Figure 1. TLE expression in the embryonic chick spinal cord.

(A) Schematic representation of the five progenitor cell (p) domains of the ventral spinal cord, termed p0, p1, p2, pMN and p3 from dorsal to ventral positions, respectively. These domains are defined by the specific expression of combinations of HD and bHLH transcription factors. Refinement and maintenance of these progenitor domains is achieved through cross-repressive interactions between pairs of transcription factors, for example between Pax6 and Nkx2.2 at the pMN/p3 boundary. In turn, each progenitor domain generates different neuronal populations, V0, V1 and V2 INs, somatic MNs and V3 INs, respectively. Like the progenitor domains, separate populations of postmitotic neurons can be defined by the expression of specific transcription factors, such as HB9 and Isl1 in MNs derived from the pMN domain or other factors in other cell types, as indicated in the right-hand column. (B–E) Sections through the spinal cord of HH stage 18 chick embryos were subjected to double-labeling immunohistochemical analysis using a panTLE antibody together with antibodies against the indicated proteins. Panels in the right-hand column show high-magnification views of the boxed areas in the adjacent panels. Arrows point to examples of double-labeled cells. Arrowheads point to examples of cells expressing only TLE. TLE expression was observed in most ventral spinal cord cells, including domains p0–p2 (region of high Pax6 immunoreactivity dorsal to the Olig2+ domain), pMN (region expressing Nkx6.1, Olig2 and low levels of Pax6) and p3 (region expressing Nkx6.1 and Nkx2.2) of the ventral area. Notice in particular how virtually all Nkx2.2+ cells also express TLE.