embryoblast (subsequently forms the embryo).

Outer layer of cells - trophoblast (will form the placenta.

Question 2

A week after fertilisation, the implanted blastocyst starts to differentiate into various types of cell.

What do the cells of the embryoblast develop into?

Answer 2

The cells of the embryoblast split into two: the yolk sak on one side and the amniotic cavity on the other.

Question 3

What sits between the yolk sac and the amniotic cavity?

Answer 3

The embryonic disc.

Question 4

What do the cells of the embryonic disc develop into?

Answer 4

The foetal pole, and eventually into the foetus.

Question 5

What are some intrinsic factors affecting foetal growth?

Answer 5

Maternal:
- height & weight
- parity
- ethnic group

Fetal:
- sex
- genes/inherited conditions
- multiple pregnancy

Question 6

How does parity affect foetal growth?

Answer 6

Higher parity is positively associated with macrosomia.

Question 7

What are some extrinsic factors affecting foetal growth?

Answer 7

Maternal:
- social class
- nutritional status
- environment: altitude
- pre-existing disease: cardiac/renal/respiratory/vascular
- pregnancy-related disease: HTN, diabetes

Foetal:
- nutrition: placental
- teratogenic: tobacco, narcotics, alcohol, medication
- infective: rubella, CMV, listeria, syphilis

Question 8

How does altitude affect foetal growth?

Answer 8

High altitude exposure increases the incidence of pregnancy complications and neonatal morbidity, such as intrauterine growth restriction (IUGR).

Chronic hypoxia slows fetal growth and reduces the pregnancy-associated rise in uterine artery blood flow.

Question 9

What surrounds the complex of the developing embryo?

Answer 9

The chorion

Question 10

What are the 2 layers of the chorion?

Answer 10

The cytotrophoblast (inner layer) and the syncytiotrophoblast (outer layer).

Question 11

What is the syncytiotrophoblast embedded in?

Answer 11

The endometrium

Question 12

Over a short time, what forms around the yolk sac, embryonic disc and amniotic sac?

Answer 12

A space called the chorionic cavity

Question 13

How are these structures (yolk sac, embryonic disc and amniotic sac) suspended from the chorion?

Answer 13

By the connecting stalk

Question 14

What does the connecting stalk go on to form?

Answer 14

The umbilical cord

Question 15

At around 5 weeks gestation, what does the embryonic disc develop into?

Answer 15

The foetal pole

Question 16

The foetal pole contains 3 layers. What are they?

Answer 16

1) Ectoderm (outer layer)
2) Mesoderm (middle)
3) Endoderm (inner)

These three layers go on to become all the different tissues of the body.

Question 17

What does the endoderm go on to form?

Answer 17

• GI tract
• Lungs
• Liver
• Pancreas
• Thyroid
• Reproductive system

Question 18

What does the mesoderm go onto form?

Answer 18

• Heart
• Muscle
• Bone
• Connective tissue
• Blood
• Kidneys

Question 19

What does the ectoderm go onto form?

Answer 19

• Skin
• Hair
• Nails
• Teeth
• CNS

Question 20

When does the foetal heart begin to form/beat?

Answer 20

Around 6 weeks gestation

Question 21

When do the spinal cord and muscles begin to develop?

Answer 21

Around 6 weeks gestation

Question 22

At how many weeks gestation have all major organs started to develop?

Answer 22

Around 8 weeks gestation

Question 23

During what phase of the menstrual cycle does the endometrium thicken and get ready for a fertilised egg to arrive?

Answer 23

Follicular phase

Question 24

What makes the endometrial tissue highly vascular?

Answer 24

The myometrium sends off artery branches into the endometrium.

Initially, these arteries grow straight outwards like plant shoots.

As they continue to grow, they coil into a spiral. These thick-walled and coiled arteries are bunched together, making the endometrial tissue highly vascular.

These are known as the spiral arteries.

Question 25

When the blastocyst implants on the endometrium, what grows into the endometrium?

Answer 25

The outer layer of the blastocyst (the syncytiotrophoblast) grows into the endometrium.

It forms finger-like projections called chorionic villi.

Question 26

What do they