Embryology 2

Question 1

What does week 3 involve?

Answer 1

• Gastrulation- Formation of germ layers
• Neurulation- Formation of neural tube
• Development of somites
• Early development of cardiovascular system

Question 2

remember quick recap of week 2…

Answer 2

the inner cell mass of trophoblast flattens itself to form a bilaminar disc (Epiblast and Hypoblast)

epiblast has amniotic cavity above epiblast
yolk sac is below the hypoblast.

so two cavities (amniotic cavity above epiblast, yolk sac below hypoblast)

Question 3

inner mass cell (epiblast and hypoblast). what other future does it have?

Answer 3

The primitive streak which is is the small dip in the midline of the epiblast. (invagination)
Primitive streak Is important for the embryo- the axis of the embryo is formed.

Question 4

describe the Formation of 3 germ layers

what is this?

Answer 4

• Epiblast cells migrate into the space between the epiblast and hypoblast layers.
• Cells then displace hypoblast.
  -This forms 3 germ layers:
   Ectoderm
   Mesoderm
   Endoderm
  • This is the formation of trilaminar disc.

Question 5

what does the formation of the trilaminar disc mean?

Answer 5

• Now the cells have become specialised.

Question 6

describe the origin of ectoderm, mesoderm, endoderm.

Answer 6

Inner cell mass of blastocyst -> bilaminar disc -> primitive streak -> cells from primitive streak invaginate > displace hypoblast -> endoderm, mesoderm, endoderm

Question 7

describe the Formation of notochord

Answer 7

From the primitive streak, cells from the primitive streak sink down to form a solid tube which invaginate until two crests and a groove is formed -> NOTOCHORD is formed.

Question 8

describe the Formation of neural tube

Answer 8

Notochord induces ectodermal cells in the midline to form a neural tube. Neural plate (in ectoderm) sinks down to form a neural tube.

Question 9

describe the Development of somites

Answer 9

• The neural tube induces the mesoderm to thicken.

Mesoderm splits into three:
- Paraxial mesoderm -> somites -> dermatome, myotome, sclerotome

• Intermediate plate mesoderm -> urogenital system (kidneys and repro)
• Lateral plate mesoderm -> body cavity and peritoneum, pleura
  o Lateral plate mesoderm splits to form a somatic and splanchnic mesoderm. Space formed in between called the intraembryonic coelom.

Question 10

what causes the mesoderm to thicken?

Answer 10

neural tube

Question 11

what does the lateral plate mesoderm further split into?

Answer 11

• Lateral plate mesoderm -> body cavity and peritoneum, pleura

o Lateral plate mesoderm splits to form a somatic and splanchnic mesoderm. Space formed in between called the intraembryonic coelom.

Question 12

what is 4th – 8th weeks-

Answer 12

Embryonic period/ organogenetic period

Question 13

describe 4th-8th week

Answer 13

• Folding into a tube (lateral folds) which started in 3rd week completed
• Neural tube – forebrain, midbrain, hindbrain and spinal cord development
• Heart starts to beat on Day 24!
• Gut formation from endoderm
• Urogenital system formation from intermediate mesoderm
• Body cavities from lateral plate mesoderm
• 43 pairs of somites form in the paraxial mesoderm and differentiate further
• Limb buds form
• Neck development – pharyngeal arches

Question 14

Further development of somites- what does each somite (paraxial mesoderm) do?

Answer 14

divides into 3:

• Dermatome
• Myotome
• Sclerotome

Question 15

dermatome role?

Answer 15

gives rise to dermis of skin

Question 16

what is myotome?

Answer 16

gives rise to muscles

Question 17

what is sclerotome?

Answer 17

forms the bones including vertebrae

Question 18

what does the ectoderm give rise to?

Answer 18

epidermis of skin and neural tube

Question 19

what does the endoderm give rise to?

Answer 19

gut and respiratory

Question 20

what kind of diagnosis might you do prenatal?

Answer 20

Ultrasound scan- 12 week anomaly scan
Blood- AFP
Invasive tests- chorionic villus sampling and amniocentesis

Question 21

what is the risk of teratogenesis?

Answer 21

• 0-3 is risk of death
• 3-8 massive risk of malformations
• 9-38 risk of teratogenesis decreases as there is decreasing sensitivity to teratogens.

Question 22

• The risk posed by a teratogen depends on 3 factors:

Answer 22

o Exposure during critical periods of development
o Dosage of drug/chemical/factor
o Genetic constitution of embryo i.e. some more susceptible than others.