Lecture 6 - Early Embryology 2

Question 1

Why is the embryonic period (weeks 3-8) so vital in development?
What damage can occur if exposure to damaging substances occur within each of the 3 phases of gestation?

Answer 1

• All the structure of the body are made, and it is the period of greatest change.
• Pre-embryonic period - lethal effects
• Embryonic period = Can cause life problems
• Foetal period = Lower sensitivity with exception to CNS

Question 2

When and where is the primitive streak formed?

What does this indicate?

Answer 2

• At the end of the 2nd week on the dorsal surface of the epiblast - at the end of the streak is the primitive node.
• Indicates the start of gastrulation
  NB: primitive node ensures we achieve left-right asymmetry.

Question 3

What is gastrulation and what occur as a result of it?

Answer 3

• Gastrulation is the migration and invagination of cells

- Displacement of hypoblast layer means bilaminar disc become trilaminar disc (ectoderm, mesoderm, endoderm)

Question 4

What does the ectoderm, mesoderm and endoderm go onto form?

Answer 4

Ectoderm = organs & structure that contact outside world, e.g.: nervous system + epidermis 
Mesoderm = Supporting tissues, e.g.: muscle, cartilage, bone, vessels etc.
Endoderm = Internal structures, e.g.: epithelial lining of GIT, respiratory tract etc.

Question 5

What appears in the midline of the trilaminar disc after gastrulation?
What is the role of this structure?

Answer 5

• The notochord
• Directs conversion of ectoderm to neuroectoderm by releasing signalling molecules, and causes production of the neural tube.

Question 6

What are somites and how are they formed?

How often do they appear and how many pairs are left in total?

Answer 6

• Somites are organisations of the paraxial mesoderm in segments.
• 1st pair appear at 20 days and they appear at 3 pairs/day, with 31 pairs left in total.

Question 7

What are the 3 main derivatives of somites?

Answer 7

From a block of mesoderm cells, organised degeneration means somites form derivatives:

• Dermatome = skin sections (dermis)
• Myotome = muscle sections (muscles)
• Sclerotome = hard tissue sections (bones)

Question 8

What does the organisation of mesoderm into somites give rise to?

Answer 8

Repeating structures such as vertebrae, ribs, intercostal muscles & spinal cord segments.

Question 9

What kind of folding occurs together to form cephalocaudal folding?
What kind of folding occurs alongside cephalocaudal folding to create an envelope structure?
What consequence does this folding have on embryo structure?

Answer 9

• Head & Tail folding
• Lateral folding
• Means ectoderm is the presenting tissue on the outside with endoderm tucked nicely on the inside.