Fate Map Of The Interface Between Somitic And Lateral Plate Mesoderm During Body Wall Formation - A New Way At Looking At Embryonic Cell Populations
We have mapped the interface between the somitic and lateral plate mesoderm using quail-chick chimeras (Nowicki et al 2003). These observational data have necessitated the introduction of new terminology to reflect the early ‘community’ relations of populations of mesodermal cells.
This animation, colored in shades of blue, yellow and green, is designed to highlight those domains and introduce the terminology.
The somitic mesoderm is depicted in blue and the early lateral plate is depicted in yellow. As the body wall begins to form, the somite population begins to expand. A significant proportion of the somitic cells maintain an exclusively somitic neighborhood. That is, they do not intermingle with cells from the lateral plate. Rather, the bulk of the somite population displaces the lateral plate, and as a coherent population, will form the ribs and intercostal muscles.
This all-somite community maintains its original composition (implied by the blue color) and we refer to this community as the primaxial domain. All of the structures in the axial system arise from somitic cells that do not mix with other cell populations and thus by definition are primaxial. (The name reflects the central position of this cell population and the structures it forms during both the development and evolution of the vertebrate body plan).
Figure 1
Figure 2
At the same time, a subpopulation of somitic cells leaves the primaxial domain (blue) and enters the lateral plate (yellow). These somite cells have changed their community and now intermingle with cells of the lateral plate. We call this new community, comprised of both somitic and lateral plate cells the abaxial domain. The change is reflected in the animation by the color change from yellow and blue to green. The appendicular system arises from the abaxial domain.
As can be seen in the cross sections of early embryos (Figure 1), the boundary between the somitic and the lateral plate mesoderm is initially exclusionary. During body wall development, the expansion of the primaxial domain pushes that boundary towards the ventral mid line. At the same time, the boundary is transgressed by some somite cells that migrate out of their parental somitic neighborhood to intermingle with cells of the lateral plate.We refer to the boundary between primaxial and abaxial domains as the Lateral Somitic Frontier (LSF).