11 - Gastrulation and development of the embryonic germ layers

Postimplantation 

Formation of the pro-amniotic cavity to gastrulation 

Polarisation of the primitive ectoderm 

• Signals from basement membrane induce polarisation of epiblast 

• Polarisation means that the cells develop apical, lateral and basal domains 

Formation of pro-amniotic cavity 

• Cells on apical surface of primitive ectoderm cells undergo programmed cell death 

• Debris phagocytosed by ectoderm cells - gives rise to cavity 

• Polarised ectoderm required for cavitation; the mechanism is unknown 

• Cell death is essential for normal development 

Anterior-posterior axis formation 

• Anterior visceral endoderm (AVE) - specialised region of the VE 

• Essential role in patterning the early embryo 

• The AVE move unidirectionally from its initial position at the distal tip of the egg cylinder to the future anterior of the embryo

Mouse embryo on day 6 (1 day after implantation) 

• The AVE cells - green 

• Cell nuclei - blue 

• Cell membranes - red 

• Expression of transcription factor LIM-1 by AVE - required for head formation - including brain 

• LIM-1 expression in the AVE cells causes these cells to secrete factors that act on the underlying primitive ectoderm, inducing formation of anterior structures 

• The AVE also secretes factors that inhibit gastrulation 

• This ensures gastrulation takes place at the future posterior part of the embryo 

• AVE - crucial role in formation of anterior-posterior axis: promotion of anterior structure formation and inhibits gastrulation 

Gastrulation 

• Is where the 3 embryonic germ layers are established: 

1. Ectoderm 

2. Mesoderm 

3. Endoderm 

Mechanisms of gastrulation in Drosophila 

• During gastrulation of embryo, surface cells invaginate to form mesodermal organs 

• The cells that will become mesoderm start to express a specific marker (brown stain) 

Molecular mechanisms that regulate gastrulation

Cadherins → Regulate ce;;-cell adhesion and important role in regulating cell movements during development 

Transmembrane proteins: 

• Extracellular domain can bind to that of another cadherin of the same type 

• Binding is calcium-dependent 

• Intracellular domain binds to cytoskeleton (requires Beta-catenin) 

• In response to pressure, the Beta-catenin translocates to the nucleus and turns on key genes requires for invagination 

• Mechanical pressure results in Beta catenin-mediated cell signalling 

• Leads to the expression of the twist gene 

• Twist = transcription factor required for invagination 

• Twist turns on mesodermal genes that are required for heart and muscle development

• The mesodermal cells also express the transcription factor ‘snail’ that turns off genes for neuronal development 

• Nuclear translocation of Beta-catenin - causes cadherin-cadherin interactions between neighbouring cells to become destabilised - as the cadherins are no longer attached to the cytoskeleton 

• Enables the cells to move more freely 

Gastrulation in the mouse embryo 

• Takes place in the posterior part of the mouse embryo 

• AVE - secreted factors that inhibit gastrulation 

• This ensures gastrulation takes place in the posterior part 

Cross sections through the embryo 

Some cells that come through the primitive streak displace the visceral endoderm - forming the definitive endoderm 

Some cells that come through the primitive streak are positioned between the ectoderm and endoderm - forms the mesoderm  

Mechanisms controlling gastrulation 

• During gastrulation, cells in primitive ectoderm need to loosen their contact with others so that they can migrate through the primitive streak 

• This cell movement is delamination 

• Ectoderm cells at the primitive streak undergo an epithelial to mesenchymal transition

• For this - E-cadherin levels need to reduce in the epithelial 

How are the E-cadherin levels reduced 

• FGF signalling is required 

• Cells at the primitive streak - synthesise and respond to FGF 

• FGF signalling blocked - E-cadherin levels high - cells can’t migrate through primitive streak 

• This results in absence of endoderm and mesoderm - embryo death 

→ Also, down-regulation of E-cadherin means that Beta-catenin is free to translocate to the nucleus & regulate gene expression 

• Beta-catenin - induces expression of key genes required for gastrulation and differentiation of the mesoderm and ectoderm 

• Eg. transcription factors twist and snail - needed for migration through the primitive streak 

• Brachyury (T) - transcription factor - for differentiation of mesoderm

Brachyury expression in the gastrulating mouse embryo

What if Brachyury is not expressed 

• Homozygous mutants - defects in mesoderm formation - lack trunk and tail - die around embryonic day 10 

Heterozygotes have a shortened tail and abnormal sacral vertebrae 

Incorrect gastrulation in humans 

• Disrupted by genetic abnormalities and toxic insults 

• Caudal dysgenesis (sirenomelia) in humans - similar phenotype to brachyury mouse mutants (insufficient mesoderm is formed) 

• Mesoderm contributes to the formation of lower limbs, urogenital system and vertebrae - abnormalities in these structures 

• Affected individuals - defects include: hypoplasia, lower limb fusion, vertebral abnormalities, renal agenesis, genital organ abnormalities