Classification scheme for genes expressed during formation and progressionof the avian primitive streak

We have systematically examined the expression patterns of thirteen genes b y in situ hybridization during the formation and progression of the avian p rimitive streak. Based on common patterns of expression, we classify these genes into three distinct groups. Group I genes, subdivided into group 1A ( Wnt8c, Slug, Vg1, and Nodal) and group 1B (Fgf8, Brachyury, and Cripto), we re expressed first in the epiblast and then, throughout most of the length of the primitive streak. Group 2 genes, namely, cNot1, Sonic hedgehog (Shh) , Hnf3 beta and Chordin, were confined to the rostral end of the primitive streak, and then, to Hensen's node. In contrast, Group 3 genes, comprising Goosecoid (GSC) and Crescent, were expressed in the hypoblast. This classif ication scheme provides a rational basis for categorizing genes expressed d uring avian gastrulation, and such systematization is likely to provide ins ight into the relationships among different genes and their potential roles in key events of gastrulation. Anat Rec 262:221-226, 2001. (C) 2001 Wiley- Liss, Inc.