Morphogenesis of the early mammalian embryo: cell lineage heterogeneity and developmental potential

The cell lineage heterogeneity is an essential component of early morphogenesis and is associated with the maintenance of developmental potential for the embryo. The mammalian embryo has long been considered to exhibit remarkable regulative capacity, presumably reflecting a plasticity in developmental potential to combat different environmental conditions. Cell contact patterns between blastomeres have a major influence on this dynamic state. While compaction and epithelial cell polarisation at the 8-cell stage is mediated by cell adhesion, contact patterns remain effective in steering developmental pathways throughout the cleavage period. The capacity to recavitate and re-form trophectoderm-like outer cells is progressively lost when inner cell masses (ICMs) are isolated from older, more expanded blastocysts which then form predominantly primary endoderm-like structures. The cell contact pattern, either asymmetric or symmetric, maintaining trophectoderm and ICM lineage pathways respectively, acts upstream of signal transduction mechanisms regulating phenotypic status.