The paraxial and other regions of the mesoderm are thought to be specified by bone morphogenetic proteins (BMPs) along an axis spanning from the center to the sides of the body.
Members of the fibroblast growth factor family also play an important role, as does the Wnt pathway.
[1] This specification process has now been fully recapitulated in vitro with the formation of paraxial mesoderm progenitors from pluripotent stem cells, using a directed differentiation approach.
[3] As the primitive streak continues to regress, somites form from the paraxial mesoderm by "budding off" rostrally.
As the primitive streak regresses and somites bud off anteriorly, new cells derived from these stem-cell like precursors constantly enter the posterior end of the paraxial mesoderm.