Localization of cells of the prospective neural plate, heart and somites within the primitive streak and epiblast of avian embryos at intermediate primitive-streak stages

By constructing avian transplantation chimeras using fluorescently-labeled grafts and antibodies specific for grafted cells, we have generated a prosp ective fate map of the primitive streak and epiblast of the avian blastoder m at intermediate primitive-streak stages (stages 3a/ 3b). This high-resolu tion map confirms our previous study on the origin of the cardiovascular sy stem from the primitive streak at these stages and provides new information on the epiblast origin of the neural plate, heart and somites. In addition , the origin of the rostral endoderm is now documented in more detail. The map shows that the prospective neural plate arises from the epiblast in clo se association with the rostral end of the primitive streak and lies within an area extending 250 mum rostral to the streak, 250 mum lateral to the st reak and 125 mum caudal to the rostra[ border of the streak. The future flo or plate of the neural tube arises within the midline just rostra[ to the s treak, confirming our earlier study, but unlike at the late-primitive strea k stages when both Hensen's node and the midline area rostra[ to Hensen's n ode contribute to the floor plate, only the area rostral to the primitive s treak contributes to the floor plate at intermediate primitive-streak stage s. Instead of contributing to the floor plate of the neural tube, the rostr al end of the primitive streak at intermediate primitive-streak stages form s the notochord as well as the rostromedial endoderm, which lies beneath th e prechordal plate mesoderm and extends caudolaterally on each side toward the cardiogenic areas. The epiblast lateral to the primitive streak and cau dal to the neural plate contributes to the heart and it does so in rostroca udal sequence (i.e., rostral grafts contribute to rostral levels of the str aight heart tube, whereas progressively more caudal grafts contribute to pr ogressively more caudal levels of the straight heart tube), and individual epiblast grafts contribute cells to both the myocardium and endocardium. Th e prospective somites (i.e., paraxial mesoderm) lie within the epiblast jus t lateral to the prospective heart mesoderm. Comparing this map with that c onstructed at late primitive-streak stages reveals that by the late primiti ve-streak stages, prospective heart mesoderm has moved from the epiblast th rough the primitive streak and into the mesodermal mantle, and that some of the prospective somitic mesoderm has entered the primitive streak and is u ndergoing ingression. Copyright (C) 2001 S. Karger AG, Basel.