|Abstract: ||Anterior-posterior (A-P) axis specification requires signalling from a specialised extraembryonic
tissue called the anterior visceral endoderm (AVE). AVE precursors are
induced at the distal tip of the pre-gastrulating mammalian embryo and move to the
prospective anterior. This movement is essential in correctly positioning the A-P axis.
Firstly, the contribution of proliferation to AVE movements has been tested. Previous
studies have suggested that differential proliferation within the visceral endoderm (VE) is
required to determine its direction by passive displacement. High-resolution imaging and
embryo culture experiments have been used to assess the distribution and requirements of
proliferation during AVE movements. Differential proliferation was not observed along
the A-P axis of the VE. However, proliferation within the epiblast was found to be
required for correct AVE movements in a Nodal-dependant manner.
Secondly, to address the function of the AVE in vivo, this tissue was genetically ablated.
Diphtheria toxin A was knocked-in to the Hex locus (an early AVE marker) by
homologous recombination in embryonic stem cells. Depending on the time of ablation
the AVE or its precursors could be ablated as demonstrated by AVE marker analysis.
Ablation of the AVE precursors prior to 5.5 dpc resulted in embryos that displayed
greatly disturbed patterning and were unable to gastrulate. AVE ablation between 5.5-6.5
dpc resulted in delayed formation of the primitive streak, epithelial to mesenchymal
transition defects and a mispatterning of the anterior primitive streak. A decrease in
Nodal signalling was found to be a likely cause for the defects observed in these ablated
embryos, indicating that AVE maintains Nodal signalling levels in the epiblast.
Together these experiments find a role for epiblast proliferation in AVE migration and
identify a novel role for the AVE in patterning the primitive streak. Therefore this
investigation provides a critical insight into how the A-P axis is established in mammals.|