L17: Intro to dev (main stages, germ layers, morphogens)

Question 1

Give the 2 hemispheres found in unfertilised egg

Answer 1

• Animal (smaller, top)
• Vegetal (larger, bottom - ‘yolk’)

Question 2

Define cleavage and gastrulation

Answer 2

• Cleavage: Cell division in early embryo
• Gastrulation: Cell movements which produce gut and 3 primary germ layers
  (morula -> blastula -> 2 then 3-layered gastrula w/ blastopore)

Question 3

Why is the animal pole more rapidly dividing than vegetal in early embryonic dev? (Xenopus)

Answer 3

• The first 2 cleavages are equal holoblastic w/ nuclei displaced animal-ward
• The 3rd is unequal holoblastic -> more rapidly dividing

Question 4

Give the 3 primary germ layers w/ their organs (vert. vs insect)

Answer 4

1. Endoderm:
   - gut, liver, lungs (V)
   - gut (I)
2. Mesoderm:
   - skeleton, muscle, kidney, heart, blood (V)
   - muscle, heart, blood (I)
3. Ectoderm:
   - skin, NS (V)
   - cuticle, NS (I)

Question 5

Induction in formation of the eye (V)

Answer 5

• Left optic vesicle removed and transplanted to lateral side
• Eyes begin as pair of diverticula from lateral aspects of forebrain
  -> develop into optic cup (forms future retina)
• Invaginated ectoderm forms the lens
• Overlying ectoderm forms cornea

Question 6

Methods for inducing signals

Answer 6

1. Diffusion:
   Secreted molecules, long range
2. Direct contact:
   Ligand on one cell directly to receptor on another, v. short range

Question 7

5 Key signalling pathways in dev (+ their main role)

Answer 7

• TGF-beta rec.s (a/p axis specification)
• RTKs (migration and proliferation)
• Wnt rec.s (parasegment boundaries)
• Hh rec.s (limb development)
• Notch/Delta rec.s (cell fate decisions in the CNS)

Question 8

Pattern formation (w/ example)

Answer 8

• Conc. of morphogen wrt to threshold values determines differentiation of cells by group
• In order to maintain a gradient at a steady state there has to be a ‘sink’, actively degrading the morphogen

e.g. A posterior-anterior gradient of Shh specifies digits in the developing limb bud (V)

Question 9

Reciprocal induction in kidney development

Answer 9

• Acts in 2 directions; the two cell types both have an influence on each other
• Kidney begins as ureteric bud (epithelium surrounded by mesenchyme - ‘metanephric blastema’)
• Mesenchyme induces bud branching whilst simultaneously, bud induces condensation of cells resulting in mesenchyme-epithelium transition (MET) to form tubules
  -> renal tubule forms, eventually glomerulus develops
• Mesenchyme secretes GDNF
• Ureteric bud expresses Ret, GDNF rec.-alpha
  -> GDNFR-a and Ret are co-receptors; binding of GDNF to GDNFR-a allows interaction with Ret; activating its kinase domain in the cell interior
  -> classical RTK
• LIF purported to be ureteric bud signal
  *EMT is the reverse process, associated w/ metastases

Question 10

Limb bud formation (key regions, specifying hind vs forelimb, evidence for ZPA role)

Answer 10

• Within the progress zone, AER (apical epidermal ridge); growth zone where most of proliferation occurs which secretes FGFs, and ZPA (zone of polarising activity); important for determining digit fate
• Hind vs forelimb thought to be decided by expression of Tbx4 vs Tbx5
• Transplantation experiments of ZPA from posterior of limb bud to anterior of another - fusion occurred w/ 2 sets of digits, therefore ZPA is inductive signal instructing what fate each digit gets (determined by [Shh])