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Monitoring Stem Cell Research 
systems, connective tissue and blood vessels, and endoderm gives 
rise to epithelial parts (e.g., the linings) of the digestive and 
respiratory systems. 
Gastrulation is a crucial event in the development of the body 
plan of the individual, and it is a stage of development common to all 
vertebrates. Our understanding of the significance of establishing 
the three germ layers has grown more complex and subtle over the 
years. Once interpreted as three completely separate paths or 
compartments of development, we now know that the progeny of the 
three layers are not totally isolated in their fates. Cartilage, for 
example, was once thought to be entirely of mesodermal origin, but 
now we know that some cartilaginous structures of the head and 
neck come from ectoderm. Even more recently, work with certain 
adult stem cell populations in culture and under special conditions 
has suggested plasticity of cell progeny from one germ layer to 
develop characteristics of cells typically from another germ layer, 
long after gastrulation has assigned the cells of different germ layers 
their different fates. Gastrulation is not the first differentiating event: 
cells begin to acquire fates for different parts of the developing 
embryo before the inner cell mass separates into epiblast and 
hypoblast, indeed some results suggest even before the blastocyst 
develops an inner cell mass and trophoblast. Yet these findings in no 
way detract from the significance of gastrulation. They rather 
facilitate our understanding of gastrulation by placing it in the 
context of the entire process of differentiation, beginning from the 
very earliest stages. 
PRE-PUBLICATION VERSION 
