Embryonic and Foetal Period

Question 1

What is neurulation?

Answer 1

• Formation of the neural tube
• Occurs by folding of thickening of ectoderm overlying the notochord

Question 2

What are the three steps of neurulation?

Answer 2

• Lateral edges of thickened ectoderm form neural folds
• Depressed midregion forms neural groove
• Neural folds fuse in midline forming the neural tube (Narrow caudal portion (spinal cord) and broader cephalic portion (brain))

Question 3

What is lateral folding?

Answer 3

• Involves folding of the flat ectoderm sheet laterally around the embryo, joining in the middle

Question 4

What is cephalo-caudal folding?

Answer 4

• Folding of cranial and caudal regions of the embryo

Question 5

Give two examples of neural tube defects. What are they usually associated with and how have the rates of NTDs changed over time?

Answer 5

• Anencephaly (if failure to close in cranial region)
• Spina bifida (if failure to close from cervical region caudally)
• Associated with mutations of VANGL genes (in PCP pathway)
• Decreased following recommendation of folic acid for pregnant women

Question 6

Describe the three layers formed from the mesoderm during early development.

Answer 6

• Paraxial mesoderm - future MSK - formed from proliferation of mesoderm cells close to midline
• Intermediate mesoderm - future GU system - lateral to PM
• Lateral plate mesoderm - made of parietal layer (continuous with layer covering amnion), visceral layer (same as parietal but covering yolk sac)

Question 7

Describe what happens to lateral plate mesoderm by end of week 4.

Answer 7

• Lateral folding occurs - ventral closure of endoderm and parietal mesoderm
• Amniotic cavity pulled down - embryo surrounded by amniotic cavity
• Parietal layer lines intraembryonic cavity. Visceral layer surrounds primitive gut.

Question 8

Describe the process of vasculogenesis.

Answer 8

• Blood islands form in visceral mesoderm at 3 weeks
• Mesoderm cells form endothelial cells to form vessels using VEGF
• Developing vessels extend into embryo.
• Needed for transport of nutrients from mother to tissues in embryo

Question 9

Give an example of a defect that can occur during vascular development in the embryo.

Answer 9

CAPILLARY HAEMANGIOMA

Question 10

Describe development of the heart from the lateral plate mesoderm.

Answer 10

• Clusters of vessel from visceral mesoderm merge to form cardiogenic areas
• Merge to form endocardial tubes - myoblasts lined with endothelium
• Tubes fuse to form primitive heart tube
• Tube undergoes elongation and looping to form chambers

Question 11

Give examples of congenital heart defects.

Answer 11

• LATERALITY DEFECTS - dextrocardia
• Aortic/pulmonary stenosis

Question 12

Describe endodermal development

Answer 12

• Endoderm - roof of yolk sac
• As folding occurs, communication with yolk sac narroes. Viteline duct forms
• Yolk sac - role in early nutrition

Question 13

What occurs by end of week 4 of embryonic period?

Answer 13

• Neural tube, primitive gut and primitive heart has formed
• Connection with yolk sac via viteline duct
• Connection with placenta via allantois

Question 14

What occurs by end of week 8 of embryonic period?

Answer 14

• All organs formed
• Increased head size
• Limbs, ears, nose and eyes form