Development of the chick embryo mesoblast. Formation of the embryonic axis and establishment of the metameric pattern

The mesoblast of the primary organizer region of the developing chick embryo at the early head process stage was examined with the scanning electron microscope. It was found that the mesoblast layer is patterned from its inception at the primitive streak. Viewed dorsally, the mesoblast region most recently traversed by Hensen's node is metameric. Each segment consists of two 175-μm-diameter circular buttons of paraxial mesoblast (somitomeres) and an enclosed axial region. These tripartite segments are stacked tandemly and mark precisely, in the ectoderm above, the limit of neural plate formation. Viewed ventrally, the metameric pattern of the mesoblast is most closely mimicked by underlying endoblast, which shows corresponding radially arranged wedge-shaped cells in somitomere-sized circular patches. At this stage of development, each paraxial somitomere is a slightly hollowed, squat cylinder, composed of tapering mesenchymal cells whose long axes are directed toward the core center. Closely timed with neurulation, somitomeres undergo morphogenesis, being first converted to triangular wedges and, finally, condensed into cubes. Anteriorly, somitomeres participate in branchiomeric development, while posteriorly, they develop into somites. Examination of segmental plates shows that they consist of about 11 tandem somitomeres not visible by light microscopy. The most mature somitomeres, closest to the emerging somites, are delineated from one another by cellular orientations and the progressive buildup of fibrous extracellular matrix. The least mature somitomeres are not as well defined, but appear initially just posterior to Hensen's node and merge medially with the notochordal process. The observations suggest that the emergence of somitomeres from the paraxial mesoblast of the primitive streak is the result of its association with nodal cells. Further, this combined association of the mesoblast heralds primary induction and establishes the metameric pattern of the basic body plan.