Anterior-posterior differences in vertebrate segments: Specification of trunk and tail somites in the zebrafish blastula

Abstract

The findings of Szeto and Kimelman (2006) begin to integrate our understanding of mesoderm patterning with vertebrate segmentation. Using a series of isochronic and heterchronic cell transplantation experiments, they show that the progenitors of the anterior trunk, posterior trunk, and tail somites are specified during a 1-h period in the zebrafish blastula by nodal, bmp, and fgf signaling. Despite this patterning, fate mapping experiments indicate that these distinct MPC populations intermingle in the late blastula and within the posterior tailbud. At the cellular level, it is not clear how such a pattern is generated nor how cells "remember" when to exit the progenitor pool and enter the segmentation program. We also do not understand how the early specification of anterior trunk, posterior trunk, and tail MPCs relates to the control of segmentation, although the two processes share a dependence on t-box genes, fgf signaling, and wnt signaling. The unresolved questions in the field involve larger issues fundamental to modern developmental biology and molecular medicine. Questions such as how signals are integrated, how one cell population can respond differently to the same signal at different times, and how cells decide to exit their progenitor stem cell niche and begin to differentiate need to be understood to both accurately model vertebrate development and to adroitly engineer stem cells. © 2006 by Cold Spring Harbor Laboratory Press.