Evidence for Stem Cells in the Mesoderm of Hensen’s Node and Their Role in Embryonic Pattern Formation

Abstract

In the chick embryo, both mesoderm and endoderm are derived from a single germ layer, the epiblast, through the primitive streak, which is the first axial structure to develop. The streak arises after about 8 hours’ incubation, at stage 2 (Hamburger and Hamilton, 1951), as a thickening at the future caudal end of the blastoderm, just beneath the epiblast. It elongates cranially over the next 9 hours, while a groove appears along its length in the midline; cells migrate laterally from it to form the early mesoderm and the definitive (gut) endoderm. At the definitive streak stage (16–18 h of incubation; stage 4), the streak extends about three-quarters of the way across the area pellucida, with the primitive groove terminating rostrally in a pit. Anterior to this pit lies a mass of cells, termed Hensen’s node, after Viktor Hensen who first described it in 1876.