Figure 2. Expression of T-box genes during gastrulation (A) and somite formation (B) in the mouse embryo.

A. Eomes is expressed in the extraembryonic ectoderm prior to primitive streak formation and transiently in the primitive streak. Expression persists in the chorion [56,57]. T is first expressed in a band in the distal extraembryonic ectoderm [22], partially overlapping with Eomes, and later in the primitive streak, node and core of the allantois [58]. Tbx6 is first expressed in the primitive streak and newly formed paraxial mesoderm but does not appear in the node [24]. B. During somite formation, T continues to be expressed in the notochord and tailbud but is downregulated to undetectable levels in the PSM [59]. Tbx6 is expressed throughout the presomitic mesoderm and tail bud but is downregulated as the somites form [24]. Tbx18 is expressed in the anterior compartment of somites as well as stripes corresponding to the anterior portion of two developing somites (s0 and s-1) in the PSM [60]. Tbx22 shows a complementary expression pattern in the posterior portion of the forming somite (s0) and in the posterior portion of the newly formed somites, fading as the somites mature [41]. Tbx2 is not expressed in the first 10–15 somites to form but at later stages is expressed in the PSM in the posterior aspect of the two forming somites and in the posterior compartment of 5–6 newly formed somites [43].