Imaging the onset of oscillatory signaling dynamics during mouse embryo gastrulation

A fundamental requirement for embryonic development is the coordination of
signaling activities in space and time. A notable example in vertebrate
embryos is found during somitogenesis, where gene expression oscillations
linked to the segmentation clock are synchronized across cells in the
presomitic mesoderm (PSM) and result in tissue-level wave patterns. To
examine their onset during mouse embryo development, we studied the
dynamics of the segmentation clock gene Lfng during gastrulation. To this
end, we established an imaging setup using selective plane illumination
microscopy (SPIM) that enables culture and simultaneous imaging of up to
four embryos (SPIM-for- 4). Using SPIM-for-4, combined with genetically
encoded signaling reporters, we detected the onset of Lfng oscillations
within newly formed mesoderm at presomite stages. Functionally, using
chemical inhibition and genetic mutants, we found that initial synchrony
and first ~6-8 oscillation cycles occurred even when Notch-signaling was
impaired, revealing similarities to previous findings made in zebrafish
embryos. Finally, we show that a spatial period gradient is already
present at the onset of oscillatory activity, providing a potential
mechanism accounting for our observation that wave patterns build up
gradually over the first oscillation cycles.