Abstract
Hoxd13, Tbx2, Tbx3, Sall1 and Sall3 genes are candidates for encoding antero-posterior positional values in the developing chick wing and specifying digit identity. In order to build up a detailed profile of gene expression patterns in cell lineages that give rise to each of the digits over time, we compared 3 dimensional (3D) expression patterns of these genes during wing development and related them to digit fate maps. 3D gene expression data at stages 21, 24 and 27 spanning early bud to digital plate formation, captured from in situ hybridisation whole mounts using Optical Projection Tomography (OPT) were mapped to reference wing bud models. Grafts of wing bud tissue from GFP chicken embryos were used to fate map regions of the wing bud giving rise to each digit; 3D images of the grafts were captured using OPT and mapped on to the same models. Computational analysis of the combined computerised data revealed that Tbx2 and Tbx3 are expressed in digit 3 and 4 progenitors at all stages, consistent with encoding stable antero-posterior positional values established in the early bud; Hoxd13 and Sall1 expression is more dynamic, being associated with posterior digit 3 and 4 progenitors in the early bud but later becoming associated with anterior digit 2 progenitors in the digital plate. Sox9 expression in digit condensations lies within domains of digit progenitors defined by fate mapping; digit 3 condensations express Hoxd13 and Sall1, digit 4 condensations Hoxd13, Tbx3 and to a lesser extent Tbx2. Sall3 is only transiently expressed in digit 3 progenitors at stage 24 together with Sall1 and Hoxd13; then becomes excluded from the digital plate. These dynamic patterns of expression suggest that these genes may play different roles in digit identity either together or in combination at different stages including the digit condensation stage.
Highlights
There are a number of developmental systems in which apparently repetitive yet discrete and distinct structures form in a particular order and position
Tbx2 and Tbx3 expression patterns are very similar with two distinct stripes: one running along the anterior margin of the wing bud, the other along the posterior margin [19,36,37]. 3 dimensional (3D) analysis (Figure 1, A–C) shows that in the posterior margin, Tbx2 expression occurs entirely within the expression domain of Tbx3 (Figure 1, C ii–iv) while, in the anterior margin, expression of Tbx3 is entirely overlapped by Tbx2 expression
Some genes may perform the function of encoding stable positional values from early bud stages while others may play different roles at different stages including the digit condensation stage
Summary
There are a number of developmental systems in which apparently repetitive yet discrete and distinct structures form in a particular order and position. Examples of such systems are body segments of insects and, in vertebrates, hindbrain rhombomeres, pharyngeal arches, and teeth. For all of these systems, specific sets of transcription factors have been identified which specify position and confer a particular character on what initially appear to be similar structures e.g the transcription factor odontogenic code for the mandibular primordium that leads to the different teeth arising in their appropriate positions [1]) and the transcriptional Hox code for rhombomere identity [reviewed 2]. There is evidence that this information is specified in the early wing bud [5,6] but the cell condensations that give rise to the digits do not form until about 2
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