Embryology

Question 1

What is the pre-implantation period?

Answer 1

Also known as the pre-implantation phase. At this point, the embryo hasn’t implanted onto the uterus yet. It is still doing cell division/ cleavage in the fallopian tubes/ovaries. Weeks one to two.

Question 2

What is the embryonic period?

Answer 2

This is the post implantation phase. The embryo has implanted on to the uterus. This is when the organs develop. Lasts through weeks 3 to 8.

Question 3

What is the foetal period?

Answer 3

Where the baby grows. Weeks 9 to 38. Then birth and post- natal development happen.

Question 4

Day 0/1

Answer 4

Fertilisation/ zygote (fertilised egg, single cell only)

Question 5

Day 2/3

Answer 5

Cleavage/ Cell division. Becomes 16 cells/ morula

Question 6

Day 4

Answer 6

• Early blastocyst
• 32+ cells
• First differentation: Inner cell mass (embryoblast) and outer cell mass (trophoblast).

Question 7

Day 5-11

Answer 7

• Implantation on uterine wall

Question 8

Day 11?

Answer 8

Bilaminar disc: refers to the epiblast and hypoblast evolved from the embryoblast. The epiblast are like jam (epiblast) and peanut butter (hypoblast) with two pieces of bread sandwiching them (the yolk sac and amnion).

Question 9

Day 12-14

Answer 9

• Placenta formation begins
• Elongation of embryo begins
• Primitive streak formation

Question 10

Day 15

Answer 10

• Epiblast cells migrate towards primitive streak
• Cells enter primitive streak and differentiate into endoderm or mesoderm
• Cells that do not migrate through primate streak differentiate into ectoderm

Question 11

What is the fate of the ectoderm?

Answer 11

▫Nervous system and sensory organs (neural ectoderm)

▫Epidermis (surface ectoderm)

▫Neural crest

Question 12

Fate of the mesoderm?

Answer 12

▫Endothelium

▫Limbs

▫Musculoskeletal system

▫Cardiovascular system, blood

▫Lymph, spleen, kidneys

▫Reproductive system

Question 13

Fate of the endoderm?

Answer 13

Gut tube

Liver, pancreas

Trachea and lungs

Auditory system

Urinary system

Question 14

Day 17-28?

Answer 14

Neurulation:

• Neural plate – thickening of neural ectoderm (trilaminar disc)
• Movement of ectoderm towards central groove
• Causes neural folds to form and rise up
• Neural folds meet to form a tube
• Neural tube sinks into embryo and opening is covered by surface ectoderm

Question 15

Day 25-27

Answer 15

•Neuropores internalised

Failure in closure of anterior neuropore-Anencephaly

Failure in closure of posterior neuropore-Spina bifida

1. Embryo folding
2. Folded embryo can then enter foetal phase – growth
3. Organogenesis of organ systems occurs in parallel