Tracing the origin of heterogeneity and symmetry breaking in the early mammalian embryo

Abstract

A fundamental question in developmental and stem cell biology concerns the origin and nature of signals that initiate asymmetry leading to pattern formation and self-organization. Instead of having prominent pre-patterning determinants as present in model organisms (worms, sea urchin, frog), we propose that the mammalian embryo takes advantage of more subtle cues such as compartmentalized intracellular reactions that generate micro-scale inhomogeneity, which is gradually amplified over several cellular generations to drive pattern formation while keeping developmental plasticity. It is therefore possible that by making use of compartmentalized information followed by its amplification, mammalian embryos would follow general principle of development found in other organisms in which the spatial cue is more robustly presented.

Fig. 1

Different ideas of the first mammalian cell fate decision and clues from “half-embryo” development. a, b The timeline of mammalian embryonic development leading to specification of the embryonic inner cell mass (ICM) and extra-embryonic trophectoderm (TE) lineages, and the different views of the fundamental question of whether a the first cues for cell fate bifurcation in the mammalian embryo emerge randomly and then become refined by spatial cues effective after from the 16-cell stage onwards; or b whether molecular cues for differentiation emerge much earlier and guide cell fate specification by affecting cell position, cell polarity, and differentiation so finally cell fate. A fundamental question underlying these two different ideas is whether it is molecular cues that guide the morphological distinction, or the morphological distinction guides molecular clues toward cell fate decisions. What then, if both exist? c The chance of a “half-embryo” derived from a 2-cell blastomere developing into a mouse is not equal^–. It depends on the number of epiblast cells generated by the embryo implantation. EPI epiblast, PE primitive endoderm

Fig. 2

Origin of cellular pattern formation from Turing’s theory to compartmentalized intracellular reactions. a Turing’s reaction-diffusion theory illustrates how two interacting chemicals with different diffusion rates can generate stable heterogeneity from a homogenous system. b, c Without changing the number of molecules in a biochemical reaction, the spatial information where b a specific type of molecule resides in the system, or c the relative sub-location of multiple molecules can lead to differential outcomes/products, thus altering the features of the system. d Illustration of the spatial mapping of the transcriptome at a single-cell and sub-cellular level by emerging technologies. e The morphological changes of a cell, due either to cell–cell contact or external forces, results in an altered reaction space inside the cell, leading to region-specific changes in biochemical reaction rates and cell properties. f, g The 4-cell mammalian embryo can be either f flattened or g tetrahedral in shape; the cell geometry and contact areas of tetrahedral 4-cell blastomere differ from the flattened blastomeres, which may trigger differential changes in intracellular reaction space related to cell fate

Fig. 3

The conditions under which stochastic events may, or may not, drive determinism. The key to whether any initially small bias in molecular expression will either be transformed into a stronger bias or neutralized will depend on the property of the molecule itself. a The transformation of a small bias into a strongly defined molecular pattern (bi-stable) would only apply to a subset of molecules (lineage specifiers) that bear the potential to trigger downstream events that are able to consolidate/amplify their influence (enhancing its color) to change the landscape of cell fate. b Otherwise, initially small biases will be diluted or reversed by subsequent stochastic fluctuations during development. Our hypothesis represents a view that differs from the classic “Waddington’s landscape” in which the valley, representing the path of a cell lineage, is already set. In our view, the landscape of cell fate could be gradually shaped by molecules that have the potential to “dig” and so alter the landscape as shown in a. These principles may apply to both embryonic as well as stem cell fate decisions

Fig. 4

Subcellular compartmentalization of lineage specifiers are key factors for cell fate decisions. a Differences between the nuclear location/accessibility of Sox2 at the 4-cell stage embryo are regulated by CARM1, which regulates the level of histone H3R26 methylation^,. This differential behavior of Sox2 leads to differential expression of lineage specifiers such as Sox21, the level of which then directs cell fate as Sox21 is repressor of Cdx2 that directed cell differentiation and is in the positive feedback loop with cell polarization^, . The identity of a potential factor present at the 2-cell embryo stage that may regulate heterogeneity in CARM1 activity at the 4-cell stage remains unknown. b Apically localized Cdx2 transcripts at the late 8-cell stage facilitate the asymmetric distribution of Cdx2 transcripts in daughter cells upon the 8–16 cell division, generating a bias in Cdx2 expression, which segregates cell fate which is further enhanced by nuclear localization of YAP^–