Morphological analysis of the development of the primary organizer in avian embryos

Abstract

AbstractScanning electron microscopic (SEM) analysis of the primary organizer region of the mesoblast of avian embryos, stages 3–5, showed that mesodermal cells gastrulated through the primitive streak are organized into distinct domains as early as stage 3. These domains initiate a metameric pattern consisting of prechordal plate and the first pair of somitomeres. The prechordal plate first appears immediately anterior to the primitive streak, along the embryonic midline, whereas the somitomeres are located on either side of the prechordal plate in the paraxial mesoblast region. The appearance of a metameric pattern during mesoblast expansion preceeds the formation of Hensen's node and full elongation of the primitive streak. By stage 4 of development the primitive streak is fully elongated, and Hensen's node is established as a raised hillock of epiblast at the cranialmost end of the axis. Also at this stage, avian embryos show the first indication of notochord formation. The notochord initially appears as a highly condensed, elliptical domain of concentrically organized chordamesoblast cells (100μm in length), deposited along the midline between the prechordal plate and Hensen's node (the source of chorda cells). This initial group of notochord cells has been termed a notochord segment. Continued regression of Hensen's node along the primitive streak results in the sequential generation of paraxial somitomeres and axial notochord segments, each segment measuring approximately 100 μm in length. By stage 5, the avian embryonic axis contains four pairs of somitomeres and four axial notochordal segments.