Gastrulation

Question 1

Week 3

“the week of threes”

Answer 1

Three cavities
Three “germ” layers
- rudimentary lineages from which all others will arise

Question 2

Gastrulation

Answer 2

• the process establishing the three germ layers and hence the origin of all tissues of the body
• trilaminar disk
• sets the axes observed in the adult

Question 3

Beginning of gastrulation

Answer 3

Primitive streak appears (in the third week) 
- primitive node with primitive pit 
Cellular rearrangement 
- migration 
- invagination

Question 4

Primitive streak, pit & node

Answer 4

• streak is a narrow groove with bulging edges
• node is located on the cranial end of the streak
• pit is located at the centre of the node
  Once the three layers are established the primitive streak regresses

Question 5

Formation the trilaminar disk

Answer 5

Epiblast cells migrate and differentiate to the edges of the primitive streak - they displace the hypoblast and create a third layer
- fate of the invaginating epiblast depends on where in the steak or node they invaginate

Question 6

Trilaminar disk

Answer 6

The ectoderm/endoderm can be seen in two places when looking at the mesoderm from below/above

Question 7

The notochord

Answer 7

• defines the phylum Chordata
• basis for axial skeleton - defines the midline
• drives the formation of the nervous system
• is what herniates when a disk slips
• it regresses - vestigial remnant in the adult is the nucleus pulposus of intervertebral discs

Question 8

Ectoderm

Answer 8

• organs and structures that maintain contact with the outside world
  E.g. Nervous system, epidermis

Question 9

Mesoderm

Answer 9

• supporting tissue

E.g. Muscle, cartilage, bone, heart and vessels

Question 10

Endoderm

Answer 10

• internal structures

E.g. Epithelial lining of GI tract, respiratory tract

Question 11

Side-specific signalling cascade

Answer 11

• action of ciliated cells at the node results in a leftward flow of signalling molecules

Question 12

Situs Inversus

heart appears on RHS

Answer 12

• commonly results from immobile cilia

- problems arise if there is both normal and mirror-image disposition

Question 13

Monozygotic Twins

Answer 13

• embryo splits after first cleavage – two embryos each with its own placenta
  OR
• inner cell mass duplicated – two embryos sharing a placenta
  Splitting can occur very late (embryos can share the same amniotic sac)
  Sometimes separation is not complete - conjoined twins

Question 14

Teratogenesis

Answer 14

• normal embryonic development is disrupted
• week 3 to 8 is the most sensitive to teratogenic insult
• each organ system will have a particular sensitive window

Question 15

Teratogenic Agents

Answer 15

Chemical and infectious agents are known to cause developmental defects

• thalidomide (limbs)
• rubella (head and neck)
• alcohol (CNS)
• certain therapeutic drugs

Question 16

Embryonic Period

Answer 16

weeks 3 to 8

• period of greatest change
• all major structures and systems are formed
• the most perilous for the developing child