The rSC begins to express <i>Prox1</i> and to acquire a tube-like morphology.

<p>Z-projection of confocal stacks encompassing the rSC from a <i>Prox1-GFP</i> eye at P4.5. VECAD immunostaining shows a region with branching of the rudimentary vessel (block arrows, equivalent to stage D in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001912#pbio-1001912-g011" target="_blank">Figure 11</a>). <i>Prox1</i> is first expressed in the rSC at late P4. The branching rSC begins to acquire <i>Prox1</i> expression as indicated by GFP⁺ cells (arrowhead). Higher <i>Prox1</i> expression correlates with transition of rSC morphology from a sprouting chain of cells to a flattened tube (arrows). (Inset) DIC merged image showing that the tubular regions of the rSC have pockets of space filled with RBCs (black arrows). Scale bar, 50 µm.